Optimum Performance of Radiator Space Heating Systems Connected to District Heating Networks via Heat Exchangers Patrick Ljunggren, Janusz Wollerstrand & Svend Frederiksen Lund Institute of Technology Department of Energy Sciences Sweden

Topics Consequences of ‘oversizing’ radiators (RADs) & Heat EXchangers (HEXs) Various RAD circuit control methods Constant or variable RAD circuit flow Various methods of upsizing HEXs ?? ºC lower DH return temperature

Minimum primary return temperature, t r1, at altered radiator supply temp., t rf, for increasing HEX area

Mode with excessive indoor air heating, simulated instantaneous temperatures space heating system 100 % oversized

Standard temperature program (constant flow) and optimized program (variable flow) no oversizing of radiators or heat exchanger

Lowered supply temperature set-point curve & unchanged flowrate (full curves) and optimized program (variable flow, dashed curves). 100% over-sized radiators and heat exchanger

Low flow balancing (design supply temperature, full curve, and reduced flow, dashed curves) 100% over-sized radiators and heat exchanger

Difference (in °C) in annual weighted return temperature from space heating heat exchanger versus reference case with a return temperature of 44.9 °C 0 % oversized HEX 100 % oversized HEX 0 % oversized radiators, constant flowrate % oversized radiators, optimized flowrate % oversized radiators, ‘low flow balancing’, constant flowrate % oversized radiators, lowered set point curve, constant flowrate ,3 100 % oversized radiators, Optimized flowrate ,2

Primary return temperature and secondary supply temperature at design heat load, assuming optimal radiator circuit flowrate ( four different assumptions )

Conclusions Radiator control should be optimised to minimise primary return temperature By combining control optimisation & HEX up-sizing gains in primary return of around 10ºC are possible