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, Estoniaprimary side defined by DH provider by means of flowrestrictor or by available differential pressure in DHnetwork. To reduce tap delay on primary side, controlconcepts with by-pass, avoiding cooling of DH water inbranch pipes and substations, and thus reducingsubstantially waiting time for DHW are available. Thereare two concepts of by-pass in relation to the heatexchanger: external and internal by-pass (see Fig. 1). Incase of external by-pass, DH water enters substation,but not enters heat exchanger and is sent back to DHreturn pipe and thus branch pipe is kept on desiredtemperature. Desired temperature is controlled bythermostatic valve situated in by-pass loop. Increasedlevel of comfort expressed by reduced tap delay can beadjusted independently on temperature of DHW onsecondary side.Fig. 1 Different by-pass strategies for IHEU: left - no-by pass; middle - external by-pass (cold HEX); right - internal by-pass(hot HEX)The set-point temperature of external by-pass isalways compromise between insufficient cooling of DHwater and additional heat consumed by customer andreduced waiting time for DHW. In case of operation ofspace heating system, the function of by-pass is tosome extend overtaken by space heating loop andthus heat for ―by-pass‖ operation is not wasted andtemperature of DH water returning to DH network iscooled sufficiently..In case of internal by-pass, by-passflow is passing through heat exchanger and keep itwarm (see Fig. 1). The benefit of this solution is evenmore reduced tap delay than in case of externalby-pass, but on the other hand, since heat exchangeris kept warm, internal by-pass solution has additionalheat losses. If substation is installed in room with needof space heating, heat losses are considered onlyoutside of heating season.Contrary to external by-pass solution, where it is not soimportant if space heating loop is installed in series orin parallel to DHW heat exchanger, in case of internalby-pass it is in importance. If space heating loop isconnected in parallel to DHW heat exchanger intraditional way, by-pass water just pass through DHWheat exchanger and is sent back to DH network withstill high return temperature, without any other use. Ifspace heating loop is connected in series to DHW heatexchanger or in parallel but with possibility to sent bypasswater flown through internal by-pass to spaceheating loop (see Fig. 2), this solution provides highlevel of comfort for users as well as proper use of heatneeded for by-pass operation.63Fig. 2 Combined by-pass concept, with possibility of useby-pass flow in space heating loopIn order to run by-pass without drawback of insufficientcooling of DH water and wasted heat also outside ofheating season, it is proposed to use by-pass flow forfloor heating, installed in bathroom and operate it allyear. From preliminary calculations it looks, that flowneeded to keep bathroom floor surface temperature on24°C will be enough as by-pass flow. Considering theuse of renewable sources of heat, the problem ofinsufficiently cooled DH water is related to reducedefficiency of these sources and whole year using offloor heating for comfort in bathroom is reasonable.Supply – supply recirculationAs an alternative solution for customers who don‘twant to use whole year bathroom floor heating,solution called supply-supply recirculation is apossibility how to use benefits of by-pass withoutwhole year heating of bathroom. In this case, district