Report Contribution of the calorific value method to the ... · Prüflaboratorium Gas. 2.2 Net and gross calorific value . The net calorific value (H. net) describes the energy released

DVGW-Forschungsstelle am Engler-Bunte-Institut der Universität Karlsruhe

Prüflaboratorium Gas

Report

Contribution of the calorific value method to the reduction of primary energy

consumption for heating purpose in Germany

DVGW-Forschungsstelle Prüflaboratorium Gas Eine Einrichtung des DVGW Deutsche Vereinigung des Gas- und Wasserfaches e.V.

Engler-Bunte-Institut Universität Karlsruhe Postfach 6980 D-76128 Karlsruhe

endisch

Autoren: Prof. Dr. -Ing. Bockhorn Prof. Dr. -Ing. Lenze Dipl.-Ing. John Dipl. Ing. Stenger

DVGW-Forschungsstelle Prüflaboratorium Gas

Table of Content

1 Introduction ....................................................................................................... 1 2 Fundamentals .................................................................................................... 6

2.1 Principles...................................................................................................... 6 2.2 Net and gross calorific value ........................................................................ 7 2.3 Volume-, mass- and enthalpy balances ....................................................... 9

3 Privileges and exceptional positions in Germany and Europe ................... 18 3.1 Standard efficiency..................................................................................... 18 3.2 Condensing technology in existing buildings.............................................. 21

4 Influencing variables and criteria for optimum gain .................................... 23 4.1 Boiler design .............................................................................................. 23 4.2 CO2 content and burner design .................................................................. 28 4.3 Water connections...................................................................................... 29 4.4 Heating systems......................................................................................... 33

5 Emissions and flue gas systems ................................................................... 37 6 Summary and Conclusions ............................................................................ 41 7 Nomenclature .................................................................................................. 44 8 Literature.......................................................................................................... 45

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DVGW-Forschungsstelle am Engler-Bunte-Institut der Universität Karlsruhe

Prüflaboratorium Gas

1 Introduction The global resources of primary energy such as coal, oil and gases are limited. The

amount of air pollution from combustion of fossil fuels such as SO2, CO, NOx, CO2,

soot and hydrocarbons some of these being injurious products concerning global

warming have to be diminished as soon and as much as possible. The KYOTO

protocol demands a drastic reduction of CO2 and toxic gas exhaust gas components,

which can be reached by cleaner combustion systems with higher efficiency in

industry, power plants, motors and domestic oil and gas combustion installations

above all.

0

2.000

4.000

6.000

8.000

10.000

12.000

14.000

16.000

1960 1970 1973 1975 1979 1980 1985 1990 1991 1992 1994 1995 1996 1997 1998 1999 2000 2003

Year

Ener

gy in

Pet

ajou

le (P

J)

PEC

EEC

TR

HH

CTS

IND

Fig. 1 : Energy consumption of Germany in the last 45 years [5,7]

The development of heating appliances with higher efficiencies and lower pollution

has made a great progress within the last 20 years. Thus the efficiencies of power

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plants increased from 35 to 40% for coal and oil fired boilers and from 40 to nearly

60% for combined cycle power plants and achieved values of about 85% for power-

heat coupling processes. In the field of domestic heat production the efficiencies

increased from about 75% to 90% with low temperature boilers and up to 103

respective 107% with condensing boilers for oil respective gas (natural gases).

In this report the present knowledge from literature and technical applications is

collected and discussed. The report presents the potential for reduction of energy

consumption and air pollution within the field of heating of buildings by use of the

calorific value method which is considered being the most efficient technology for

heating purpose.

The increase of efficiencies reduces the usage of natural resources such as coal, oil

and natural gases and decreases the emission of toxic and greenhouse-phenomena

effecting exhaust gases. In the western part of Europe, especially in Germany, the

energy consumption has increased very slowly during the last 30 years (Fig. 1) and

the emission of toxic and greenhouse gases like NOx, SOx and CO2 has decreased

(Fig. 2). The efficiency of the use of energy - pointed out by the decrease of the ratio

of energy consumption to gross social product – has increased dramatically during

the last 25 years (see Fig. 3).

0

1000

2000

3000

4000

5000

6000

1970 1975 1980 1985 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

Year

CO2

(Mt),

NO

x (k

t), S

O2

(kt)

CO_2NO_xSO_2

Fig. 2 : CO2-, CO-, NOx-, SOx- emissions in Germany in the last 30 years [5]

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0

0,5

1

1,5

2

2,5

3

3,5

1970 1980 1985 1990 1993 1994 1995 1996 1997 1998 1999 2000 2001Year

PEV/BIP (kW h/Euro)

Fig. 3 : Ratio of energy consumption to gross social product in

Germany in the last 25 years. [7,8] Fig. 4 shows which of the atmospheric trace gases contribute by which fraction to the

Greenhouse effect. It demonstrates the importance of CO2 and CFC but also

methane. The guidelines for reduction of the green house gases, especially CO2, are

the national CO2-reduction aims, which are shown in Fig. 5 for the different energy

consuming sectors. The figure makes clear the importance of the household energy

consumption and the need for more effective energy use in this sector.

Fig. 4 : Ratio of trace gases on the anthropogenic green house effect [2]

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Fig. 5 : Aims of national CO2-reduction in the different parts of energy use. [2]

Following the progress in development of high efficient condensing boilers the use of

these boilers increased dramatically during the past 12 years and will increase further

on, as can be seen in Fig. 6 a-c for Great Britain (c), France (b) and Germany (a).

Fig. 6 a

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Fig. 6 b

Fig. 6 c Fig. 6 a - c: Development of the market for different systems in a) Germany,

b) France and c) Great Britain between 1992 – 2006. [6]

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The forecast of the future increase in demand of condensing boilers in Great Britain

for instance is due to an ordinance which implies that after the 1st of April 2005 all

new installations of boilers for the household heating must be condensing boilers. [6]

However, also in Germany the number of condensing boilers has increased

continuously during the past 10 years and will increase further.

2 Fundamentals

2.1 Principles The calorific value method is based on the condensation of parts of the water vapour

of the flue gases. It is a very efficient method of converting natural gas, LPG´s and/or

fuel oil into useful energy – especially in central heating systems with low return

water temperatures - by combustion processes. An earlier and still used method is

low-temperature technology: it follows the principle of operating the boiler only with

that temperature, which is required to cover the current heating demand of more or

less low temperature level and to prevent corrosions caused by condensation of

water vapour in the boilers and the heating gas flues. That leads necessarily to

higher flue gas temperatures (80 < t < 150°C depending on flue gas constituents) in

the boiler and at the boiler exit. The temperature at the boiler exit is a measure for the

combustion efficiency, as will be shown later.

During operation of low-temperature boilers and boilers equipped with steely and/or

cast iron combustion chambers and heat transfer channels, condensing of hot gases

and subsequent wetting of the heating surfaces must be avoided. The calorific value

method operates to quite different rules: condensing of the flue gases is highly

desirable and is necessary to turn the latent energy contained in flue gas water

vapour, in addition to the sensible flue gas energy into useful low temperature heat

for central heating systems with radiators or floor heating pipes and/or domestic hot

water stores (DHW).

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2.2 Net and gross calorific value The net calorific value (Hnet) describes the energy released during complete

combustion, if the water created in the process is turned into vapour. The gross

calorific value (Hgross) defines the energy released during complete combustion

including the evaporation heat contained in the water vapour of the flue gas.

Table 1 provides an overview of the fuel characteristics, which are relevant to the

utilisation of the calorific value method. In the past, the evaporation heat could not be

utilised, since the relevant technical prerequisites and demands have not yet been

available. Most combustion systems worked at high temperature and high flue gas

temperature levels. Therefore, the net calorific value (Hnet) was used as reference for

all efficiency calculations. Referring to Hnet and utilising the additional evaporation

heat thus can result in standard efficiencies above 100 %. Because of mostly used

guidelines, standard efficiencies in heating technology continue to refer to the net

calorific value (Hnet).

Following the data given in the principle sketch of a natural gas condensing boiler

(Fig. 8), the equations given in chapter 2.3 and the values from table 1 the gross and

net efficiencies can be calculated depending on the enthalpies

ηnet = Hprofit net / Hcomb. net = ( Hcomb. – Hb – Hfl ex)/ Hcomb.net η gross = Hprofit gross / Hcomb. net = ( Hcomb. – Hb – Hfl ex + Hcond ) / Hcomb.net

It follows that η gross >ηnet if tfl < td and normally according to definition ηgross > 100%.

The calculations give a theoretical energy efficiency gain for natural gas, compared to

low-temperature technology, of 11 %. With fuel oil the additional energy efficiency

gain by application of the calorific value method is only about 6 % caused by the

lower H/C ratio (Fig. 7)

The heat losses exhausted via the flue as residual heat of the flue gases will be

substantially reduced compared to low-temperature boilers. This is because the flue

gas temperature can be substantially reduced (from 150°C(flue gases containing

SO2)/80°C(older boilers) to 50°C(condensing boilers)/30°C(condensing boilers with

floor heating)) by application of condensing boilers.

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100%

109%

96,2%

100%

104%

Net calorific value 100%

Gross calorific value 111%

11%

11%

6%

0 10 20 30 40 50 60 70 80 90 100 110

Natural Gas

Low Temperature Boiler

Gas-Fired condensingBoiler

Oil

Oil fired condensingBoiler

Net calorific value

Gross calorific valueKWKWater vapourBoiler lossflue Gas loss

1%

1%

1%

1%

3%

1%

Fig. 7 : Total efficiency (with and without heat losses) for different fuels and

boiler technologies

Fig. 7 shows the general differences of enthalpy ratios between gross and net

calorific values. A comparison of the efficiency of oil and gas fired condensing boilers

is made as well as a comparison of efficiency for low temperature and condensing

boilers fired with natural gas.

Principally one has to distinguish between efficiencies η and standard or norm

efficiencies ε. The efficiency η is the energy converted during the time, when the

burner is running with full or reduced load. Only the conditions and losses during the

operation time of the burner are considered. There are different efficiencies for a

boiler system like reaction-, furnace-, combustion- and total efficiency. For domestic

boiler systems operating with gas or oil the reaction efficiency is 100% (no unburned

fuel, fuel containing ashes, soot and/or CO), the furnace efficiency about 97-99%

(low boiler wall temperatures and good insulations and following small losses by

radiation and convection outside the boiler), the combustion efficiency 90–108% and

the total efficiency - the product of the previous efficiencies - 88-107% related on the

net calorific values.

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The standard or norm efficiency ε is the sum of profits during a certain time – for

instance a year. Within this time the burner has periods of full, half and zero loads

and the ambient temperature changes with the seasons. During the year the boiler

system has additional heat losses caused by the starting procedure and the standby

times, when more or less hot boilers have losses by radiation and convection to the

colder surrounding, depending on where the boiler is installed. The procedure to

calculate or to proof the efficiencies η and standard or norm efficiencies ε by

experiments is described in DIN 4702 T2 ; T8, in EN 303 [11] and in Viessman

handbook for heating systems.[3]

2.3 Volume-, mass- and enthalpy balances Following the data in the principle sketch of a natural gas condensing boiler (Fig. 8)

one can make the volume-, mass- and enthalpy-balances and calculate the efficiency

factors.

Most fuels primarily consist of carbon(C) and hydrogen (H) compounds. During the

combustion of natural gas, LPG´s and fuel oil the carbon and hydrogen reacts with

the air component oxygen (O2) to carbon dioxide (CO2) and water (H2O).

The general equations for volume- (m3/m3fu) and mass-balances (Kg/Kgfu) for

gaseous fuels are:

vfu + va = vfl and with v * ρ = m one gets mfu + ma = mfl vfu + λ * va = vCO2 + vH2O + vN2 + λ * va with the excess air factor λ

For natural gas –100% methane (CH4) is taken for simplification of the calculations –

the following combustion equation for a boiler system as shown in Fig. 8 applies:

CH4 +2 O2 ->2 H2O +CO2 + energy (heat)

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2H2O

CO2

O2

AIR 3,76 N2

CH4

O2

AIR 3,76 N2

7,52 N2

QBoiler loss 1%

Qprofit 98%

Qprofit,kond 11%

QFlue gas loss 1 %

Qprofit,total

flue gas dry

Fig. 8 : Volume (mass) and enthalpy-balance for natural gas condensing boiler

Considering air as oxidiser and the use of excess air (λ) (lean combustion)

λ = va / vstoich (values for domestic boilers with fan driven burners are usually λ = 1,05 – 1.25

and for atmospheric burners λ = 1.8 – 2.2 ) one gets

1CH4 + 2* λ *(O2 + 3,76 N2) = 1*CO2 + 2*H2O + 8,52 *N2 + (λ-1) * va

Condensate will be formed from the water vapour (2H2O) in the hot gas, if the

temperature near the walls on the hot gas side of the heating surfaces falls below the

water vapour dew point. The excess air factor λ can be calculated from the measured

dry volume-concentration of CO2 and / or O2 with following equations

λ = CO2 max,dr / CO2 meas,dr and λ = 21 / (21-O2 meas,dr)

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The enthalpy balance is Hfu + Ha = Hfl dr + HH2O + Hcond. HCH4 + λ * 2*(HO2 + 3,76*HN2) = HCO2 + 2*HH2O + 8,52*HN2 +(λ-1)*Ha

Hloss = Hfl ex + Hboilerloss – Hcond.

= (vCO2*cp CO2 + vH2O*cp H2O + vN2 *cp N2) * (tfl ex – ta in) + Hboilerloss

– Hcond.theor* α (tfl ex) with

α = vcond. actual / vcond. theor. Values of vcond. theor can be found in Tab.1 The values of α can be taken from Fig. 11

and Fig. 12, which show for nearly stoichiometric conditions ( λ ~ 1.1 ) the

dependence of α, η and vH2O on the flue gas temperature at the exit of the boiler.

Table 1 Energy levels of different fuels [1,3]

Gross calorific value HGross kWh/m3

Net calorific value Hnet kWh/m3

HGross/Hnet

HGross-Hnet

kWh/m3

Volume of

condensate (theoretical)

kg/m3 1)

Vair

m3/m3 1)

Vflue gas,wet

m3/m3 1)

Vflue gas, dry

m3/m3 1)

CO2,max,fuel

vol % dry

f

Town gas 5,48 4,87 1,13 0,61 0,89 3,88 4,54 3,59 12,1 0,5762

Natural gas LL 9,78 8,83 1,11 0,95 1,53 8,4 5,4 7,7 11,8 0,5619

Natural gas E 11,46 10,35 1,11 1,11 1,63 9,8 10,9 8,9 12,0 0,5714

Propane 28,02 25,8 1,09 2,22 3,37 23,8 25,8 21,8 13,8 0,6571

CH4 11,07 9,95 1,11 1,12 1,61 9,52 10,52 8,52 11,7 0,5589

Fuel oil EL2) 10,68 10,08 1,06 0,6 0,88 11,2 11,8 11,2 15,5 0,7381

1) relative to the fuel volume 2) for fuel oil EL, details refer to the unit “litre”

The different chemical consistencies of natural gases and fuel oils result in different

C/H-ratios and λ depending water vapour dew point temperatures, at which water

vapour in the hot flue gas condenses and which can be calculated by the partial

pressures of water vapour in the flue gas.

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15

20

25

30

35

40

45

50

55

60

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

CO2 contents [vol %]

Wat

er v

apou

r dew

poi

nt [°

C]

Natural gas (95% CH4)

Fuel oil EL

57

47

Atmospheric burner Pressure jet burner

λ=2,25 λ=1,14 λ=1,15

gas gas oil

Fig. 9 : Water vapour dew point in dependence of CO2 and λ for natural gas

and fuel oil (EL) [3]

In the near stoichiometric range, the water vapour dew point temperature tdew point for

natural gas is approximately 57 °C and depends on excess air factor λ (Fig. 10) and

for fuel oil EL it is approximately 47 °C. These values can be taken from Fig. 9 and

Fig. 10 in dependence on excess air factor and CO2 meas,dry . Boilers with atmospheric

burners are operated with very high excess air factor and low concentration of CO2 in

the flue gas (Fig. 9). These burners are the injector burners, which work with high

excess air factors to enable a secure and complete combustion. Further the

combustion chamber is open to the surrounding allowing a draught into the boiler

depending on the load and the combustion chamber temperature, which usually can

not be prevented by the flow safety duct. However high excess air factors dilute the

flue gas and reduce the water vapour concentration and consequently the dew point

temperatures, the rate of condensation and the enthalpy profit as it is shown in

Fig. 10 and Fig. 11.

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Fig. 10 : Dew point of flue gas (NG) in dependence on excess air factor λ

Fig. 11 : Water content m, rate of condensation α enthalpy ratio η and

enthalpy profit for condensation in dependence on the flue gas temperature at the boiler exit

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Fig. 12 : Efficiencies and mass flow of condensed water in dependence on

return flow temperatures

Fig. 13 : Excess air factor and dew point in dependence on CO2* for gas and

oil flames

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A more general way for calculation of the flue gas dew point temperatures for every

fuel uses the ratio of the maximum CO2 –flue gas concentration of a fuel (gas, oil) to

the maximum possible CO2 –flue gas concentration CO2*max (21%), resulting from

stoichiometric reaction of pure C with air (21% O2). From this ratio a factor

f(CO2 fuel) = CO2 fuel / CO2* max (21%) is formed. With this factor f it is possible to

calculate a hypothetical CO2*fuel,λ − flue gas concentration for every fuel and the

related excess air factor. Finally the dew point for every fuel in dependence on

CO2*fuel,λ and /or λ can be calculated.

Fig. 13 shows CO2* and tdew point in dependence on λ and/or CO2*. Division of the

known concentration CO2 meas. by factual fuel , which can be extracted from table 1

provides CO2*actual fuel and /or λ. The dew point temperature for the actual fuel can

now be read off on the scale on the right site of figure 13.

Two examples:

NG:CO2meas.=10,5% / f(=0,5714) gives CO2*=18,375%, λ= 1,14 and tdew point NG=570C

Oil: CO2 meas.=13,5% / f(=0,7381) gives CO2*=16,26%, λ= 1,30 and tdew point oil= 450 C.

Following the equations above it is obvious, that the energy gain of a condensing

boiler compared to a low-temperature boiler is not just the result of the condensing

energy gain. To a large extent, it is a consequence of the lower flue gas loss resulting

from a smaller excess air factor λ and therefore a reduced flue gas flow and the

lower flue gas exit temperature tfluegas exit, which is only a few degrees higher than the

return water-temperature. A basic energy assessment can be made using the boiler

efficiency equations (chapter 2.1). Compared with a conventional boiler, the

condensing boiler efficiency formula is expanded by the condensation proportion,

which is defined by the fuel-specific constant values Hgross and Hnet, as well as by the

variable condensation-ratio α which is a function of temperature and excess air factor

λ. The factor α provides the ratio between the actual volume of condensate in a

condensing boiler and the theoretically possible (flue gas temperature=00C) volume

of condensate. The lower the flue gas temperatures the higher is the actual volume of

condensate and therefore the condensate factor α and the more effective is the

condensing boiler. At the same time, the lower flue gas temperature in a condensing

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system, compared to a low-temperature boiler, also reduces the flue gas loss and

increases the efficiency at an average of 1% per ∆T=200C reduction of flue gas

temperature.

This means that condensing boilers achieve an improved energy utilisation by

lowering the flue gas losses, as well as by gaining condensation energy. A simple

equation which only needs some summary-factors - coming from the calculations of

enthalpies and flue gas concentrations (CO2, λ, O2) - for the calculation of flue gas

losses Hflue gas and efficiency η is given by the German pollution law (table 2). [9]

Hfl/Hnet = ( tfl exit – ta )*(( A1 / CO2 dr ) + B )

η = 1 – (( Hfl + Hboiler ) / Hnet ) + ( α ( Hgross – Hnet )/ Hnet )

with the factors A1, A2, and B for different fuels and different flue gas concentration

measurement.

Another very simple equation for the efficiency η is given by Siegert [4]

η=100-(F*(tfl exit-ta)/CO2, dry))

with t in °C, CO2,dry in % and F factor for fan or in ( ) for atmospheric driven boilers

Table 2: Fuel correction values acc. to the 1st BimSchV Germany Fuel oil

EL

Natural gas

Town gas

Coke gas

LPG and LPG-air mixtures

A1 (CO2Meas.) A2 (O2 Meas.) B F

0,5 0,68 0,007 0,59

0,37 0,66 0,009 0,46 (0,42)

0,35 0,63 0,011 0,38 (0,35)

0,29 0,6 0,011 0,36

0,42 0,63 0,008 0,35

For ground floor heating with low return water temperatures the flue gas-losses

Hfl are about 2 –3 %, the boiler-losses Hboiler about 1% and the profit by condensation

is about 6-7%. The part-load efficiencies are increased too with ground floor heating

caused by the lower return water temperature as will be shown in Fig. 14 and Fig. 15.

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Fig. 14 : Part-load efficiencies for various boilers depending on boiler

load for low-temperature and condensing boilers

Fig. 15 : Dependencies of efficiencies on outside temperatures

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The part load efficiencies shown in Fig. 14 and Fig. 15 has been fixed by the method

described in DIN 4702 1-10 and has been compared with real expense

measurements for different houses , flats and rooms and the differences between the

two methods were small (about 1-3%) [2].

3 Privileges and exceptional positions in Germany and Europe

3.1 Standard efficiency In Germany the standard efficiency is defined in DIN 4702-8 [11] and is used to

calculate e.g. the yearly energy efficiency of modern boilers. It is defined as the ratio

between the heat available p.a. and the combustion heat supplied to the boiler

(relative to the net calorific value of the fuel).

Fig. 16 : Calculation of the standard efficiency acc. DIN 4702 [11]

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A process was determined by DIN 4702 [11], which will lead to comparable details

based on standardised test facility measurements. For Germany five working loads of

the boiler, which are illustrated in figure 16, were defined relative to the annual

heating output.

For each work load stage the same heating output defined as product of boiler load

and heating time is calculated. Two temperature pairs result for each of the five levels

defined by DIN 4702 (one pair based on radiator central heating: design basis

75/60 °C; one pair based on a floor heating system: design basis 40/30 °C acc. to

EN 677). For each, part-load efficiency is determined on the test stand. To calculate

the standard efficiency, the five actual part-load efficiency levels are averaged. This

results in comparable values, which generally reflect a realistic boiler operation in

Germany.

The boiler design should ensure that at the lowest likely outside temperature the heat

demand can be fully covered. For Germany, these design temperatures are –10 to –

16 °C. However, in Germany such low temperatures described above are rarely

reached during daytime operation, hence the boiler must only provide its full output

for a few days each year. For the remainder of its operating time, only a fraction of its

rated output needs to be provided. For the total year the largest part of the required

heating energy in Germany therefore, relates to temperatures above freezing (0 to 5

°C), which are perhaps lower ( 0 - -50C) in other countries.

Fig. 17 : Proportion of heating load subject to outside temperature

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For Germany this results in the average boiler load over a period of twelve months

being less than 30 % of rated output.

Fig. 18 and Fig. 15 permit a comparison of part-load level efficiencies of different

boiler configurations, particularly for low average loads. The advantages offered by

condensing boilers are significant, especially in the case of low average loads.

Reduction of the load results in substantial losses for a boiler operating at a constant

temperature, since the boiler temperature must be maintained at a high level, even if

the temperature level of heating system demands only a low output. One result is a

substantially higher proportion of radiation losses as part of the overall energy

requirement and hence a reduced level of efficiency. Condensing boilers, on the

other hand, provide an especially high level of efficiency in cases of low average

loads, because the condensing effect is particularly successful due to the low

temperature level of the heating- and return-water.

Fig. 18 : Part load- and standard-efficiencies for various boiler designs [3]

Fig. 18 illustrates a comparison of the part load efficiencies ε for various types of

boilers (curves A – E) in dependence on specific combustion loads and/or outside

temperatures. Specific combustion loads and outside temperatures are coupled,

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because the lower the outside temperatures the higher the combustion load has to

be. The curves D and E in fig.18 show part load efficiencies for very old boilers with

on/off-regulations, high and constant boiler water temperature (curve E) or lower

temperature limits but still no regulation devices (curve D). These very low part load

efficiencies for boilers E and D are caused by high flue gas and boiler water

temperatures, high boiler wall losses by radiation and convection and no modulation

of burner load with long standby times during which the boiler looses a lot of to the

colder surrounding despite the boiler is installed in one of the rooms to be heated.

Such kind of gas burning boilers are inadmissible today (DIN and ENEF) and no

longer offered on the European market, where only low temperature and condensing

boiler technologies are allowed.

On the other side the condensing boilers, mostly modulating systems, with low boiler

water and flue gas temperatures, low standby times and low heat losses caused by

radiation and convection offer extremely high part load efficiencies especially in the

range of low and medium values of specific combustion load, which cover a great

portion of running time and of fuel consumption during a year.

The figure also shows the standard efficiencies ε for the different boilers e.g. boiler E

84%, D 90%, C 96%, B 106% and A 109% and therefore points out the possibility of

saving energy and decreasing flue gases and air pollutions by a change in boiler

design.

3.2 Condensing technology in existing buildings Condensation energy can be utilised not only with reduced loads, i.e. low heating

system and low return water-temperatures. Even with heating systems designed for

75/60 °C, the actual temperature in the return falls below the dew point when

operating at load levels up to 90 % and higher or outside temperatures as low as –11

°C. This enables the system to operate in the condensing range for more than 90 %

of its operating range, even with a high design temperature of 75/60 °C according to

Fig. 19.

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Fig. 19 : Flow/return temperature, subject to outside temperatures, condensing

gain for 75/60°C and 40/30°C ( floor heating system)

The applicability of low-temperature heating systems such as under floor heating

systems (40/30 °C), where condensing operation is ensured at every outside

temperature for gas and oil burning boilers (Fig. 19 , lowest – blue - curve), is even

better.

It is known from experience that older buildings are frequently equipped with

oversized radiators and/or floor heating systems, as the insulation of the buildings

has increased for instance, what is common in Germany since 30 years. For new

buildings it is possible and perhaps expedient, to take larger (oversized) radiators or

larger heat exchange surfaces of the heating pipes for floor heating systems with the

aim of increased heat output in the rooms at lower flow and return temperatures but

constant temperature difference. The oversizing in existing buildings is partly due to

an over-generous design during the initial installation, and also to the thermal

insulation measures introduced over the intervening years: Retrofitted doubled

glazed windows, cladding and roof insulation have substantially reduced the heating

demand. However, the radiator size has remained unchanged. This enables the flow

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and return temperatures to be significantly reduced from the original design (e.g.

90/70 °C).

In the case of oversized ground floor heating systems only the heating water flow can

be reduced keeping the incoming flow temperature constant (400 C), which will

diminish the return temperature and increase the condensation level. It is very simple

to measure the ratio of over sizing with some thermometers in the heating water flow,

return flow, room and outside the building as described in [3,11].

4 Influencing variables and criteria for optimum gain 4.1 Boiler design Application of the calorific value method in boilers for heating purpose improved with

increasing condensation rate of the water vapour contained in hot flue gases. This

enables the latent energy in hot gas to be converted into usable heating energy and

to reduce the flue gas exit temperatures..

Fig. 20 a-b: Boiler design features [3]

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The main difference between low temperature boilers and condensing boilers (Fig. 20

a-b) is, that the wall temperature of the combustion chamber of low temperature

boilers always should be higher than the dew point temperatures of the different flue

gases, which depend on fuel and on acid building compounds of the fuel (mainly on

the sulphur content of the fuel). Contrary to the flue gases in low temperature boilers

which flow upwards or horizontally in the chimney flue gases in condensing systems

should flow downwards to transport the condensate, according to the gravity effects,

with the help of flue gas flow friction to the bottom of the boiler and to the condensate

exit.

Fig. 20 c-e: Principal constructions of condensing boiler systems [2]

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Fig. 20 c-e shows different constructions of condensing boilers with bottom (c), roof

(d) and cylindrical burners and different systems of flue gas – water heat exchangers

(Flue gases flows over the pipes through which the water streams (c+d) or water

flows around the flue gas channel (e)).

Conventional and low temperature boiler designs are unsuitable for this task. In

conventional low-temperature boilers, the heating surfaces are designed to prevent

condensation of hot gases inside the boiler tfl > td as described above.

Condensing boiler design is quite different: Hot gases are guided downwards as near

as possible to the return connection, leading to maximum cooling. Hot gas and

heating water should flow in a counter current pattern in order to utilise the low

temperature of the return water to provide maximum cooling of the exhausted hot

gases. At the same time, modulating burners with suitably intelligent controls should

be utilised, to enable automatic matching output to the current heating demand. The

selection of suitable materials should ensure that the condensate created will not

cause the boiler to suffer corrosion damage by acid condensate (SO2-, CO2-, NOx-

content).

During combustion, constituents of the fuel (fuel oil or natural gas) and of the

combustion air create compounds, which shift the pH value (degrees of alkalinity or

acidity) of the condensate up to acid levels. The CO2 created during combustion can

build carbonic acid and the nitrogen N2 contained within the fuel and air reacts to

become nitric acid (NO, NO2). Using fuel oil for combustion can create a particularly

aggressive condensate, as the sulphur content of fuel oil is responsible for the

creation of sulphurous and sulphuric acid (SO3, SO4). Therefore, all heat exchanger

surfaces which come into contact with condensate must be made from materials

which remain unaffected by the chemical attack of the constituents of the

condensate. For some years now, stainless steel has proved to be the ideal material

for this purpose. For fuel oil and natural gas, different stainless steel alloys are

available (alloying elements are, amongst others, chromium, nickel, molybdenum,

titanium), which were matched to the characteristics of condensate. This enables

these materials to withstand the corrosive attack of condensate without further

treatment and ensures a long lifetime of the boilers.

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Because the sulphur content in natural gas in Germany and Europe without regard to

odoration materials is low (3mg/m3), the problem of acid condensate is less

important. Normally odorisation is made with THT or mercaptane which contain

sulphur. The sulphur content is 20 – 40 mg/m3gas, not affecting too much the lifetime

of the boiler. Studies have shown, that sulphur content in the gas of 60 mg/m3 or

higher will cause corrosion on the surfaces in contact with the flue gas and/or

condensate. Additionally there is a new product for odoration of gases in Germany,

which works without sulphur and can therefore reduce the overall sulphur content in

the gases and flue gases of gas driven boilers for more than 60%. Also Nitrogen

oxides and their acid condensates can be reduced by low NOx burners (matrix full

premixing burners at low excess factors λ=1,05) in cooperation with well designed

combustion chambers with low wall temperatures and high heat transfer by radiation

from the flue gas to the water heater material.

In the past the calorific value method was rarely utilised with fuel oil on account of its

high sulphur level, but the legislation in Germany has changed increasing the general

availability of low-sulphur fuel oil. This clearly opens new opportunities for application

of oil-fired condensing boilers as low-sulphur fuel oil only contains about 50 ppm

sulphur (In former times fuel oil acc. to DIN 51603-1 [11] contained about 2000 ppm

sulphur).

Only the availability of low-sulphur fuel oil will make and have made the increasing

application of oil-fired condensing technology possible: condensate is significantly

less acidic, and heating surfaces are substantially less contaminated. However, for

oil-fired condensing boilers the following fuel-related requirements will still have to be

observed:

Higher residual levels than for natural gas combustion (ash, soot, nitrogen

oxides and sulphur)

Acidic condensate because of the residual sulphur and higher (compared to

natural gas combustion) NOx- content depending on burner/flame system

A specific design for condensing boilers can counteract these factors. Due to the

higher corrosion potential of the condensate, materials with higher acid resistance

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are used (stainless steel 1.4539). Waste water currently still needs to be treated in a

neutralising system in Germany.

The use of stainless steel allows for an optimum geometric design of the heat

exchanger surfaces. To transfer the hot gas energy efficiently to the heating water, it

is essential that hot gases are in intensive contact with the heating surface and

conduct the flue gas heat to the boiler walls and from the boiler walls to the water and

vice versa. Fig. 21 shows the temperature profiles in a low temperature and a

condensing boiler. According the equations for heat transfer per area

q = αtotal ( tfl – treturn temp.)

qfl-wall = αfl ( tfl – twall)

qwall = λ/s ( ∆ twall)

qwall-water = αwater ( twall – twater )

with α = convective heat-transfer coefficient a function of velocity, turbulence of flue

gas or water and wall conditions and λ = thermal conductivity a function of material

the transferred heat can be calculated.

Fig. 21 : Typical temperature profiles in a heat exchanger [2,4]

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To achieve an optimal heat transfer from flue gas to the heating water two options

are available:

The heating surfaces may be designed to allow the hot gas to have a high velocity

and turbulence level and to achieve homogeneous mixing in transversal direction to

avoid flows with higher temperatures in the core and to maximise heat exchange.

Smooth pipes are unsuitable for this purpose, because at the surface of the pipes

exists thick laminar boundary layers with low Reynolds-number and low temperature

gradients. Instead of this deviations and changes in the cross section must be

created to allow flow separation to enhance turbulence to force the development of

thin turbulent boundary layers with high temperature gradients in the flue gas – wall

boundary.

To prevent reverse flow of condensate into the combustion chamber, hot gas and

condensate should flow in the same direction downwards. This allows gravity to

support the downward flow of condensate droplets. Therefore the condenser

generally is arranged below the combustion chamber and hot gases exit the heat

exchanger downwards (Fig. 20 b-e).

4.2 CO2 content and burner design For efficient utilisation of the calorific value method it is important, that combustion

takes place with a high content of flue gas or a low level of excess air, since the dew

point temperature is influenced by the dilution of flue gas with air, which is

proportional to the excess air factor λ and can be measured by CO2 and/or O2

content of the hot flue gases (Fig. 9, Fig. 10, Fig. 11 and Fig. 13).

The dew point temperature should be kept as high as possible, to allow condensation

even in heating systems operating with a high return temperature. Therefore, the

designer’s aim should be to maximise CO2 content in the flue gas and accordingly

little excess air or low O2 content in the flue gas. The CO2 content which is achieved

with actual boiler designs is besides the CO2,max-value that is specified by the fuel

primarily subject to the burner design (diffusion, premixed/partly premixed flames

and/or fan driven or atmospheric burners Fig. 9). Another influencing factor that has

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to be kept in mind is the physical condition of the fuel, because liquid and solid fuel

combustion mostly needs higher excess air ratio than gaseous fuels to prevent soot

and CO formation in the flue gas.

For this reason atmospheric burners should not be utilised for condensing boilers as

these tend to operate with high excess air levels, resulting in low CO2 concentration

in the flue gas , which leads to low condensation temperatures within the hot gas as

explained in chapter 2.3 and shown in Fig. 9.

The residual energy in the flue gas at flue gas temperatures of 50 °C or below is

generally insufficient to ensure the function of the chimney or the flue gas system by

natural draught. In this context it is important, that fans for modulating devices are

speed controlled to enable the air volume to be matched to the gas volume flow near

to the stoichiometric value (λ =1,02 – 1,05). Only by these means the high CO2 content can be ensured in modulating operation too. For wall-mounted gas-fired

condensing boilers, the power consumption of such fans is approximately

50 kWh/p.a., leading to annual running costs of about 6-8 $.

4.3 Water connections The hydraulic system has to ensure that the return temperature falls significantly

below the dew point temperature of the flue gas to make sure that water vapour will

condense. One essential measure to achieve this is to avoid increasing the return

flow temperature by mixing with the hot flow as it is usually done by using a four-way

mixer, which is installed in the most systems in Europe For application with

condensing boilers three-way mixers could be used instead. They conduct the return

water from the heating circuits directly to the condensing boiler, i.e. without raising

the temperature.

Three-way thermostatic valves should be excluded as well, since they cause the flow

from the boiler and the return-flow to be directly connected and therefore lead to

raising return-flow temperatures. Modulating circulation pumps automatically match

the flow volume to the system requirements, and thus prevent an unnecessary raise

of the return flow temperature and thereby support the utilisation of condensing

technology.

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In some cases, a distributor without differential pressure or a low loss header can be

omitted. In older boiler configurations, low-loss headers (see Fig. 22) were used to

ensure the presence of a minimum circulation volume inside the boiler. Modern

condensing boilers no longer demand this installation since the capacity regulation

(see Fig. 23) will be used.

Fig. 22 : Low loss header functions [3]

Fig. 23 : Influence of sizing on capacity/ spread at floor heating [3]

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If the maximum permissible boiler throughput is less than the circulating volume

inside the heating circuit, e.g. in floor heating systems, the installation of a low-loss

header may be necessary. Here the higher heating circuit volume flow should be

balanced with the boiler circuit volume flow to avoid raising the return temperature

and ensures high heat transfer from the water flow to the walls of radiators or to the

material of floor heating systems.

The capacity of boiler circuit or heating circuit pumps must be matched, so that the

higher volume flow is circulated in the heating circuit, to reliably prevent a mixing of

warm flow water with the return. The flow temperature sensor should be installed

downstream of the low loss header after the colder return water has been mixed into

the flow to record temperatures which are relevant to the system. Careful design and

control are required, if the use of a low loss header cannot be avoided to achieve the

best possible condensing effect.

Rules for designing with wall-mounted boilers:

For cascades with several boilers a low loss header is generally required.

When balancing the low loss header, the volume flow on the boiler side should

be adjusted to approximately 10 to 30% lower than the volume flow on the

heating system side (lower return temperature).

The low loss header should be sized for the maximum volume flow which may

occur in the system.

Any DHW (domestic hot water store) cylinder which may be integrated into the

system should be connected upstream of the low loss header as the highest system

temperatures occur there thus reducing the loading time.

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Fig. 24 : Drinking water heater with layer charging store [2]

Fig. 25 : Efficiencies at the drinking water heating with condensing technology

and different water stores [2]

Fig. 24 shows the scheme of a condensing boiler in conjunction with a layer charging

domestic hot water store and Fig. 25 points out the dependence of the efficiency of

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the domestic water heating with condensing boiler systems and different water stores

on the hot water consumption. If no mixer is installed, connection of the DHW

cylinder downstream of the low loss header (Fig. 22) will result in the unregulated

heating-up of the heating circuits.

The condensing gain of heating systems with radiators or floor heating is also

influenced by the design of the pump with regard to capacity or spread. Fig. 23

illustrates this influence for a floor heating system: In an existing system

(Q = constant) reduction of the capacity (V) to 50%, increases the spread and

decreases the return water temperature. However, the average radiator or floor

temperature will initially fall.

If the flow volume is increased to renew the original temperature conditions after heat

is transferred to the room, the spread doubles if the average temperature stays

identical and the return temperature drops correspondingly. This significantly

improves the condensing effect. In reverse it follows that high capacities reduce the

spread and can therefore reduce the condensing effect.

4.4 Heating systems. As shown in Fig. 7 utilisation of the calorific value method in condensing boilers

provides great advantages and increase of efficiency compared with older systems or

low temperature boilers. The lowest losses and highest efficiencies exhibits the

combination of condensing technology with ground floor heating systems, which will

have efficiencies of 105 – 107% for natural gas. These high efficiencies are caused

by the low return temperatures in floor heating systems, which are possible when the

floor heating systems is well designed. There are different rules and limits in the DIN-

and/or EN- regulations ( DIN EN 1264, 4702 T3, T6, T8,…)[11] for instance for the

maximum flow- and/or floor surface-temperatures (29(35)°C) or for the temperature

differences (spread) in Germany and the European countries. On the other hand

there are different floor materials and insulations, which will influence the heat

transfer from the heating water pipes to the floor and at least to the room (Fig. 26 -

Fig. 28).

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Fig. 26 : Diagram of productivity of a floor heating system with PVC shielded

Cu Pipes [1]

Fig. 27 : Baseline for floor heating system [1]

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Fig. 28 : Example of special lines and limits for heat power in dependence on

temperature differences [1]

The DIN and EN describes the rules and regulations for design that should in the

case of new installations be considered to attain at least a good heat transfer, the

desired room temperature at every outside temperature, but also to allow for the

lowest return temperature. As shown earlier low return temperatures lead to a high

efficiency caused by a high condensation rate and low flue gas temperatures, low

boiler wall temperatures and following low flue gas (1%) and boiler (1%) heat losses.

To attain a high heat gain from a constant heat flow a good insulation against colder

rooms (cellar), high density of pipes in the floor (large heat exchange area in the floor

material), a high lateral heat exchange between the pipes (∆t < 0,5 - 2°C), a

homogeneous temperature distribution in the floor and a low heat resistance

(δ/λ < 0,15m2°C/W) of the floor materials is needed. The figures 29 - 31 show the

dependencies on these parameters.

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Fig. 29 : Heat losses to unheated walls, floors and roofs (cellar,…) in

dependence on power, isolation and cellar temperature [1]

Fig. 30 : Diagram for calculation of temperature spreading with flow

temperature as a function of over temperature of the heating

medium [1]

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Fig. 31 : Increasing of heating medium temperature with increasing heat

resistance of floor materials [1]

5 Emissions and flue gas systems

The particularly clean combustion achieved e.g. by modern matrix radiant burners

ensures that the emissions of most condensing boilers substantially fall below the

emission limits prescribed by all known regulations in Western Europe. In some

cases emissions fall below technically verifiable levels. The extremely low emissions

achieved by the matrix radiant burner are a result of the complete premixing of gas

and air as well as the low combustion temperature, which results from the large

semicircular reaction surface for instance. A high proportion of the released energy is

dissipated as infrared radiation from the reaction zone directly, which becomes

relatively cold (about 1000 – 1200°C). These low combustion temperatures

significantly reduces NOx formation, because NOx (thermal NOx) is generated at

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temperatures higher than 1400 – 1500°C in a high concentration. Blue-flame burners

should be used for oil-fired condensing boilers, since these create extremely low

emissions, when a good heat exchange by radiation to relative cold combustion

chamber walls is guaranteed. A conventional chimney normally is unsuitable for the

installation of condensing boilers because of the low flue gas temperature (<85 °C)

that is not always adequate to ensure a thermal up-draught within the flue gas

system and the danger of condensation of the residual humidity in the flue gas

system. Therefore condensing boilers are frequently equipped with a fan and are

operated with positive pressure inside the combustion chamber. Compared to

conventional chimneys, these conditions lead to quite different requirements:

- During operation, there is no requirement for resistance against soot

combustion, etc.

- The temperature load remains quite low.

- The system may operate with positive or negative pressure.

- Condensation needs to be expected.

These conditions can be met by simple flue pipes which are made from plastic,

stainless steel, ceramics or glass. Before commencing work on the flue gas system,

(in Germany) the installer should confer with the district chimney sweep [where

appropriate]. In general, a decision should be made, whether the condensing boiler

should be installed within the

- living accommodation (occupied)

or

- outside the living accommodation (boiler room).

The system may be installed inside the living accommodation, if the flue pipe is run

inside a protective pipe and is surrounded by secondary air (BF system, balanced

flue operation). Using a connector, which provides secondary ventilation up to the

duct (operation with interconnected room air supply), a condensing boiler may, in

exceptional cases, also be installed inside the living accommodation, when it is

operated as open flue system. Different possibilities for flue gas systems for

condensing boiler systems are shown in Fig. 32.

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Fig. 32 : Flue gas systems for condensing boilers for balanced flue operation

[3]

Outside the living accommodation, the flue pipe may also be installed inside a boiler

room without secondary ventilation. However, the boiler room would then need to be

provided with an adequately sized ventilation aperture to the outside (according to

TGI ’86/96).

For a rated output up to 50 kW is ventilation aperture quantifies:

150 cm2 or 2 x 75 cm2

and for a rated output above 50 kW or multi boiler systems:

150 cm2 and an additional 2 cm2 for every kW output above 50 kW.

If an open-flue boiler (boiler type B) is selected, the combustion air will be drawn from

the room, where the boiler is installed. Special measures need to be taken for living

accommodation to make adequate volumes of air available for combustion without

sacrificing the ambient climate (interconnected room air supply). The flue pipe should

be coaxial until it enters the duct, thus the combustion air supply is affected via the

outer pipe casing. Any escaping flue gases are therefore directly piped back to the

boiler.

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Generally the following conditions apply:

Permissible: - Gas-fired devices may be installed on the same floor

- Living accommodation with interconnected room air supply

- Ancillary rooms with interconnected room air supply (storage rooms, cellars,

work)

- Ancillary rooms with outside wall apertures (ventilation air/exhaust air)

150 cm2 or 2 x 75 cm2 at the top or bottom of the same wall up to QN = 50 kW

- Attics, but only with adequate minimum chimney height, (according to

DIN 18160 – 4 m above inlet)

Not permissible: - Stair wells and common hall ways, exception: detached and semi-detached

houses with a low build height (top edge of the floor in the upper storey <7 m

above ground level)

- Bathrooms or toilets without outside windows with duct ventilation

- Rooms where explosive or combustible materials are stored

- Rooms with mechanical or single-duct ventilation according to DIN 18117-1. Balanced flue boilers (device type C) draw combustion air from outside the building

shell. For this purpose, either the available cross section of the duct, inside of which

the flue pipe is installed, will be used or a coaxial pipe, through the inside of which

the flue gas flow is exhausted, whilst combustion air is drawn in through the outer

pipe casing. In either case the flue pipe installed inside the boiler room (flue gas

connection) is surrounded by an outer casing, inside of which the flue pipe is

surrounded by secondary ventilation. Principally it is possible to connect several

condensing boilers to one flue pipe.

Options are, for example, installations in residential or living rooms, in non-ventilated

ancillary rooms, inside cupboards and niches without clearance to combustible

materials, as well as in attics (pitched attic and side spaces) with direct outlet of the

flue pipe/ventilation pipe through the roof. Pipes must be routed through a duct if floor

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boundaries are breached. This duct must be implemented according to fire protection

class F90; for low duct heights F30 will be sufficient. The boiler room must be

provided with a condensate drain as well as the blow-off line for the safety valve.

Electrical interlocks with extract fans (extractor hoods, etc.) are not required for

balanced flue operation.

6 Summary and Conclusions The global resources of primary energy such as gas, oil and coal are limited and

therefore the consumption should be decreased drastically. Simultaneously the large

and increasing amount of pollution by flue gases such as SO2, CO, NOx,

hydrocarbons and soot, some of them being responsible for the global warming, have

to be diminished as soon as much as possible. The Kyoto protocol demands a drastic

reduction of all exhaust gases influencing the global warming. This can be achieved

by cleaner combustion systems with lower energy consumption at the same useful

energy output. Therefore combustion systems with higher efficiencies and further a

more efficient use of residual heat of the flue gases by heat recovery are needed. In

the industry and the field of power plants in Europe there has been a big progress in

decreasing energy consumption and simultaneously a reduction of emission of toxic

gases and gases causing the global warming. In the field of domestic heating with

coal, oil and gas and in the field of traffic (oil, gasoline, gas driven motors and

engines) it is still a big task and necessity, to reduce energy consumption and

emission.

One possibility of reducing energy consumption in the field of household especially

domestic heating is beside the insulation of the walls of the buildings and the use of

regenerative energies the development of better and more efficient heating systems

like condensing boilers. As has been shown in this report the energy consumption in

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the field of heating buildings, flats and rooms can be reduced about 12 – 17%, if the

heating system is optimally designed.

The report describes the fundamentals of the calorific value method and the

dependencies of a heating system utilising this method on fuel, CH-ratio of the fuel,

combustion conditions expressed by excess air factor or CO2 concentrations in the

flue gases and on the rate of condensation as a function of excess air ratio and

return flow temperatures, which are low in the low temperature boilers and extremely

low in floor heating systems with excellent regulation devices. However, the

efficiencies η of the heating system and/or standard-efficiencies ε are also

dependent on burner and flame designs for example premixed, fan driven and

atmospheric burners. Finally the heat exchange and transfer systems such as

radiators, convector heaters or floor heating systems with their different return flow

temperatures can have a positive influence on the different efficiencies first of all in

the range of medium and low loads, in which the system is operated mostly during

the heating of a year depending on the regional climate.

The report demonstrates that for a high efficiency η and good standard efficiency ε of

the process combustion should take place at low excess air ratios, with forced and

controlled premixing by fans and the rate of vapour condensation off the flue gas

caused by low return temperatures in the boiler should be high. A necessary

condition for the effectiveness of all these positive and energy saving process

features is a precise and with the given conditions (climate e.g. outside temperatures

during the year and constitution of building e.g. isolation) fitted design of the boiler

and heat exchange/ transfer system with continuously regulation of maximum and

minimum load in dependence on the heat consumption with or without domestic

water heating system.

If for instance room-heating by electricity – made by coal or oil burning power plants,

by gas or oil driven combined cycle power plants or by power-heat-coupling-

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processes – is compared with condensing boiler systems 70, 50, or 20% of primary

energy can be saved. At the same time CO2 -pollution will be reduced with a factor of

about 5,35 /3,75 for coal/oil burning power plants, of about 2,4/1,7 for oil/gas driven

combined cycle power plants and of about 1,63/1,25 for oil/ NG driven for power-

heat-coupling processes. These values point out the benefits of the calorific value

method, whose fundamentals are well known. Layouts for heating systems with

ondensing boilers are already existent, need further development of these systems,

the expenditures for materials and money are relative small.

Summarizing all these arguments and data the calorific value method is an excellent,

reasonable, well developed and prospective technique for heating of buildings. It

permits energy savings and reduction of emissions of environmental injuring and

global warming causing exhaust gases.

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7 Nomenclature Symbols A Factor B Factor cp specific heat F,f Factor H enthalpy m mass flow q heat flow density Q heat flow s thickness t temperature V,v volume flow α condensation ratio α heat transfer coefficient δ difference η efficiency λ excess air factor λ heat conductivity factor Subscripts a air b boiler Comb combustion Cond condensation d dew point dr dry gas ex exit fl flue gas fu fuel g gas gr gross in entrance meas measured net net stoich stoichiometric w wall

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8 Literature

1. Recknagel, Sprenger, Taschenbuch für Heizung + Klimatechnik Schramek Oldenbourg Indusrieverlag

2. Jannemann, Theo Kompendium Gas-Brennwert-Technik Vulkan Verlag Essen 3. Viessmann Fachreihe Brennwerttechnik

Viessmann 4. Viessmann, H. Viessmann Heizungs Handbuch Gentner Verlag, Stuttgart, 1987

5. VIK VIK Daten zum Energieverbrauch

6. Rogatty, W. Die Gasheiztechnik im europäischen

Vergleich IKZ – Haustechnik, Heft 11, 2005

7. BMWI Zahlen und Fakten. Energiedaten 2000 und 2003 Keitz und Fischer Druck GmbH

8. Wieheu,o. und Spitzner, h. Energie, Gesamtkonzept Bayern zur

Energiepolitik Bayr. Staatsministerium für Wirtschaft, Infrastruktur, Verkehr und Technologie, 2004

9. Feldhaus,G. und Hansel, H.D. Bundesimmissionschutzgesetz BimSchG

Deutscher Fachschriften Verlag, BraunGmbH, Wiesbaden

10. Wolf, D.,Teuber, P., u.a. Betriebsverhalten von Heizungsanlagen mit

Gas- Brennwertkesseln, Fachhochschule Braunschweig Wolfenbüttel, 2004 DBU – AZ 14133

11. DIN / EN German / European rules of standardization

Nr. 1264 1-4, 4725, 1266, 4701, 4702, 4703, 442, And many others, Beuth Verlag, Berlin

Report Contribution of the calorific value method to the ... · Prüflaboratorium Gas. 2.2 Net and gross calorific value . The net calorific value (H. net) describes the energy released

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