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, Estoniagrowth are assured by circulation line of DHW, but notproperly designed or maintained DHW circulation isquite often responsible for increased risk of Legionella[11]. Another disadvantage of DHW circulation is bigheat losses, sometimes even bigger than net energyneeded for DHW heating [8]. The 10 sec waiting time isnot rule and for some people it is a long time, for somepeople short, but this value is used to evaluate testedconcepts if they are fulfilling requirements for high levelof users comfort or not. An overall tap delay can bestudied from different angles. From dynamic point ofview, tap delay consists of transportation time neededfor ―new volume‖ of water travel to tap and dynamicthermal behaviour of passed components, i.e. pipes andsubstation. From point of view related to location, itconsists of three parts, tap delay in branch pipe (pipefrom DH pipe in street to users substation), in DHsubstation and in DHW system in building. A tap delayin branch pipe and substation are related to DH networkand substation‘s control system strategy, while tapdelay in DHW pipes in buildings without DHWcirculation are defined only by thermal capacity of pipes,volume of water in individual pipes, nominal flow and tosome extend also by their insulation.Tap delay in DHW system in buildingFor DHW systems with individual feeding pipes andoverall volume of pipes lower than 3 L, DHW circulationis not needed, because waiting time for DHW withdesired temperature is not critical. In Table 1, transportdelays for individual fixtures in typical house built in pilotLEDH project in Larch Garden - Lystrup, Denmark [11]are presented. It should be mentioned, that data areonly transport delay, without dynamic behaviour ofcooled pipe. From Table 1 can be seen, that reasonablydesigned close locations of fixtures, not so far awayfrom substation, lead to maximal transport delay around6 sec, for basin. The total volume of DHW systemconsists of 0.99 L in pipes and 1.1 L in HEX (typeXB37H-40). It means, that it is possible to install longerpipes or more fixtures and still fulfil requirement of DHWsystem with volume lower than 3 L. The velocity offlowing water is below 2 m/s and thus problems withnoise propagation during tapping are avoided.Table 1 – Transport delay for nominal flows for individualfixtures due to DS439, in DHW system in typical house inLystrup, for pipes with inner diameter 10 mmfixturenominalflow(L/min)lengthtofixture(m)volumeinpipes(L)velocity(m/s)transp.delay(s)shower 8.4 2.2 0.17 1.8 1.2basin 3.4 4.1 0.32 0.7 5.8kitchen 6 6.3 0.49 1.3 4.962Tap delay on primary sideA transport delay on primary side consists of delay inbranch pipe and delay in DH substation. While tap delayin DHW installations in building is for DHW systemwithout circulation uniquely determined, tap delay onprimary side varying as control strategies for substationcontrol varies. From energy consumption point of view,the best solution is a control strategy without by-pass(see Fig. 1). In this case, DH water staying in the branchpipes is cooled down to temperature of ambient ground(if tapping wasn‘t performed for long time) and DH waterin substation to room temperature. In general, waitingtime for DHW is influenced by controller used insubstation. Basic principles of controllers areproportional flow controller and thermostatic controller.Each controller has own advantages anddisadvantages, thus best solution is to combine bothcontrollers [12]. In case of proportional flow controller,ratio between primary and secondary flow is fixed toprovide DHW with desired temperature and it means incase of using LEDH primary and secondary flow will bevery similar. If proportional flow controller is used forsetup without by-pass, user will face long waiting timefor DHW. Waiting time for this case can be seen fromTable 2. For branch pipe with inner diameter 15 mm (asis designed in Lystrup for IHEU), even transport delay toreach substation for nominal flow for basin, kitchen sinkand shower will be 31.6, 17.7 and 12.6 sec,respectively. This solution is from comfort point of viewand water savings completely unacceptable. If wedecrease inner diameter of branch pipe to 10 mm,transport delay is decreased roughly to one half of valuefor pipe with inner diameter 15 mm, but it is still longtime. In case of combined proportional flow controllerand thermostatic controller, from beginning of tappingthermostatic part assures opening of valve onapproximately full capacity until desired temperature ofDHW is reached.Table 2 – Transport delay for nominal flows for individualfixtures due to DS439, in branch pipe, 10 m long, for typicalhouse in Lystrup, data simulate using proportional flowcontroller without by-passfixturenom..flow(L/min)innerpipeØd(mm)volumein pipes(L)velocity(m/s)transp.delay (s)basin 3.4 15 1.77 0.3 31.6kitchen 6 15 1.77 0.6 17.7shower 8.4 10 0.79 1.8 5.6shower 8.4 15 1.77 0.8 12.6bath 12.6 15 1.77 1.2 8.4Full opening from beginning of tapping leads to muchhigher flow rate on primary side than on secondary andtime delay is decreased substantially. This solution canbe used for short branch pipes with reduced diameters.But it should be mentioned, that transport time in branchpipe will be always limited by maximal allowed flow on