12th International Symposium on District Heating and Cooling

The <strong>12th</strong> <strong>International</strong> <strong>Symposium</strong> on District Heating and Cooling,September 5 th to September 7 th , 2010, Tallinn, EstoniaLOW TEMPERATURE DISTRICT HEATINGCONCEPTReduced risk of Legionella by use of system withminimal volume of DHWSince LEDH is mainly developed for low energybuildings already designed with low temperature spaceheating, the lowest acceptable forward temperature ofLEDH system is defined by requirement for DHW supplytemperature. The hygienic requirement for heating ofDHW is due to recent standards 50 °C for single-familyhouses and 55 °C for multi-storey buildings [3] whereDHW circulation is used. In case of using circulation,temperature of recirculated water should never fallbelow 50 °C. These requirements are based on need toavoid Legionella growth in DHW pipes and storagetanks. It is widely believed, that Legionella grow intemperature range between 46 °C – 20 °C, in systemswith high volume of water. Mentioned temperaturelevels are made in order to assure comfort and hygienicrequirements in furthest tap away from a heat source. Itis important to say, that there is high level ofdiscrepancy among different results and nationalstandards focused on Legionella.Due to German Standard W551 [4], temperature ofDHW can be below 50 °C and not cause Legionellapromotion, if total volume of DHW system connected toone heat source is lower than 3 L. From literaturestudied, it can be concluded that requirements toproduce DHW with temperature higher than 50 °C aredefined for an old fashion DHW building installations,which can be characterized as systems with verticalriser, branched pipes with bigger diameter (increasingwater volume of the system), using DHW circulation.For new and renovated buildings, DHW installations aredesigned in much better manner, with individualconnection of DHW pipes between each tap and sourceof DHW and with maximally reduced pipe diameter,defined by requirements for noise propagation andpressure drop.Due literature, danger of Legionella growth in DHWsystem is influenced by temperature of DHW, nutrientsin DHW, laminar or turbulent flow in the DHW pipes andwater stagnation [5]. Several on site measurementswere performed in buildings using DH for DHW heating.From results of Martinelli [6] and Mathys [7] can beconcluded, that Instantaneous Heat Exchanger Unit(IHEU) tend to have much less problems with Legionellathan traditional units with DHW storage tank. Bothstudies concluded, that these findings are caused by thefact that in IHEU, DHW is produced with temperature60 °C, while in case of storage units only withtemperature 50 °C. But is necessary to mention, that incase of traditional DHW storage tanks, overall volume ofDHW in a system is much higher than in case of IHEUsystem. Due to our knowledge, there is not reported61investigation of Legionella in DHW system using IHEU,producing DHW with temperature below 50 °C andreduced volume of the system below 3L.For single family houses with appropriate close locationof tapping points, volume of DHW in IHEU and pipes willbe lower than 3 L and thus temperature of 50 °C onprimary side will not cause Legionella problems. Formulti-storey buildings, district heating substations foreach flat is a state of the art solution [8]. In this case,each flat has own completely separated DHW system(with volume of water below 3 L) and thus hasincreased users comfort and no huge DHW systemswith circulation, where Legionella is forming andspreading [9]. The other advantage of using flat stationin multi-storey buildings is individual metering of eachflat and complete control over space heating and DHWpreparation, which is positively affecting energy savings.With properly designed DHW building installations,supply temperature of LEDH will be defined byrequirements for users comfort. These requirements arediscussed in following text.Users comfort in DHW supplied by LEDHAnother important question, when concerning DHWsystems is level of user comfort. From comfort point ofview, requirements for temperature and waiting time forDHW can be specified. Due to Danish Standard DS439―Code of Practice for domestic water supplyinstallations‖, [10] temperature of DHW should be 45 °Cin kitchen and 40 °C in other taps, provided withnominal flowrate and desired temperature reachedwithin ―reasonable‖ long time, without significanttemperature fluctuations. It is a question, if requirementof 45 °C degrees for kitchen tap is not too high, butargument of problems with fat dissolving from dishescan be objected and should be investigated. Based onmentioned standard, desired temperature of DHWflowing from fixture is 45 °C. But in order to definedesired forward temperature of LEDH system, weshould be aware of temperature drop in DH network, inuser‘s substation and in DHW installations in building.The temperature drop in DH network is not in focus ofthis paper, so our goal is to find needed temperaturelevel at the entrance of substation to produce 45 °Cfrom tap in building. Desired temperature will be foundby experimental measurement of LEDH substation laterin article.Beside temperature requirements, users comfort isinfluenced by time needed for DHW to reach a fixtureafter tapping was started. This waiting time is infollowing text called ―tap delay‖. Due to DS439,suggested value for tap delay is 10 sec and it is definedin order to avoid wasting of water and to protect usersagainst too long waiting times for DHW. In large multistoreybuildings with centralised preparation of DHW,short tap delay and measures avoiding Legionella