Heat pumps in older buildings: when they work and when they do not

The real criterion

Heat pumps in older buildings have a poor reputation dating from an era when units were designed for 35 degrees of flow temperature and anything above that collapsed their efficiency. Modern units with R290 refrigerant reach 70 degrees and still deliver a seasonal performance factor of 3.0 to 3.4 at 55 degrees.

The relevant question is therefore not how old the house is, but: how hot does the water in the heating circuit have to be on the coldest day of the year for every room to reach its target temperature?

How to find out yourself

There is a test that costs nothing but a cold weekend. On a day near the design temperature — around minus ten degrees in most of Germany — lower your heating's flow temperature step by step, open every thermostat fully, and watch over 24 to 48 hours whether the rooms hold their temperature.

If the house stays warm at 50 or 55 degrees, it is heat-pump-ready without any insulation work. If it only stays warm at 70, individual radiators are undersized — usually fixable for a few hundred euros per room, and far cheaper than insulating a façade.

The economics

The arithmetic is pleasantly simple. One kilowatt hour of heat from a heat pump costs: electricity price divided by seasonal performance factor. From a gas boiler: gas price divided by efficiency.

At a 27-cent heat pump tariff and an SCOP of 3.2, that is 8.4 cents per kilowatt hour of heat. At 11.5 cents for gas and 88 percent efficiency, it is 13.1 cents. The heat pump is cheaper even though the electricity costs more than twice as much as the gas — because it moves energy rather than burning it.

On top of that, the carbon price on fossil fuels is scheduled to keep rising. Every year shifts the comparison further towards the heat pump.

The subsidy

The German BEG subsidy for a heating replacement consists of a 30 percent base grant, a 20 percent speed bonus for replacing an old but functioning system, a 30 percent income bonus for households below €40,000 of taxable annual income, and a 5 percent efficiency bonus for natural refrigerants or ground source.

The sum is capped at 70 percent, and eligible costs at €30,000 for the first dwelling. The maximum grant is therefore €21,000.

What you do not need

Underfloor heating is not required. A complete façade renovation is not required. A buffer tank in series is actively harmful in most installations, because it raises the flow temperature unnecessarily.

What you do need is a room-by-room heat load calculation and a hydraulic balance. Together they cost a low four-figure sum and decide several thousand euros of running cost in the years that follow.