National Energy Action Energy House Tests

Comparing Air Source Heat Pump and Gas Boiler Space Heating Energy Costs

Gas boilers have been replaced by air source heat pumps (ASHPs) under the Energy Company Obligation Phase 4 (ECO4) scheme. However, there are concerns that this could increase energy bills, particularly in less energy efficient hard to treat (HTT) homes, where higher flow temperatures may be required, reducing ASHP efficiency.

As fuel poverty disproportionally affects occupants of HTT homes, National Energy Action commissioned tests at the Energy House to compare HTT home winter space heating provision and running costs of a typical ECO4 ASHP retrofit with those of a modern gas boiler. Modifications were also made to the ASHP to assess the potential for improving existing and future HTT home ASHP retrofits.

Winter day space heating costs for the ECO4 ASHP were 19% higher than a gas boiler programmed to operate for two hours in the morning and seven hours in the evening (SAP heating pattern). However, the SAP heating pattern resulted in unacceptably cold bedroom temperatures throughout most of the night. As the ASHP was programmed to provide 24-hour heating, with a 2 °C setpoint reduction (setback) between the morning and evening heating periods, healthy temperatures were maintained overnight.

Use of an overnight setback with the gas boiler increased heating costs (including gas standing charge) to 2% greater than the ECO4 ASHP. Total space heating costs for the modified ASHP system were lower than any of the gas boiler heating patterns tested.

The findings demonstrate that well designed and properly commissioned ASHP retrofits can deliver lower space heating costs than gas boilers in HTT homes. It was also found that use of a specialist ASHP tariff, which reduces the gas-to-electricity price ratio for ASHP energy use, could make ASHPs substantially cheaper to run than gas boilers.