Methodology

The calculator compares options using one consistent model and user-provided home inputs.

Core model inputs

  • Heated floor area and insulation profile
  • Building age and current heating setup
  • Electricity price and climate assumptions
  • System-specific efficiency and coverage assumptions

Results are estimates, not quotes. Installation-specific factors and local market pricing can materially change final economics.

Existing heat pumps and replacements

Ground-source, air-to-water and air-to-air can be selected as existing heating. We estimate their seasonal efficiency at 95% of the new-pump profile for pumps under 10 years, 80% for 10–15 years or unknown age, and 65% for older pumps. These are indicative generation assumptions; age alone does not establish performance. Annual heating electricity can calibrate delivered heat demand for all-electric setups. Exclude household electricity and use a normal year when the pump worked. A manually entered delivered heat demand takes precedence.

Air-to-air covers about 65% of space heating in mild conditions, falling to about 45% in cold conditions. We allocate 20% of delivered heat demand to hot water, which air-to-air does not supply. Uncovered heating and hot water use the sources you select, defaulting to electricity. A functioning hydronic heat pump remains the backup when adding air-to-air.

Confirmed borehole reuse reduces the indicative ground-source investment by a planning allowance of £14,000 in the UK or €10,000 in Ireland. These allowances are model assumptions, not installer quotes. A complete quote for replacement with the same pump type overrides the estimate before grants. An existing hydronic heat pump avoids an added distribution-retrofit charge.

A working pump may be recommended to keep. Unknown condition is treated as working. If the pump needs replacing, keeping it is excluded and normal previous running costs are shown only as a reference. Air-to-air alone is excluded as a replacement for a failed hydronic heat pump. Further replacements and future repair costs are not included, and replacing with the same pump type has no assumed home-value uplift.

Who maintains the model?

David Shore, an IT engineer and heat pump owner, builds the comparison tool. His ownership experience informs practical questions, but model values are estimates rather than measurements of a particular home or product. A qualified installer should check any property-specific conclusion.

UK gas boiler and grant assumptions

For a gas boiler, we divide estimated heat demand by an 84% assumed boiler efficiency and multiply by a gas unit price. The default is 7.33p/kWh, the Ofgem average direct-debit rate for July–September 2026. Gas standing charges and other gas use are excluded. You can adjust the unit price or enter your own monthly heating cost in the results.

The calculator includes no grant by default. If the home is in England or Wales and you confirm eligibility for the Boiler Upgrade Scheme, you can include the relevant grant in the results. Eligibility and grant values depend on the property and the replacement system; an installer must confirm them. See the current grant amounts and eligibility rules.

Ireland assumptions

Irish electricity and fuel defaults use SEAI’s 1 July 2026 domestic fuel cost comparison. The electricity default is 34.38c/kWh (household band DD); the gas default is 13.00c/kWh (household band D2). Oil uses discounted kerosene at 11.72c/kWh delivered and an assumed 75% boiler efficiency. The gas boiler estimate assumes 90% efficiency. Standing charges, maintenance, and other household energy uses are excluded from the heating comparison.

Irish heat pump efficiency ranges are chosen within SEAI’s published typical seasonal performance bands. Air-to-water installation uses an indicative €14,000 before grants, within the €10,000–€16,000 range in SEAI homeowner research. Other system costs and any distribution retrofit are rough planning allowances, not local quotes. The location control reflects the broad coastal and inland temperature differences described by Met Éireann, not address-level weather data. Irish home-value effects are not included.

No Irish grant is included unless you enter a confirmed amount for a specific system. The calculator caps entered amounts at SEAI’s published maximum for that system and dwelling type. Grant eligibility depends on the home, installation design, and approval before work begins; see the SEAI heat pump grant guidance.