Heat pump size calculator

Check whether a heat pump’s available output covers the home’s assessed design heat loss. Choose an installer’s heat-loss figure directly, or use a professionally measured heat-transfer coefficient with the design temperatures. This tool does not guess heat loss from floor area or the age of the home.

Enter the indoor temperature, outdoor temperature and water flow temperature for the proposed design. Water flow temperature is the temperature supplied to the heating system. Use equipment output at those same conditions. Choose a published model test point only when the exact product and test temperatures match; otherwise use an assessed output supported by your installer.

Use installer-assessed design heat loss directly, or professionally measured heat transfer coefficient × (indoor minus outdoor temperature) ÷ 1000. Available equipment output must be taken at the stated outdoor and flow temperatures. Margin = available output minus design load. No arbitrary age-band, area or oversizing factor is used.

A heat-transfer coefficient is the number of watts lost for each degree of temperature difference. Its unit is W/K, or watts per kelvin. A Celsius temperature difference has the same size as a kelvin difference. Multiply the coefficient by indoor minus outdoor temperature, then divide by 1000 to get kilowatts.

Kilowatts, written kW, measure heating power. Compare design load and available output in the same unit. A positive margin means output exceeds the entered load at those conditions. A negative margin means output is below it. The margin is not a recommended extra capacity or a guarantee that the whole system will work well.

The result keeps the selected conditions with its working. Use that record when discussing a quote. Do not replace a manufacturer output figure with the electrical input rating: heat delivered and electricity consumed are different quantities.

These illustrative inputs are separate from your saved answers. Prices, use and performance are held constant for this worked scenario; they are not personal defaults.

Use an installer heat-loss assessment or a measured heat-transfer study for this property. The coefficient must cover the intended building. Do not use a number chosen only to make a preferred unit appear large enough. Personal design values start empty; examples are not submitted defaults.

Published model output applies only at the stated air and water conditions, including humidity where recorded. Confirm the exact product against the linked maker table. The calculator does not interpolate between test points or infer seasonal performance. An assessed output is needed where the source does not match the proposed design.

A capacity margin alone does not assess room emitters, controls, hot-water requirements or safe installation. It is not a room survey or a sizing certificate. Display values are rounded while the calculation keeps unrounded figures. Incomplete or invalid edits preserve the previous valid estimate with a warning.

Use the UK property assessment and the maker’s record for the product being considered. The source links and checked dates show where the method and published test values came from. They do not confirm that your home or a proposed installation has been assessed.

This capacity check does not use a tariff, grant amount or assumed running cost. A published output is evidence for its stated test conditions only. Keep those conditions beside the quote and ask the installer to explain any difference from the intended outdoor and water temperatures.

A qualified installer should assess each room, the heat emitters and equipment output at design conditions. Ask for the heat-loss report and the performance table used to choose the unit. Check that the quoted model is the same model shown in that table.

Bring the saved inputs and capacity margin to the installer. Ask them to explain any shortfall and how the complete system meets the design load. Follow the maker instructions and local installation requirements.