Size a heat pump to the building's calculated heat load at design outdoor temperature, not to the old boiler. Typical specific loads are 30–50 W/m² for well-insulated homes and 80–120 W/m² for unrenovated older buildings: a modern 150 m² house needs about 4.5–7.5 kW, a poorly insulated one 12–18 kW. A room-by-room EN 12831 heat load calculation beats every rule of thumb.
How do you calculate the heat load of a house?
The correct method is a heat load calculation to EN 12831: for each room, transmission losses through walls, windows, roof and floor are added to ventilation losses at the design outdoor temperature (typically −8 to −16 °C in central Europe, milder near coasts). The result — in kW — is what the heat pump must deliver on the coldest expected day. Never copy the old boiler's rating: boilers are routinely oversized by 50–100%, and an oversized heat pump is a genuinely worse machine, not a safety margin.
What size heat pump per square metre?
Specific heat load per square metre of heated floor area gives a fast first estimate before the formal calculation:
| Building standard | Specific load | 150 m² home |
|---|---|---|
| Passive house | 10–15 W/m² | 1.5–2.5 kW |
| New build (current codes) | 30–50 W/m² | 4.5–7.5 kW |
| Renovated older building | 50–80 W/m² | 7.5–12 kW |
| Unrenovated, pre-1980 | 80–120 W/m² | 12–18 kW |
Another cross-check works from fuel bills: annual gas consumption in kWh divided by roughly 2,000–2,300 full-load hours (central European climate) approximates the design load. A house burning 18,000 kWh of gas a year lands near 8 kW.
Why is oversizing a heat pump worse than undersizing?
An oversized unit short-cycles: it reaches the target temperature in minutes, stops, and restarts continuously, multiplying compressor wear and cutting seasonal efficiency. Even an inverter compressor helps only within its modulation range — a 16 kW unit that can throttle to 5 kW still cycles in a house that needs 3 kW during most of the season. Mild-weather operation dominates the year: a correctly sized unit runs long, quiet, efficient cycles at part load, supported by a buffer tank where zoning or defrost volume demands it — see do I need a buffer tank with a heat pump.
Should the heat pump cover 100% of the design load?
In most of Europe, yes — monovalent sizing to the full load is standard and avoids fossil backup entirely. In cold climates or hard retrofits, a bivalent system sizes the heat pump to roughly 70–85% of the design load and lets a backup heater cover the coldest 1–5% of annual hours; the heat pump still delivers well over 90% of the yearly energy. German BEG funding rules, for example, require a seasonal performance factor (JAZ) of at least 3.0 — achievable only when sizing, emitters and flow temperature are planned together, since every 1 °C lower flow temperature adds roughly 2–3% efficiency.
Does domestic hot water change the size?
Usually not by much. Domestic hot water for a family of four averages only 0.2–0.3 kW continuous, and a properly sized cylinder decouples DHW from the space-heating peak: the heat pump reheats the tank in off-peak windows. What matters more is choosing a cylinder with a large coil surface designed for 55 °C heat pump operation. Solimpeks, manufacturing in Konya since 2001, builds heat pump-ready cylinders and Solibuffer vessels from 50 to 1,000 L precisely for this pairing.
Frequently asked questions
Can I just replace my 24 kW gas boiler with a 24 kW heat pump?
No. Combi boilers are sized for instant hot water, not for the building's heat load, which is typically 6–12 kW. Matching the boiler rating would give a heat pump two to three times too large, causing short-cycling and poor efficiency.
What happens if my heat pump is slightly undersized?
On the few coldest days a small electric backup element covers the gap — a modest running-cost penalty for a handful of hours a year. Slight undersizing generally costs far less over the unit's life than the cycling losses of oversizing.
Do I need bigger radiators for a heat pump?
Often one or two radiators need upsizing so the system can run at 45–50 °C instead of 70 °C. A room-by-room heat load calculation identifies exactly which emitters limit the flow temperature — frequently only 10–20% of them.
Who should perform the heat load calculation?
A qualified installer or energy consultant, using EN 12831 or the national equivalent (MCS in the UK, DIN/TS 12831-1 in Germany). Many subsidy schemes require the documented calculation as part of the funding application.