Legionela_ArtigoCIBW62_2009

182 III.4AnInvestigationintoFactorsAffectingthe DesignTechniquesUsedtoControl Legionella in Water Systems Paul AngusBEng (Hons) EngTech, FCIPHE, ACIBSE RPSenior Public Health Engineer, WSP Ltd. Steven IngleDSc, MSc, I Eng, FCIPHE, ACIBSE, FSoPHE, LCGI, RP Director, Ingle Project Design Ltd. Derek KingMPhil BEng (Hons) Cert Ed CEng MCIBSESenior Lecturer, Liverpool John Moores University John TurnerMSc, CEnv, I Eng, MCIWEM, FIHEEM, FCIPHE,ACIBSE.Director, Public Health Engineering Direct Ltd. Abstract Overthepasttenyearstherehasbeenmuchdevelopmentinvariousandnovel meansofcontrollingLegionellainwatersystems,whilsttryingtomaintain compliance with the current water and health & safety regulations. However, as most large projects now are value engineering driven, in the current economicclimatethereisoftenaninsistenceonre-interpretingtheCodesof Practice to provide designs with cost advantages.In many instances this saving maynotprovidethebestoverallcostandlowrisksolutionforaparticular installation. 183Decisionmakersintheconstructionprocess,drivenbyfinancialconsiderations, appeartoooftentoinsistonthemostcosteffectivedesignsolutions,aslongasthe spirit of the design code indicates that such a solution is acceptable. Thispapersuggeststhatthisphilosophyencouragesundesirablecompromises.An environmentisfosteredwheredecisionsmayoftenbemadeonavalueengineering andprofitbasis,whileleavingrealpublichealthrisksandnon-sustainable,non-maintainable systems in their wake. Introduction It is intended that this paper will critically analyse some methods of control of legionella bacteria in hot and cold water systems. Thepaperwilladdress,inparticular,butnotbeconfinedto,theimpactofusing temperature as a sole means of Legionella control in domestic hot water systems based on value engineering design.Under examination also are the issues of management and operationalcostsofutilisingthispractice,and,inthecontextoftheconstantquestfor CO2reductionandsustainability,whethersuchatechniquecanbeconsidered environmentally friendly. Furthermore,thepaperwilldiscusstheuseoftraceheatingwithoutsecondary circulation pipework, and the associated risks and advantages of this application, along with the benefits or otherwise of alternative treatments. When considering cold water systems, the present Codes of Practice accept the widely heldviewthatattemperaturesbelow20oC,Legionellabacteriaremaindormant2.Furthermore,itiscommonlyacceptedthatcoldwaterpipeworkwillremainatsafe temperatureswithouttheneedforanyparticularpracticalsteptoguaranteethis(other thaninsulatingpipework,whichhaslongbeenstandardprocedure).However,global warmingisbeginningtocauseappreciablyhigherairandgroundtemperatures.In addition, changed rainfall patterns may mean that water supplies in the UK must come fromalternativesources,evenfromdesalinationprocesses(asintheMiddleEast).Thus cold water may well be supplied to buildings at temperatures closer to the higher end of what may be considered safe. 22 HSE. (2000) Legionnaires Disease: The control of legionella bacteria in water systems. Approved Code of Practice and Guidance L8, Health and Safety Executive. 184Bacterial growth in hydrodynamic systems Research has long shown that the use of heat and chlorine treatments to control bacterial growth in water pipework has given rise to certain bacteria developing higher tolerances tobothheat3andchlorine4.Legionellabacteriahavebeenisolatedfromnatural environmental locations where temperatures have been as low as 5oC and as high 63oC3.Isolationinsystemsattemperaturesmaintainedat30oC54oCiscommon,however legionella bacteria are not found at temperatures above 71oC5.In addition, it has been shown that thermophilous bacteria, Pseudomonas aeruginosa and other similar bacteria are becoming increasingly common and may partner the Legionella bacterium.These bacteria are believed to cause an increase in health problems, particularly those related totheskin6.Free-livingamoebaeareoftenfoundalongsideLegionellaeandother pathogenicbacteria,amoebaeofthegeneraNaegleriaandAcanthamoebacanbe pathogenictohumans.Thesecancauserapidfatalmeningoencephalititis,when introducedthroughthenasalmucosae,aswellasbrain,eye,pulmonaryandkidney infections7. Itisanacceptedcontroltechnique in hydrodynamic systems that a temperature should bemaintainedatwhichbacteriallifecannotbesustained.Suchfactorsaskilltime (thetimetakenfortemperaturetokillbacteria)andresidencetime(thetimethat waterstaysinthesystembeforebeingdrawnoff),however,arealsocritical.Ifthe residence time in a system is shorter than the kill time (and this can easily be the case at 60oC) then this is clearly unsatisfactory.At 60oC it takes 32 minutes to kill Legionella bacteria, while at 55oC this period is increased to 5-6 hours3. 3WadowskyRMetal.(1985)EffectofTemperature,pH,andOxygenlevelsonthemultiplicationofnaturallyoccurringLegionellapneumophilainPotableWater.Applied&EnvironmentalMicrobiologyMay1985p119712054RidgwayHF.OlsonBH.(1982)Chlorineresistancepatternsofbacteriafromtwodrinkingwaterdistributionsystems.Applied&EnvironmentalMicrobiology44(4)972987.5WadowskyRMetal.(1982)HotwatersystemsassourcesofLegionellapneumophilainhospitalandnonhospitalplumbingfixtures.Applied&EnvironmentalMicrobiology43;110411106OvesenK,SchmidtJorgensenF,&BaghL.(1993)Bacteriagrowthinhotwatersystems.DanishNationalBuildingResearchInstitute7TyndallRL,DomingueEL.(1982)CocultivationofLegionellapneumophilaandfreelivingAmoebae.Applied&EnvironmentalMicrobiology44;954959185It is also known that dead bacterial cells can provide a food source for living bacteria5, andthatbacteriacansurviveinanaerobicconditions(althoughtheydonotgrowor multiply readily in such conditions3).Mutant strains have also been shown to develop, dependinguponthequalityofthesourcesupply,andmaterialsutilisedinthesystem installation, galvanised steel, copper and plastics being the most common.Corrosion in galvanised steel in particular isaccelerated at temperatures above 60oC, as is the build up of debris from the killed bacteria in all materials at these elevated temperatures.The unevensurfacesofcorrodedpipeworkandsettleddetritusthenprovidenutritionand breedinggroundsforbacteria.Plasticshavebeenshowntoactivelyincreasebacterial growth rates across all temperature ranges6. Inmanycases,legionellabacteriaarefoundinthesedimentincoolingtowers, calorifiers or pipework systems even at 65oC high colony counts have been found in sludge in calorifiers8.In cooling towers, where water is treated using biocides, it is not unusualforthebiocidestohavetobealternatedtoensurethatthebacteriadonot become tolerant. Standingwaterisknowntobeundesirableinhydrodynamicsystems:longerresidence timesleadtostagnationandthisisknowntoincreasethegrowthrateofbacteriaas morenutrientsbecomeavailable.Heattransferbetweenhotandcoldwaterservices pipeworkincloseproximityisalsoinevitableifwaterisleftstandinginthepipework forlongperiods,nomatterhowwellpipesareinsulated,andthisisevenmore unhelpful.Stagnant water in a system is considered to represent a fluid category 3 risk under the current water regulations9. Ifengineerscouldinterpretsuchmicro-biologicalresearchlanguageandrelatethisto practicalandeffectiveengineeringdesignreality,theycouldpossiblymakedesign recommendations with confidence and rebut the constant arguments in favour ofvalue engineeredsolutionsandcostcutting,whichmaywellcompromisepublichealthand sustainability. Influences on design techniquesHot water supply accounts for as much as 30% of all energy consumption in buildings such as hotels, hospitals and residential homes.It is estimated that 65-70% of hydrodynamic systems in public buildings utilise heat as ameansofpreferredcontrol of contamination10 and this approach is seen to comply 8SchmidtJorgensenF.(1991)Danishinvestigationsonbacterialcontaminationinhotwatersystems.DanishNationalBuildingResearchInstitute.BuildingServicesDivision.9 HMSO. (1999) The Water Supply (Water Fittings) Regulations 1999 10 Arrowsmith, M. (2006) Legionnaires Disease Risk Minimisation, IHEEM Journal, August 2006. 186withalldesignguidanceandlegislation.Inpractice,however,particularlyinlarge systems, it is difficult to ensure that balancing achieves the required temperatures at all drawofflocations,orallowsthewholesystemtobeproperlypasteurisedintheheat cycle.Itisdifficulttobecertainthatfrequentstagnationdoesnotoccuratlocations with low water usage (e.g. washbasins in hospitals).In addition, designers seldom fully addresstheimportanceoftemperaturemaintenanceofthecoldwaterdistribution network, and the consequences should cold supply temperature be allowed deviate. Overrecentyearstherehavebeensignificantchangestohowwestore,heatand distribute water around buildings.These changes are mainly influenced by the ongoing pressurestoreducespacetakenupbymechanicalservicesequipmentandvalue engineeredsolutionstoreducecosts.Thuscentralisedplantroomshavetendedto becomethenorm,coupledwithboostedorpressurisedunventedhotandcoldwater networkswithextensivedistributionsystems.Off-siteprefabricationofpipeworkhas alsobeenintroduced,oftenwithlittleconsiderationofpossibleheattransferbetween services,coupledwiththemoveawayfromcopperandothermetallicpipework materials (common in the UK) to plastics.Strangely enough, there are many examples of where strainers or filters are added to a system to protect equipment, but these are not considered as a necessity to help reduce system contamination and protect public health on incoming water mains. Selection of pipework sizes is another factor which should be examined when looking at establisheddesignpractices:Currentpracticeistousetheloadingunitsmethod describedinBS6700,andintheCIPHEandCIBSEDesignGuides.Inthismethod pipeworksizesareselectedbasedontheinter-relationshipsbetweenseveraldynamic properties of water and the pipework material, though flow velocity and its relationship to pipework generated noise is used as the main limiting factor.Normal practice when using this method is to maintain velocities below 1 m/s, or exceptionally 1.5 m/s.It is, however, some time since any study was carried out as to the extent that noise generated inpipeworkisactuallylikelytobeproblematicinmodernbuildings,giventhat installationpracticeisconstantlyevolving.Forinstance,pipesaremorelikelytobe located centrally and in ducts, remote from working areas of buildings, and all pipework is routinely thermally insulated.Furthermore, at larger pipe sizes, there is evidence that the loading unit method becomes unreliable and over-sizing is routine.This, as well as being non-sustainable, is likely to increase the chance of stagnation and precipitation of debris.Itmaythusbepossibletorelaxtherequirementtokeepnoiseatlowlevelsso thatflowvelocitiescanbeincreased,andthiswouldpromoteausefulscouringaction within pipes.Consequently there would be less likelihood of debris and detritus settling andprovidingnutritionorbreedingplacesforbacteria.Thewelcomeby-productof suchatechniquewouldreducepipeworksizesandthuscontributefurtherto sustainability. 187More recently, the pressure to reduce carbon footprints and make savings of energy and water have moved designers into new areas of risk and managing that risk.For example intheUK,BREEAMpointscanbeearnedbyutilisingsolenoidvalvescontrolledby Passive infrared detectors (PIRs) to isolate infrequently used hot and cold water supplies tosanitaryfittings11.Thereisnoguidancetoinsistthatthegreatestofcareshouldbe takeninlocationofthesevalveswhilethereisquiteclearlyariskthatpipeworkdead legsandpossiblestagnationmayinadvertentlybetheresultofusingthismethod.It could certainly be argued here that public health is potentially compromised for the sake of sustainability! There is also a constant paradoxical question for engineers as to how it can be possible to reduce the fuel costs of producing hot water, when increasingly elevated temperatures are advised (in preference to biocides to which bacteria grow immune) in an attempt to controlbacterialcontaminationofsupplies.Howmuchbenefitistherethen,fromthe emergence of condensing boilers, which perform at their highest efficiency with return temperatures of 40-45oC? Whenquestioned,manypracticingdesignengineersandfacilitiesormaintenance managers will not know exactly the quality of the water entering their building, though mostwillaffirmthatitcomplieswiththeRegulatoryDrinkingWaterStandards12.Giventhecurrentfinancialclimateanddrivetowardssustainability,thechancesof reducing resultant levels of settled suspended matter, scale formation and corrosion, all contributors to fouling and microbiological activity, seem minimal. From a water conservation perspective it is noted that rainwater harvesting and re-use of greyandevenblackwaterisgatheringpace,certainlyasagainitattractsBREEAM points.Fromapublichealthviewpoint,however,howsensibleisitthatwateris broughtintobuildingsthatcouldpotentiallycontainhighnumbersofpathogenic bacteriaforpossibleaerosoldispersionwithinconfinedspacessuchastoiletcubicles?Thefutureriskoferroneouscross-connectionbetweenre-cycledwaterpipeworkand potable water pipework is also often not considered in the design of such systems. Legionella in cold water systems Legionellaisoftenconsideredasbeingaproblemassociatedonlywithhotwater installations.As previously stated however, it is necessary to ensure that temperatures incoldwatersystemsaremaintainedideallybelow20oC2,althoughthecurrentUK waterregulationspermitutilitiescompaniestosupplyattemperaturesupto25oC9.11 BREEAM. (2008) The Code for Sustainable Homes 12ProceedingsofLegionellaSeminar.(2009)OldTraffordCricketGround,OldTrafford, Manchester, 05/11/2009 188Cold water piped installation systems are required to be insulated and kept as far away as possible, from sources of heat such as hot water pipe-work, warm air ductwork and electrical services.Practically, this is difficult to achieve since service risers and ducts areoftenofrestrictedsizeandareusuallyexpectedtoaccommodateallservices,thus muchmaydependupontheskillandingenuityoftheinstaller.Suchconcernsmay easilybeoverlookedonsite,especiallywherecoldwaterpipeworkisinstalledwithin ceiling voids or plant rooms. Itisofupmostimportancetoconsidercarefullythelocationofthecoldwaterstorage cistern.TheinstallationitselfshouldcomplywiththeWaterSupply(Waterfittings) Regulations 19999 and Water Byelaws 2000 in Scotland13.It is the responsibility of the design engineer to ensure the cistern is located in a cool, dry place, being protected from extremes of temperature by thermal insulation.The cold water storage requirement for everydaypurposessuchasdrinking,washingandcooking,isnormallydeterminedon thebasisofcompensatingfora24hourinterruptionofsupply,asindicatedbythe appropriate British and European Standards14.Where a larger storage volume is needed for particular requirements such as catering, this may require additional treatment such asregularorcontinuouschlorinedosing,chlorinedioxidedosingorothermeansof disinfection such as ionisation. Figure 1: Cold Water Storage and system distribution temperatures (Source: Copper Development Association) 13 HMSO. (2000) Water Bye Laws (Scotland). 14 BSI (2006) BS6700 Design, installation, testing and maintenance of services supplying water for domestic use within buildings and their curtilages. 189In countries where high ambient temperatures are usual, such as in the Middle East, the incomingcoldwatermainssupplytothebuildingcanoftenbecharacteristicallyin excess of 20C.As discussed earlier within this paper, such temperatures provide ideal conditions for Legionella bacteria to breed within pipework and components like tanks.Inordertocombatthisproblem,thewatermustroutinelybedisinfected,usually chemically or by ionisation treatment, before distribution.In addition storage tanks are oftenlocatedinthebasementsofbuildings,takingadvantageofconstantcooler temperaturesunderground.As global warming takes effect, such techniques are likely to be required in parts of the world where the problem of cold supply temperatures were previously seldom considered, such as in northern Europe and North America. Legionella in hot water systems In practice it is known that around half of the reported cases of Legionnaires disease are associated with contaminated hot water, though these outbreaks are not normally widely reportedbecausethenumberofcasesinvolvedisrelativelysmall.Itishelpfulto examine the situation in homes and commercial buildings separately. Housing Therehavebeenonlyoccasionallinksbetweenthehotwaterservicesinhousingand Legionnairesdisease,despitesurveysofdomesticsystemsshowingthereareindeed traces of legionella.The main reasons for this are: 1.Itisdifficulttolinkpersonstoanyonesource:householdsareunlikelytocontain more than one susceptible occupant, and since an epidemic would not be recognized unlesstwoormorecasesaroseinthesamehousehold,GeneralPractionersand epidemiologists do not search for a common link.2.There is rapid turnover of hot water in domestic systems, with residence time in the pipeworkandcomponentstendingtobeveryshort.Inadditionthereisanever increasinguseofinstantaneouswaterheatersorcombinationboilers.Themodest size of a five person British house (around 80 m) also means that there tend not to be long dead legs of piping. 3.Domestichotwatersystemstendtocompriseofpredominantlycopperpipework andstoragevessels(thoughplasticshavebeenintroducedincreasinglyinrecent years).Ithasbeenshownthatmicro-organisms find it difficult to colonize copper pipeworkandcomponents:microbiologicalcolonizationinpipeworkalwaysstarts atthewallsurfaces,andcopperanaturalbiocidewhennewinitiallyinhibitsthis 190initiallyuntilthesurfacebecomesoxidised15,16.(Itis,however,worthmakingthe pointthatinhardwaterareas,thisprotectionquicklydisappearswhenmineral deposits coat the wall surfaces.) Once such a storage cylinder is heated and at steady state, stratification becomes evident and a temperature gradient exists, which is shown in Figure 217. Figure 2: Temperature profiles in an indirectly heated water cylinder (Source: Whiteside, 1990) Thepotentialproblemareainsuchhotwatercylindersliesinthebase.Debrisfrom killedbacteriawilldroptothebaseduringtimeofnondrawoffandwillcollectthere overtime,thusprovidingnutritionforbacteria,and,duetothecoldfeedwaterbeing suppliedinthisregionofthecylinder,temperaturesatthebasecanbelukewarm (around30oC)forlongperiodsoftime.Ifthebaseofthecylinderdoesheatuptoan acceptabletemperature,suchatemperaturemaynotbemaintainedforasufficiently long period to kill the bacteria.The UK Health & Safety Executive (HSE) recommends 15RidealS.andBainesE.(1904)Thesuggesteduseofcopperdrinkingvesselsasaprophylactic against water borne typhoid. Royal Sanitation lnstitute Journal (London), 25 591-595 16 Place, F.E. (1905) Water cleansing by copper. Journal of Preventive Medicine, (London), 13, 379 17 Whiteside, D. (1990) DHW: heat loss from tanks. Building Services Journal 12 (l), 57 191thatinlargerstoragevesselsade-stratificationpumpbefittedtocirculatewaterfrom the top of the cylinder to the bottom via the cold feed2.Inatypicaldomestichotwaterdistributionsystemthermalinsulationisusually sufficienttominimizeheatlossesandmaximizeoutlettemperaturesatthetaps.Self-regulatingtraceheating,whichisarelativelynewtechnique,claimedbymanytobe costeffective,mayalsobeconsideredinconjunctionwiththethermalinsulation, particularlyifdeadlegsexceedregulations.Thistechniqueoffersinstanthotwaterat the outlets, at some energy cost, but with less wastage of water since when users operate the taps they do not need to let the tap run while waiting for hot water. Instantaneouselectricwaterheatersareusedfordomesticshowers.Theseare connecteddirectlytothemainscoldwatersupplyandsupplywaterisdirectedovera powerful (up to 12 kW) immersion heater, emerging hot and at pressure.Such units are also available as spray taps for hand washing. Ithasbeenshownandiswellknownthatlimedepositionratesinhardwaterareas increasedramaticallyabove60C,whereasinsoftwaterareasthereisnodeposition problem. However, corrosion is accelerated at the higher temperatures18. Commercial buildings The hot water system in commercial buildings starts with a large storage vessel known asacalorifier.Inlargebuildingsthiscontainsaheatingcoilthroughwhichhotwater from the boiler plant flows.Some plant contains both LPHW and steam.Fourfactorsaredesirableinthedesignofacalorifiertominimizetheriskof contamination with legionella: 1.Thebulkstoragetemperatureshouldbereasonablyuniformthroughoutthevessel whenfullyheated.Inoldersystemsitiscommonforthelowest100mminthe vicinityofthecoldinletfeedtobelukewarm.Thispotentiallylukewarmzonein the base, with the debris mentioned above, can provide a most suitable environment forthemultiplicationoflegionella.Atechniqueofpreventingthisistoincludea de-stratification pump (as recommended by the HSE in Approved Code of Practice L82) which continually circulates water from the top to the bottom of the calorifier.Alternatively,thesecondaryreturnpipeofthehotwatersystemcanbeconnected intothecoldfeedpipeatthebottomofthecalorifierinsteadofitsnormal connection point two-thirds of the distance from the bottom. 18ASHRAE(1987)HVACHandbookAmericanSocietyofHeatingRefrigerationandAir Conditioning Engineers, Atlanta, USA 1922.Thecalorifierandentiredistributionsystemshouldbecapableofbeingheatedto 70Cforpasteurisationpurposes,andmustbecapableofmaintainingthis temperature for some hours if necessary. 3.Calorifiersmusthaveeasyaccessfordraining,dismantlingandcleaning2.All calorifierswhichareheatedbypressurisedhightemperaturewatermustbe inspected by law to ensure that the heating coil is able to withstand the pressure and somustbefittedwithanaccesshatch.Thishatchcanalsobeusedforcleaning purposes when the calorifier is drained down ready for inspection.Some engineers, however,specifynohatchwherethereisnoheatingcoil,whichistechnically permissible, but is seen as bad practice. 4.Volumetricsizingiscloselylinkedwiththeheatrecoverytimeoftheheat exchanger in the calorifier and therefore calorifier should be adequately sized for its duty.A two hour recovery period is usual but the storage volume can be reduced if ashorterrecoverytimeisdesired,dependantontheenergyinputavailable.Ifthe unitisunder-sizedinvolume,thenattimesofhighhotwaterflowdemand,the outlettemperaturewillfallandwaterattemperatureslowerthanthedesign temperature will pass through the system, carrying with it initially dormant bacteria fromthemainssupply.Subsequentlyifthewaterinthebaseofthecalorifieris contaminatedthenthecontaminationwillbespreaddownstreamthroughthe distribution network.This passing of live bacteria into the distribution network will of course allow the bacteria to colonise in dead legs and areas of low velocity. In practice the current design guides are over generous in their sizing and there is little riskofunder-sizinginnewinstallations19.Ithasbeenfoundthatthehighest dischargetemperatureduringdrawoffisachievedbyuseofvesselswithaheight fourtimestheirwidth,thesedimensionsprovidingoptimumstratification characteristics during discharge20. 5.On pressurised unvented hot water systems care must be taken to allow maintenance of the expansion vessel.The expansion vessel should maintain a temperature below 20oC and be constructed of a material that does not support bacterial growth. For buildings in continuous use (such as hospitals, hotels etc.) it is usual to provide two calorifiers to allow for maintenance: a service and standby calorifier both sized at 100% capacity may be provided, or alternatively, two calorifiers sized at two-thirds capacity.This latter option is preferable since the standby unit will come online more frequently 19CorlessDK.(1990)Hotwaterservicesforamodernhospitalwardunit.BuildingServices Engineering Research and Technology 11 (2), 57-63 20Cole,RL.andBellingerF.O.(1982)Thermallystratifiedtanks.ASHRAETransactions,88(2) 1005-1017 193attimesofhighdemandandthiswillensurethatwaterisnotallowedtostagnate.In systems where the standby unit is offline for considerable periods, it will remain full of water and may be heated slightly by leakage at the valves controlling flow to the heating coil.Thus,suchstandbycalorifiersmustbeconsideredcontaminatedandshouldbe pasteurisedbeforebeingbroughtbackintoservice.Pasteurisationisachievedby bringingthewatertemperatureuptoaminimumof60Candmaintainingthis temperatureforatleast2hours.Duringpasteurisationhotwatermustbecirculated fromthecalorifiertothevariousoutletpointsaroundthebuildingandbacktothe calorifier via a return pipework loop.The loop takes the water from the hottest part of the calorifier, the top, and returns it to the calorifier cold feed pipe.A small water pump is provided to circulate the water gently but sufficiently fast for the heat losses to permit the water return temperature to be 50C or more when the water leaves the calorifier at 70C. Commercialkitchensalsodeservespecialmentionbecausetheyrequirewaterat82C for dish washing rinsing.This can be achieved by controlling the calorifier at 82C for the dish washer and blending cold water to provide temperatures around 40C at normal outletstopreventscalding,orbymaintainingthecalorifierat60Candproviding specialistlocalheatingforthedishwashers.Bothsystemsarelikelytobelegionella-free at these temperatures. A particular problem with high temperature calorifier operation is the increase in water volume with temperature, and this relationship is illustrated in Figure 321.This increase involumemayforcethewateratthebottomofthecalorifierbackintothecoldfeed supplyaswellasthroughtheopenventpipe.Sincethewateratthebaseofthe calorifier is likely to be the most contaminated then this expansion can contaminate the feedpipe.Thiscouldsubsequentlyleadtorapidrecontaminationaftercalorifier cleaning.Non-return valves (NRVs) are recommended by the HSE (Approved Code of PracticeL8)atthecoldfeedinlettocalorifiers2,however,carefulthoughtisneeded whenpositioningthese,sinceanincorrectlocationcouldresultinwastageofheat energyinthewaterandwateritself.Inpressurisedsystemstheexpansionvesselwill containwateratidealtemperaturesforbacterialcolonisation,thuscarefulthoughtis neededtoincorporatethispieceofequipmentsuchthatbacteriacannotenterthe distribution pipework. 21OughtonD.(2008)Faber&Kell'sHeating&Air-conditioningofBuildings,Butterworth Heinemann. 194 Figure 3: Expansion volume of heated water (inlet 4oC) (Source: Oughton, 2008) InpublicbuildingstemperaturechecksunderHSEL8requirements2arenormally carried out at as part of the maintenance regime at tap outlets to ensure that hot water is delivered within a reasonable time, and that the delivery temperature is not below 46C.In hospitals special guidance defines reasonable time as within 1 minute of opening the tap the water is expected to reach 50C, although if a blending device is used to lower the outlet temperature to prevent scalding of vulnerable users, then 50C at the hot inlet to the blending valve is recommended22. Maintenanceforpublicbuildingsmustincluderegularinspectionofshowerheadsto check that they are clean and, if dirty, they should be cleaned immediately to avoid the colonisation of the debris with micro-organisms.Taps or showers which are rarely used shouldberemovedalongwiththeassociatedpipeworktominimizethedangerof lukewarm water remaining stagnant for long periods. It is seen as good design practice that the tap or outlet most frequently used should be at the end of any dead leg to ensure regular flow through the length of the supply pipe.It has long been standard practice to insulate both hot and cold pipework and ensure that the hot pipe is located above the cold so that convection heat loss from the hot pipe will not readily affect the cold supply. Earlyguidelineswereconcernedwiththesmallvolumeofwarmwaterleftinshower hoses after use.There were proposals that self-draining showers were inherently safer thanconventionalshowers.Experimentsonshowerswithaself-drainingvalveand conventional showers without such a valve showed little difference to re-colonisation by 22DHSS(1990)ControlofLegionellaeandsafehotwatertemperatures.HealthNoticeHM (90)12, HN (EP) (90) 195bacteria after sterilization.After 10 days the re-colonisation was stable and very similar forbothtypesofshower23.Ithasbeenfound,however,thatremovalofdeadlegs, regular changing of the mixing valve components, and regular flushing of shower hoses and heads produces significant reductions in legionella24.Secondary return systems and trace heating The conventional approach to the design of hot water services in commercial buildings recirculateshotwaterthroughasecondaryreturnsystemasdepictedinfigure4.This approachensuresthatwaterflowsconstantlythroughthepipeworkanddoesnot stagnate even if a tap is not used for some time.It ensures that water is maintained at design temperature around the network, and it allows the designer to keep supply dead legs to outlets as short as possible.These systems prevent the inconvenience of waiting forhotwatertoarriveaswellaspreventingbacterial contaminationinlengthsofpipe containingtepidwater.Continuouscirculationisnormallyachievedbypumping, although in some cases this is programmed to switch off at night. Figure 4: Representative sketch of secondary return type hot water distribution An alternative method of maintaining temperature around the network is to provide self-regulatingelectricaltraceheatingalongasingledeliverypipe.Theoperating temperatureofthepipeisdeterminedfromtheknowledgeoftheheatingtape characteristicswithrespecttotemperature,andknowledgeoftheheatloss characteristicsofthepipematerial.Thepoweroutputofthetapefallswithhigher 23 Humphrey TJ. (1989) Microbial contamination of hospital showers and shower water: the effect of an automatic drain valve. Journal of Hospital Infection, 13, 55-61 24 Makin T. and Hart CA. (1991) The effect of a self-regulating trace heating element on legionella within a shower. Journal of Applied Bacteriology, 70, 258-264 ControlHotstoragevesselCirculatingpumpSecondaryflowpipeSecondaryreturnOpenventpipeDrawoffpoints196temperaturesandtheheatlossfromthethermallyinsulatedpipeincreaseswithhigher pipetemperatures.Thesetwocharacteristiccurvescanbeplottedandtheintersection betweenthesedeterminestheequilibriumtemperatureofthetraceheatedpipe.This procedure is illustrated in Figure 5. Figure 5: Determination of operating temperature of trace heated pipework The installed piping scheme is much simpler than the conventional one and, on the face ofit,appearstobemoreeconomicallyappealing.However,theenergycostsof electricalpumpingandpipeheatlossesoftheconventionalsystemmustbecompared with the energy costs of running the single pipe trace heated system.The capital costs of single pipe with trace heating and the re-circulating system should also be compared.Theeconomiccaseisoftenstrongestwhenthehotwaterserviceisalongandnarrow building where the return pipe simply returns the water.It is less attractive in a square buildingwherethehotwatercircuitmaybeonepiperunninginalooparoundthe building. Therearetwootherapplicationsoftraceheatingforhotwaterpipes.Thefirstisto remedy deficiencies in existing equipment or systems.If there are long dead legs in the distributionsystemthenelectricaltraceheatingcanbeincorporatedsimplyalongthis particularlengthofdeadleg.Experimentsusingthetechniquetomaintainthe temperature in the dead legs in a shower to 50C were very successful provided that the circulatinghotwaterwasgreaterthan45C.Thetemperaturewasmaintainedvery closely ( 1.5C) and legionella were eradicated or severely reduced by the technique25. 25 Makin T. and Hart C.A. (1990) The efficiency of control measures for eradicating Legionellae in showers. Journal of Hospital Infection, 16, 1-7 197Thesecondisforplaintraceheatingusedforpasteurisation.Thereisagrowing practice to recommend regular elevation of hot water pipes to 77C for 15 minutes and to draw this water from each tap or shower head to disinfect the components18,26.Care mustbetakenifthispracticeisfollowedtoensurethatthedepositionofcalcium carbonatewillnotbeexcessiveorthatthecorrosionratewillnotbecomehighinsoft waterareas.Insomenaturalwaterswherezincisusedasanaturalprotectivecoating on steel a reversal of polarity occurs around 60-65 C.The zinc then becomes cathodic to steel stimulating attack which results in localised pitting corrosion27,28. A suggestion This paper suggests that general design good practice for public buildings is often being compromisedbytoomuchfocusonprojecteconomics.Furthermoretheclientoften does not seem to benefit from any cost savings realised, and the end user is left to pick upthecostsfortheongoingfailureorpoorperformanceofsystemsafterhandover (although it must be said that in speculative design and build projects, clients are more likelytoseesomeofthesavings).Currently,projectmanagersareheedingmany manufacturersclaimsofmoreinexpensiveinstallationcostsandthusareconstantly requestingdesignerstocarryoutvalueengineeringofprojects.Capitalcostsmay subsequently be reduced but life cycle costs seem to be ignored altogether. One such aspect which raises concern in the context of this paper is the recent tendency to introduce trace heating as a means of temperature control, and this technique is now beingpromotedasaneconomicalternativetotraditionalsecondaryreturntypehot waterdistributionsystems.Thereareanumberofissuesthatareraisedbythe introduction of this type of system: Maintenance of design temperatures throughout distribution networks; Scalebuildup,stagnationandallowingsuspendedmattertosettleoutthus creating corrosion spots and bacterial breeding grounds; Energy use and life cycle costs; Carbon footprint and sustainability. 26 Fliermans CB. and Nygren JA. (1987) Maintaining industrial cooling systems free of Legionella pneumophila. ASHRAE Transactions, 93 (2) 1405-1415 27 BRE. (1968) Durability of metals in natural waters. Building Research Station Digest 98 Second Series 28InstituteofPlumbing(1988)PlumbingEngineeringServicesDesignGuide,Instituteof Plumbing, Hornchurch 198Theinstallationofcentralwaterheatingplantwithdistributionpipeworkrunning throughout the building is generally considered by engineers as the most cost effective, efficient and reliable solution to providing hot water supplies in public buildings.The cost effectiveness and reliability of this tried and tested system is borne out by the long life (usually 30 or more years) of low stress components such as traditional calorifiers, tube bundles, boilers and so on.The question of cost effective and reliable methods of maintaining temperatures in the distribution pipe networks at safe levels is where there may be some disagreement. Itisclaimedthattemperaturemaintenancetapewillcutbothinstallationandrunning costs in comparison to a re-circulating hot water distribution system, whilst maintaining temperaturewithindeadlegs,andwillthuspreventLegionellagrowthandallowfull compliance with current legislation.The main cost argument is clearly the removal of thetraditionalinsulateddomestichotwatersecondaryreturnline,withitsassociated balancing valves and circulator pump.The function of the return pipe is to maintain the temperaturewithinthesystemsuchthatwaterisavailableatthepointofusequickly, avoidingwastageoflargequantitiesofwaterwhilstwaitingforhotwatertoarrive traceheatedsystemscouldmakethesameclaim.Toensuremaintenanceofsafe temperaturesthroughoutsystems,thesamequantityofheatmustbedeliveredasthat whichislostduetonaturalcoolingtotheambientsurroundings.Inthetraditional system standing losses in primary energy are claimed to be considerably higher than in systems using trace heating; in addition, installation costs for trace heating systems are lower and thus the overall capital costs for a traditional system are usually far higher. Engineers have a duty to their client and to the environment and should consider wider aspects in relation to the installation.In fulfilling these responsibilities it is only proper thatdetailedcomparisonsbetweenthetwosystemsoftemperaturemaintenancebe made. Theenvironmentalimpactandsustainabilityofthesystemshouldbefully considered:unlessthetraceheatinginstallationistobepoweredbyelectricity generated from renewable sources, then heating water using electricity instead of gas is inherently non-sustainable and energy inefficient. Thechoiceofprimaryfuelisanimportantdecidingfactor:withintheUK purchasing costs of fuels generally place the use of natural gas in front of others inanurbanenvironment.Ifanalternativefuelmustbeused,orelectricityis generated on-site, this may radically change the dynamic of the decision making process. Ongoingoperationalcostsmustbedeterminedbycarryingoutaseriesof detailedcalculationsandforecasts,detailingexactlyhowtheoperatingcostsof the options compare;Detailed knowledge of the relative life cycle costs for all components is required toenabletheengineertomakedesign decisions with confidence for example 199thelikelyongoingmaintenance,repairandreplacementrequirementsandcosts must be considered; Inanincreasinglylitigiousworldtheengineerneedstobeconfidentwhen specifyingeithersystem,thatsafetemperaturesaremaintainedandthe requirements of legislation and design codes are met. Conclusions Traceheatinginstallationsonthe largerscalehavelittlerecordedhistory.Itisknown that they can be effective in treating relatively short dead legs such as shower runs and remoteappliances;thereis,however,scantevidencetosuggestthistechniquecanbe effectively scaled up to meet all criteria required for public health.For instance, it is not knownexactlywhathappensinatraceheatedpipewhenpositionedverticallywhen waterisnotmoving:doesstratificationanditsassociatedproblemsoccurasitdoesin storage cylinders? Furtherresearchisessentialifengineersaretohaveconfidenceinutilisingthetrace heating technique as a reliable solution.There are at present too many unknown factors relatingtothepointsmadeearlier:maintainingdesigntemperaturesthroughout distribution networks, scale build up, stagnation, settlement of suspended matter, energy use, life cycle costs, carbon footprint and sustainability. In addition to all the likely technical problems engineers face when employing the trace heating tape technique in large scale centralised systems, other matters must also be the borne in mind: Any outbreak of Legionnaires disease is normally sensationalised by the media, and in todayssocietyscultureofblame,engineerscouldwellfindthemselvesculpable (corporate manslaughter is a constant fear).ThepublicaregenerallyunawarethatLegionellaisacommoninhabitantofwater distributionsystems.Theincorrect,generallyheldassumptionisthatLegionellaisan unwelcome invader of poorly maintained water systems and that negligence plays a role in its presence29.This is of course a half truth, borne out by the fact that most outbreaks ofLegionnairesdiseaseareindeedrelatedtooccasionswherebasicmaintenanceand health and safety procedures have been neglected.Thisisironic,becausetoofewpublicbuildingshaveanydiagnostictestingregimeor results available to alert staff that there is a problem.Diagnostic tests, because of their high cost and alleged complexity, are not recommended in the present Health and Safety 29StoutJE,YuVL.(2001)Legionellainthehospitalwatersupply;Apleafordecisionmaking based on Evidence-Based Medicine. Infection Control Hospital Epidmiol. (22) 670-72 200advice,unlessaparticularproblemissuspected.Manywouldarguethatinmany instancesLegionellosisgoesundiagnosed,andmortalityisincorrectlyattributedto other causes29. The key issue here is the question of whether the presence and colonisation of legionella leads to Legionellosis. Shouldengineersperhapsbeconcentratingtheireffortsonthesourceoftheproblem? Should they seek to understand the true status of the incoming water supply, and what implications this has on the particulars of any installation and how it is to be operated?Thefitandforgetapproachtowatersupplies,whichmanymightargueisthenorm today,hasimmeasurableconsequences.Therefore,shouldengineers,asstandard practice,attempttoreducetheamountofsuspendedmatter,organicloadingsandthe potentialforscalingthroughadditionalon-sitefiltrationandassociatedtreatment regimes?Shouldmaintenanceregimesencouragethecheckinganddrainingdownof sludge from the bottom of calorifiers more regularly than at six month intervals?Rather than relying on heat treatment in isolation to offer protection at a cost, should engineers alternativelyadoptanendofpipesolutionapproach,whereeffortsareconcentrated on control and reduction, localised to areas perceived as high risk?TheuseofUV,ultra-filtration,copper-silverionisation,silver-hydrogenperoxideand so on offer many capital and operational cost advantages, yet are least often utilised as a preferred method. There may well be an argument for localised hot water generation, with the acceptance of plateheatexchangers(orsimilar)delivering almost instantaneous hot water.These takeuplittlespace,reducestorageproblemsandcangreatlyreduceandsimplifythe requirementforsecondarypipeworkinstallationandmaintenance.Thecostofthe primarypipeworkisalsolowersincecheapermaterialsmaybeused,andthe distributiontemperaturecouldbemaintainednetworkwidewithmoreconfidence.In additionturnoverofwatersupplywouldbeguaranteed,thusreducingstagnationand associatedproblems.Thesupplycouldutiliseasinglemains(pressurised)coldwater supply, which could be routed externally, therefore limiting heat gains and maintaining the quality again by guaranteeing the turnover. Finally, until environmental cultures are performed routinely, hospitals in particular will continuetoexperienceLegionnairesdiseasewithitsattendanthighmortality.The disease will remain under-diagnosed and undetected unless diagnostic testing is carried outmorefrequently.Onlyifitisknownwithconfidencethatcolonisationlevelshave increasedcantrulyappropriateandcosteffective,reliabledisinfectionmeasuresbe recommended29.