Davis1_TAM04

Measuring Pressure and Airflow in Ducts

Affordable Comfort 2006TAM 04

Bob Davis, Paul Francisco, Bruce Manclark, Frank Spevak

Why Measure Air Flow?

• Capacity and efficiency of ac and heat pumps• Safety and longevity of gas furnaces• Comfort: Is the conditioned air getting to

where the occupants want it (even if we have perfect flow at the furnace)?

Why don’t we just trust the system will work?

• Guessing usually doesn’t work• ACs and heat pump require certain air flows to

obtain their efficiency and capacity ratings• Knowing the airflow on gas furnaces is helpful

in solving temp rise issues (too much or too little)– Gas input pressure– Orifice size

Measuring Air Flow(Air Handler End)

• Knowing the flow within at least 15% will tell you if the system is starting out ok– Is it capable of delivering rated capacity?– How much margin is there if the ducts aren’t perfect?

• If the flow is too high/low, you can adjust it (usually) and remeasure.

• If you are close to (or below) the line on flow, and you can’t make adjustments, maybe you need a different air handler, a cleaner coil, duct revisions, or some other remedy.

Lack of Air Flow Through Fan

• Fans are just like water pumps– The greater the

resistance the fan operates against, the less air the will move

– Even if fan curve is known, impossible to know CFM exactly,

– Tests will tell you if you are in the ballpark or the parking lot

Variable Speed (ECM) Fans• Have been touted as the cure

for the common cold• Do (usually) maintain flow

over a wide range of ESP• Can use as much energy as

(or more energy than) standard fans if pushed to limit

• May not be as durable as standard fans if run at high ESP

Measuring External Static Pressure• External Static Pressure

(ESP) is resistance (supply and return) the fan must operate against

• The greater the pressure, the less the flow

• External static over 0.7”might indicate a problem

Easiest Fan Flow Measurement is with Flow Plate

• The True Flow is a quick and accurate method of measuring airflow

• It is also expensive• As found supply static P

vs static P with plate in is needed for possible correction

Alternate Methods

• Flow plates are expensive

• They don’t always fit• They usually take

longer and aren’t as accurate

Duct Blaster Matching

• Measure normal supply static (NSOP)

• Attach Duct Blaster to cabinet

• Block off return• Match NSOP• Measure flow through

DB

Gas Furnace Air Flow

• CFM=(input * EFF)(1.08X temp. rise)

• Example:• 80,000Btu/hr.* 80%

1.08 * 55F • 64,000

59 • CFM=1,084

Electric Furnace/HP Fan Flow• Measure element energy use

(clamp around or read electric meter (spin test))

• Temp rise can be problematic– How good is device?– How many measurements did

you take?– Avoid “seeing” elements

• Make sure you know which flow you are measuring (check supply plenum static P if you suspect different flows)

Adjusting Fan Flow (PSC)

Adjusting Fan Flow

• Can be as easy as selecting a different speed on the fan or control board

• Can be much harder if ducts are inadequate…

Measuring Air Flow, Register End

• Air must be delivered at sufficient quantities at the right temperatures to produce comfort.

• Comfort, not efficiency is what customers demand

• Even if the fan flow is perfect, the people can still be unhappy

Measuring Register Flows

• Results can vary tremendously between different instruments– diffuser type (louvers, net free area)– air velocity– local turbulence– device calibration– Technician skill

• Know the limits of the equipment before drawing conclusions

How Close is Close Enough?• Within 20% is usually okay unless you are a geek.

– If you are measuring register airflow it’s probably because of a comfort complaint

– So you are usually trying to measure to the improvement you make to airflow to a register, not know the exact CFM

• Knowing exact CFM can help if you are pretty sure you are way under (and may need another run)

Hot Wire Anemometer

• Hot wire anemometer infers velocity by how much heat is removed from the resistor. Requires multiple, time consuming measurements.

Vane Anemometer• Vane anemometer measures

velocity by revolutions of a propeller. Must average several readings per grille. Requires net free area of register. Can measure wide range of velocities and is not restricted by register size.

Hot Wire Flow Hood

• Hot Wire flowhood uses a capture hood to funnel air past a hot wire anemometer grid. Very fast to use. Sensitive to register type.

Turbine Flow Hood• Turbine flowhood

funnels air through a known area. Measures air velocity by revolutions of a propeller. – Good to excellent at low

flows (<200 CFM). – Poor on returns. – Confused by some

grilles.

Know Your RegisterInstruments that do not utilize capture hoods require the net free area of the grille or register. This is the actual space between the blades. Use engineering data when possible. If not available, use these multipliers to go from the nominal area to effective area.

Round Diffuser X .6

Rectangular Diffuser X .6

Four-way Diffuser X .4

Volumetric Capture Hood• Garbage bag formula:

• ________________ X 0.156 ft3/gallon X 60 sec/minute • (bag size in gallons) = _______ CFM• _______________________________________________

seconds to fill bag

• Example:• 55 gallon X 0.156 ft3/gallon X 60 sec/minute • ____________________________________ = 129 CFM• 4 seconds to fill

Volumetric Capture Flowhood(under $1)

Garbage Bag Flow

0255075

100125150175200225250275300325350375400425450475500525550

1

1.25 1.5

1.75 2

2.25 2.5

2.75 3

3.25 3.5

3.75 4

4.5 5

5.5 6

6.5 7

7.5 8

8.25 8.5

8.75 9

Seconds to fill bag

CFM

33 gallon55 gallon

Pressure Grid Flow Hood

• Pressure grid flow hoods average pressure through an orifice to determine airflow. – Fast to use– Sensitive to register type– Often not designed for

residential flows– Hard to fit into some spaces

Summary

• Knowing what’s going in to and out of a forced-air system is key to– Knowing whether you have the capacity you think you

have– Knowing whether you have the efficiency you think you

have (need more measurements but you’re more than half there)

– Knowing how much to adjust flow and if you’ve made enough of an adjustment

– Knowing why someone isn’t comfortable inside the house