Time and time again, homeowners contact Renewable Heating Hub with the same concern. Is the heat pump they have been quoted actually the right size for their home?
We have explored this repeatedly, including in The Heat Pump Sizing Mess and our explanation of what a heat pump’s advertised output really means. One recurring source of confusion is the number on the quotation. A unit described as “7 kW” does not necessarily deliver 7 kW at the temperatures your heating system has been designed around. Published output depends on the exact model and operating conditions, and it can be higher or lower than the headline figure.
For example, a capacity quoted at 7C outside with water leaving the heat pump at 35C tells you about that particular test point. Your home might have been designed for -3C outside and a 45C heating flow temperature. You need the figures for those conditions to make a useful comparison. Two models carrying the same advertised kilowatt rating can produce different amounts of heat at the same outdoor and flow temperatures.
That is the motivation behind this tool, giving homeowners a clearer starting point for checking the heat pump in their proposal. It compares the published heating output we have available for the selected model with the heat loss you enter, at your outdoor design temperature and heating flow temperature. Your design temperature is the cold outdoor condition used in the heat loss calculation, while the flow temperature is the temperature of the water supplied to your radiators or underfloor heating. Use the figures from your installer’s design and ask for them if they are missing.
The comparison can highlight a possible capacity shortfall or a large difference worth discussing with your installer. Sizing also needs attention in milder weather. If a heat pump cannot reduce its output sufficiently, it may start and stop frequently. A figure above your entered heat loss does not establish that a unit is excessively oversized. Minimum output, controls, hot-water requirements and the rest of the system design all need consideration. This calculator does not carry out that wider assessment.
Select the brand and exact model code, then enter your calculated heat loss and both design temperatures. Please read the source notes alongside the result. Coverage varies by model and some conditions are not available; estimates and data limitations are identified. The aim is to help you ask a more useful question before agreeing to a proposal: “Can you show me the manufacturer’s output figures for this exact unit at my home’s design conditions, and explain how the complete system has been sized?”
This is for guidance purpose only and to act as a quick sanity check. Please read the limitations below and ask your installer to confirm the complete system design.
Guidance and limitations
This calculator provides general guidance and a quick sanity check of the heat pump information in your proposal. It is not a heat loss survey, a heating system design, a product recommendation or a guarantee of heating performance, comfort, efficiency or running costs.
Results depend on the information you enter and the published data available for the exact model and variant. Some figures are estimated between published points or read from manufacturer graphs. Data and calculations may contain errors or omissions, use different test conditions or become out of date. Defrost, supplementary heaters, quiet modes, controls and other operating limits can affect the heat available.
The checker does not validate your heat loss calculation or assess the complete installation, including radiators or underfloor heating, pipework, water flow, hot-water demand and commissioning. A result above your entered heat loss does not prove that a system is correctly sized or designed, and a figure below it should be discussed with your installer before drawing conclusions.
Ask a suitably competent installer or heating system designer to confirm the exact unit’s capacity at your design conditions, the defrost and supplementary heating basis and the complete system design. Do not rely on this tool alone when buying equipment, changing a design or judging an installation.
Spotted an error or want a model added? Please email editor@renewableheatinghub.co.uk. Include the manufacturer, full model code, conditions checked and a link to the relevant manufacturer’s technical document where possible. Corrections, questions and requests for additional heat pump models are welcome.
