Modelling for Heat Pump Sizing

A heat pump must be sized to meet a home's heat loss on a cold design day, while running efficiently for most of the year at part load. Modelling for sizing therefore starts with a room-by-room heat loss calculation: for each room, the heat escaping through walls, roof, floor, windows and ventilation, plus the heat needed to warm incoming air, at an external design temperature for the location. Summing those room losses gives the whole-home heat load that the heat pump, together with any supplementary heat, must meet.

That calculation differs from an annual energy model. An annual model estimates total energy over a year under standard conditions, while a sizing calculation estimates peak power on a cold day. A home can have a modest annual demand but a high peak load, for example where it is poorly insulated and exposed, or a high annual demand but a moderate peak, where it is heated for long hours. Sizing from annual demand alone, without a peak calculation, risks a system that is wrong on the coldest days, when it matters most.

Several inputs dominate sizing. Fabric performance and areas set most of the loss, ventilation rate adds a further share, and internal and solar gains are usually treated conservatively for sizing, because a cold, dark design day offers little of either. The design temperature, which reflects local climate rather than a national average, matters too: a home in a colder, windier location needs a larger system than an identical home in a milder location. Flow temperature is a further key input, because a heat pump's output and efficiency fall as flow temperature rises, so a system designed for low-temperature radiators or underfloor heating can use a smaller, more efficient unit than one forced to run hot.

Radiator and emitter sizing interacts with heat pump sizing. Existing radiators sized for a high-temperature boiler may be too small for a low-temperature heat pump, delivering too little heat at the lower flow temperature. A sizing exercise should therefore check emitters room by room, not merely the heat pump unit, and should identify which radiators need upsizing or replacement. A correctly sized heat pump connected to undersized radiators will still leave rooms cold.

Before accepting a proposed size, ask for the heat loss calculation in writing: room-by-room losses, design temperature, fabric assumptions, ventilation rate and flow temperature. Check the fabric assumptions against the home, particularly wall type and insulation, and check whether the calculation assumes fabric improvements that have not yet been carried out. If insulation is planned, the sizing should state whether it sizes for the home as it is or as it will be after insulation, because the two give different answers.

Also ask about part-load performance and controls. A heat pump that is oversized will cycle on and off, reducing efficiency and comfort, while one that is undersized will rely heavily on supplementary electric heat on cold days, raising running costs. A good sizing proposal explains how the chosen unit handles mild-weather part load, how controls will manage it, and what supplementary provision exists for extreme cold.

By place, sizing conventions differ. In the UK, MCS-based sizing guidance and heat loss methods set expectations for certified installations, with design temperatures varying by region. In EU member states, national standards and installer schemes set their own sizing conventions and design temperatures. A size calculated under one country's conventions should not be transferred to another without checking the local design temperature and method.

Climate change and future fabric work also affect sizing choices. A system sized for today's fabric may become oversized after insulation, while a system sized for a warmer future climate may struggle on a cold snap. Those tensions are best handled by a sizing that states its assumptions clearly, so that later fabric changes can be assessed against the installed capacity.

A common mistake is to size a heat pump by replacing like with like: choosing a unit with the same kilowatt rating as the old boiler. Boilers are routinely oversized and run at high temperatures, so their rating is a poor guide to a heat pump's required size. Another mistake is to size from annual bills, which reflect occupancy and behaviour, rather than from a peak heat loss calculation.

A further mistake is to ignore emitters and controls in sizing. A well-sized unit with undersized radiators, poor balancing or crude controls will underperform, and the fault will be blamed on the heat pump when the real cause is the system around it.

Keep the heat loss calculation, the emitter schedule, the design temperature and the installer's sizing statement together with the installation records. That file supports future servicing, future fabric changes and any query about whether the system was sized correctly.

Questions to ask include: what is the room-by-room heat loss, at what design temperature and flow temperature, and which radiators need changing for that flow temperature. Those questions reveal whether a sizing is a calculation or a guess.