Heat pump selection
Heat pump capacity check against design heat loss (MIS 3005-D)
- where
- required heat pump output (kW)
- whole-house design heat loss (kW)
- extra allowance (%)
- heat pump output as % of requirement
- heat pump output at design conditions (kW)
Explain this equation
Checks a chosen heat pump against the house's design heat loss. Use it to make sure the unit can heat the home on the coldest design day without being badly oversized.
P_d is the whole-house loss. Any extra allowance, such as for warming up after a setback, is added as a percentage. P_c is what the heat pump actually delivers at the design outdoor temperature and design flow temperature, from the manufacturer's capacity tables. The ratio as a percentage shows how well it matches.
A simple percentage comparison of capacity against need.
Sense check: an 8.5 kW unit against an 8.48 kW loss is just over 100%, a good match. Under 100% falls short at design conditions; well over 150% risks short cycling in mild weather.
Watch out: do not use the nameplate output. Heat pump output falls as it gets colder outside and as flow temperature rises, so read it at the design point.
Checks that the heat pump you have picked can deliver the design heat loss at the design outdoor temperature and the design flow temperature, as MCS requires.
Enter your values to see the result.
Good to know
Read the heat pump output from the manufacturer’s capacity table at the design outdoor temperature (for example −3 °C) and your design flow temperature (for example 45 °C), not the nominal rating at 7 °C. If the heat pump also heats the hot water cylinder, check reheat separately (see the cylinder reheat card) rather than inflating the space heating load.
Common mistake
Choosing by the nameplate kW. A unit badged 8 kW may give noticeably less at −3 °C outdoor and 50 °C flow.
Limits
A capacity check only. Defrost losses, minimum output and hot water priority are covered in the manufacturer’s data and MIS 3005-D.
Sources
- MCS MIS 3005-D Heat pump design standard (heat pump to meet the design heat loss at the design outdoor temperature; room temperatures and outdoor design temperatures; design to BS EN 12831-1)
- BS EN 14511 Air conditioners, liquid chilling packages and heat pumps for space heating and cooling: rating conditions and test methods (COP)
- CIBSE Domestic Heating Design Guide (2026 edition; design to BS EN 12831-1, room temperatures, air change rates, pipe and emitter sizing)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow MCS standards, manufacturer data, BS 7671, the ENA connection rules and Building Regulations. System design and DNO applications need a competent, certified installer.