The current Department for Education Output Specification sizes heat pumps similarly to natural gas boilers, which is inappropriate due to differences in part load efficiencies and the high capital cost of over-sizing. A more considered approach prioritising seasonal efficiency and reducing primary energy use is needed. Cundall’s research on a typical secondary school’s heat load profile assessed three heat generation approaches: air source heat pumps sized to 120% and 100% of peak load, and to the 99.6th percentile peak load with an electric boiler. The research shows that smaller heat pumps improve seasonal CoP and reduce primary energy use.