Primary circuit

Primary circuit describes the heating-water circuit on the heat-source side of a primary and secondary hydraulic arrangement. In boiler and heat pump schematics, it usually carries water through the heat source and towards a low-loss header, buffer tank, plate heat exchanger or closely spaced connection before the secondary circuit carries heat to radiators, underfloor heating or other loads. CIBSE (Chartered Institution of Building Services Engineers) Journal and Hamworthy describe low-loss headers as separating primary and secondary circuits so pumps and flow rates interact less. The term is not the fixed electrical circuit supplying a heat pump, a pipe-sizing calculation or proof that the system is correctly designed.