Heat Pump System Design and Sizing
Core concepts
Design starts with a room-by-room heat loss calculation at the design external temperature, not with a rule of thumb or a like-for-like boiler replacement. You must understand fabric heat loss (U-values × areas × temperature difference), ventilation and infiltration losses, and how the total at design conditions fixes the heat pump's rated output. Know the difference between design flow temperature and seasonal performance: lower flow temperatures (35–45 °C for underfloor, 45–55 °C for well-insulated radiators) raise COP and reduce running cost. Understand emitter sizing - radiators must be selected for the lower flow temperature and the actual room heat loss, often meaning larger emitters than the existing boiler system. Domestic hot water is a separate load: cylinder volume, reheat time, legionella cycle and the cylinder's heat loss all affect sizing. Also grasp bivalent operation, defrost penalties in cold humid weather, and the importance of a buffer or volumiser where system water volume is low.
Traps
Questions often present a heat loss in kW and a heat pump output at a different rating condition (e.g. A7/W35 vs A2/W35) - always match the rating point to the design condition. Another trap is sizing to the boiler's output rather than the calculated loss, or forgetting to add a margin for DHW and pipework losses. Watch for candidates who quote COP when the question asks for SPF (seasonal, includes defrost, standby and pump energy).
Worked example / rule of thumb
A dwelling with a calculated design heat loss of 8 kW at −3 °C external, using 50 °C flow, needs a heat pump whose output at that condition is at least 8 kW. If the manufacturer quotes 9 kW at A7/W35 but only 7.2 kW at A2/W45, the unit is undersized for the design point. Rule of thumb: a well-insulated home typically needs 40–60 W/m²; always verify by calculation.