What Is a Heat Pump?
A heat pump is a heating system that moves thermal energy from one place to another, rather than generating heat by burning a fuel. The most common residential type is the air source heat pump (ASHP), which extracts heat from outdoor air — even when temperatures are low — and upgrades it using a refrigerant cycle to deliver heat at a temperature suitable for space heating and hot water.
Heat pumps are electrically powered. Because they move heat rather than generate it, they can deliver more thermal energy than the electrical energy they consume — a ratio expressed as the coefficient of performance (COP). A well-installed ASHP in suitable conditions might achieve a COP of 3, meaning it delivers three kilowatt-hours of heat for every one kilowatt-hour of electricity consumed.
When Do Heat Pumps Work Well?
Heat pumps perform best when the difference between the source temperature (outdoor air) and the delivery temperature (the heating circuit) is small. This means they are most efficient when the outdoor temperature is relatively mild, and when the heating circuit operates at a lower flow temperature than a conventional gas boiler system.
Traditional radiator systems in older UK homes are typically designed to operate at flow temperatures of seventy to eighty degrees Celsius. Heat pumps work best at flow temperatures of forty-five to fifty-five degrees. This means that to use a heat pump efficiently, either the existing radiators must be oversized to deliver adequate heat output at lower flow temperatures, or the system must be upgraded with larger radiators or underfloor heating.
In a well-insulated new build or a heavily retrofitted older property, a heat pump can be highly effective. In an older, poorly insulated property with a standard radiator system designed for a gas boiler, the economics and performance are less straightforward.
Insulation First
For a heat pump to be cost-effective in an existing property, the building's heat loss must be reduced first. Improving insulation — loft insulation, wall insulation (where appropriate), and draught-proofing — reduces the demand the heating system must meet, which makes a heat pump more viable. Installing a heat pump in a poorly insulated building simply means the heat pump must work harder for longer, increasing running costs.
The government's guidance is to insulate first, then heat pump — and this is genuinely sound advice from an engineering perspective.
Running Costs
Heat pump running costs depend heavily on electricity prices relative to gas prices. At times when electricity is four to five times more expensive per unit than gas, the COP advantage of a heat pump must be at least four to break even on energy costs compared with an efficient gas boiler. This is achievable in a well-suited property but not guaranteed in all circumstances. As the grid decarbonises and electricity becomes cleaner, the environmental argument for heat pumps strengthens regardless of cost.
Planning and Installation Requirements
Air source heat pumps are permitted development for most domestic installations in England, subject to conditions including that the unit is not installed on a wall or roof facing a road and is not installed in a conservation area or on a listed building. Ground source heat pumps (which extract heat from the ground via buried pipework) do not typically require planning permission either, but the ground works are more involved and require suitable land area.
Installation must be carried out by a qualified engineer, ideally accredited under the Microgeneration Certification Scheme (MCS), to access certain government incentive schemes. Building regulations compliance for the electrical and heating system changes is also required.
Heat Pumps in the Context of Renovation
The ideal time to consider a heat pump is when undertaking a significant refurbishment that includes improving the thermal performance of the building envelope and replacing an existing heating system. At RCB Design and Build, we can coordinate heat pump installation as part of a wider refurbishment package where the building fabric is being upgraded. Contact us to discuss energy efficiency improvements as part of your renovation project.