Home energy guide
Plan a boiler-to-heat-pump upgrade
Replacing a boiler involves more than choosing a heat pump with a similar number on the label. A good design brings together the home's heat loss, radiators, hot water, controls and electricity tariff. This guide explains what to assess, how to compare running costs and which questions to ask before accepting a quote.
Updated
Understand what changes when the boiler goes
A boiler burns fuel to produce heat. A heat pump uses electricity to move heat from outside into the home. An air-to-water system supplies radiators or underfloor heating and usually a hot-water cylinder; an air-to-air system heats rooms through indoor units and needs separate consideration for hot water. This guide focuses on replacing a boiler with an air-to-water system.
Heat output and electricity input are different ratings. A heat pump producing 10 kW of heat does not necessarily draw 10 kW of electricity. Its input depends on efficiency at the operating conditions, and additional heaters or pumps can add demand.
Build a picture of the home you want to heat
Gather a year's fuel use in kWh, the current boiler details, room sizes, insulation improvements and hot-water needs. Note cold rooms and the temperatures you normally maintain. A low bill caused by heating only a few rooms is not a reliable estimate of the energy needed to heat the whole home comfortably.
Gas consumption can include cooking and hot water as well as room heating. Boiler losses mean fuel kWh is not the same as heat delivered. Bills help sense-check an estimate, but annual fuel use alone cannot establish the maximum heat needed on a cold day.
Record planned extensions, insulation work or occupancy changes before the design is finalised. If a heat pump is already installed, separate electricity and heat-meter records can help assess it; avoid adding its demand again when it is already in the whole-home electricity readings.
- A full year of fuel use, with estimated readings and unusual periods identified.
- Room dimensions, construction, glazing and insulation, including recent improvements.
- Usual room temperatures, heating schedule, household size and baths or showers used.
- Current radiators, underfloor heating, cylinder and any equipment you want to retain.
Size the heat pump from a heat-loss assessment
A room-by-room assessment calculates heat escaping through walls, roof, floor and windows, plus heat lost through ventilation. It uses an outdoor design temperature appropriate to the location and agreed indoor temperatures. The resulting heat loss in kW helps establish required output during cold weather.
A boiler's headline rating is not a direct heat-pump sizing rule: a combi boiler may have a high rating to produce hot water quickly. Ask for the proposed heat pump's available output at the design outdoor and water temperatures, rather than relying only on the model name.
The designer should also explain hot-water recovery, minimum output in milder weather and any backup-heater allowance. Floor-area rules can be useful for an early discussion, but cannot replace the survey or determine the final unit.
Check radiators, pipework and flow temperature
Flow temperature is the temperature of water leaving the heat pump for the heating circuit. Radiators deliver less heat with cooler water, so each room's emitter needs checking against its heat loss at the proposed temperatures. Some existing radiators may be adequate; others may need upgrading. Underfloor heating is another option, rather than a requirement for every home.
Ask for a schedule showing which emitters stay and which change. Pipework and water flow also need assessment. Buying a more powerful heat pump does not by itself make an undersized radiator heat a room adequately at the chosen flow temperature.
Understand COP and seasonal efficiency
Coefficient of performance, or COP, is heat delivered divided by electricity used at particular conditions. At COP 3.5, 1 kWh of electricity produces 3.5 kWh of heat. Outdoor temperature and the temperature of water being produced affect performance, so a favourable test-point COP is not a year's running-cost prediction.
Seasonal COP (SCOP) describes performance over a defined seasonal test pattern. A seasonal performance factor (SPF) can describe a designed or measured system over a longer period. Ask which figure is being used and whether it includes hot water, circulation pumps and backup heating. Different measurement boundaries make figures difficult to compare.
The average efficiency of 3.5 used below is an illustrative assumption, not a guarantee. Ask for a prediction that fits your location, emitters and heating and hot-water requirements, and test how costs change at a lower efficiency.
| Term | Useful question |
|---|---|
| COP | At what outdoor and water temperatures was this figure measured? |
| SCOP | Which climate and flow-temperature conditions does the seasonal rating use? |
| SPF | Is this a prediction or a measured result, and which electricity uses and heat outputs are included? |
Convert fuel use into an early electricity estimate
For a simple starting calculation, multiply the fuel used for heating and hot water by an assumed seasonal boiler efficiency. That estimates useful heat. Divide that heat by the assumed seasonal heat-pump efficiency to estimate electricity. Use the same comfort and hot-water needs in both cases.
This is an energy estimate, not a heat-loss calculation or equipment size. Actual boiler efficiency, fuel used for cooking, changes in comfort, system losses and auxiliary electricity can all change the answer. If the figures do not include all those uses, identify what has been left out.
Compare the cost of a kWh of heat
Compare useful heat rather than just the fuel unit prices. Boiler heat costs gas price ÷ boiler efficiency; heat-pump heat costs electricity price ÷ heat-pump efficiency. Express a 90% boiler efficiency as 0.9 in that calculation.
At illustrative rates of 7p for gas and 25p for electricity, boiler heat at 90% efficiency costs about 7.8p per kWh. Heat-pump heat at efficiency 3.5 costs about 7.1p. The energy-cost break-even efficiency is 25 × 0.9 ÷ 7 = about 3.21. Change either tariff and that threshold changes too.
| System assumption | Annual energy cost |
|---|---|
| Gas boiler, 90% efficiency, gas at 7p | 18,000 kWh of gas: £1,260. |
| Heat pump, average efficiency 2.5, electricity at 25p | 6,480 kWh of electricity: £1,620, or £360 more. |
| Heat pump, average efficiency 3.5, electricity at 25p | About 4,629 kWh of electricity: £1,157, or £103 less. |
| Heat pump, average efficiency 4.0, electricity at 25p | 4,050 kWh of electricity: £1,012.50, or £247.50 less. |
Plan hot water and the outdoor unit
A typical air-to-water installation needs stored hot water rather than the instantaneous supply of a combi boiler. Discuss cylinder size, available space, reheat time and busy periods such as consecutive showers. Hot-water performance can differ from room-heating performance, so ask how both are included in the estimate.
The outdoor unit needs suitable space and airflow. Ask the installer to check positioning, noise, access for servicing, condensate drainage, electrical supply and any permissions or grid notifications. A map or photograph can start the discussion, but does not settle the installation layout.
Agree the hot-water controls and any required hygiene cycle with the installer. Keep those manufacturer and installer settings in the handover information rather than changing them solely to chase a cheap electricity window.
Use controls and tariffs without losing comfort
Heat pumps can work efficiently with longer heating periods and lower water temperatures. Weather compensation adjusts the heating-water temperature as the outdoor temperature changes. Ask the installer to explain the heating curve, room controls and a suitable setback when you are out or asleep.
A time-of-use tariff may help, but a cold night, a higher flow temperature or extra heating to recover from a deep setback can change efficiency. Compare the price of useful heat and the comfort achieved, not just the cheapest electricity rate.
Consider solar, a battery and other household loads
Solar can supply some heat-pump electricity when generation and demand coincide. Heating demand is generally greatest in winter, when solar generation is lower, so annual solar output should not be treated as electricity available for every heating hour. A household battery can move energy between hours, not practically store a summer surplus for winter.
Compare the heat pump on a suitable tariff before adding the costs of solar and storage. Include EV charging and other household demand, battery losses, charging windows and inverter limits. Keep the energy and installation costs of retained equipment separate from new purchases.
Compare complete quotes and funding
Compare itemised prices for the same scope. A heat-pump unit price is not an installed-system price, and a quote with fewer radiator or cylinder changes may describe a different design. Specify which equipment will be retained, removed or replaced.
Keep the gross installation price, any confirmed grant and the amount you pay visible separately. Funding routes and eligibility differ across the UK: check the Boiler Upgrade Scheme for England and Wales, Home Energy Scotland for Scotland, and local advice for Northern Ireland. Confirm application requirements before starting work.
Simple payback needs a reviewed installed price and plausible annual savings. If a boiler would otherwise need replacing, compare the additional cost over that alternative as well as the total project cost. Include financing, maintenance, equipment life and possible replacements when considering longer-term value.
- The room-by-room heat-loss report, design temperatures and proposed emitters.
- Heat-pump model, output at design conditions, cylinder, pipework and electrical work.
- Outdoor siting, base or supports, drainage, permissions and supply checks.
- Removal of old equipment, commissioning, certificates and a controls demonstration.
- Predicted annual electricity, seasonal efficiency and any backup-heater allowance.
- Warranty, servicing costs, aftercare and a plan to review performance during the first winter.
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