Concepts · Chapter 12
Sizing done right: Heizlast & sizing
Why heat pumps are often quoted too big — and how to spot the right size.
Sizing done right: Heizlast & sizing
The most common expensive mistake with a heat pump is the wrong size — usually too big. This chapter shows how to spot the right kW figure.
The Heizlast is the basis
The heat load (per DIN EN 12831) is the calculated power your house needs on the coldest design day. It should determine the heat pump's kW sizing. An offer without a heat-load calculation is guessing — and, in doubt, guessing big.
Don't size on the old heat load
After insulation and window replacement, demand drops sharply. A real example: heating demand fell from about 39,000 kWh to about 12,000 kWh per year. A renovated ~200 m² older house then often needs ~8–12 kW, not the 16 or 22 kW sometimes quoted.
Insist on a Heizlast that reflects the renovated state — not the old oil-boiler consumption.
Why too big is harmful
An oversized heat pump short-cycles: it meets the small demand too quickly, switches off, restarts — constant on/off. That lowers efficiency (JAZ) and wears the compressor. A tightly and correctly sized, modulating heat pump runs quietly and efficiently.
Have the radiators checked too
The kW figure alone isn't enough. What matters is whether your radiators deliver enough heat at a low flow temperature. Have every installer assess the heating surfaces — and scrutinise single-pipe systems (series wiring) especially, because they force high flow temperatures.
A rough whole-system sizing check
- Household base use: ~4,000 kWh/yr
- Heat pump: ~3,000–4,000 kWh/yr
- Possible EV: ~2,000–3,000 kWh/yr
- → often sensible: ~14–16 kWp PV and 10–15 kWh battery, modular if you might expand.
In short: demand a real Heizlast for the renovated state, don't be talked into an oversized heat pump (short-cycling!), and have the radiators assessed at low flow.
Next, an often-misunderstood point: backup — what "backup-capable" really means.
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