Heat pumps in apartment buildings: what efficiency in existing stock really depends on

The short answer

A heat pump works in an existing apartment building too – the Fraunhofer ISE field study “WP-QS im Bestand” measures an average seasonal performance factor of 3.4 for air-to-water units, and the building’s age played no part in that. What matters is not the age of the building but the temperature at which it heats and warms its drinking water.

And this is exactly where the apartment building differs from the single-family house: space heating can almost always be brought down to a low flow temperature, but the central hot water in an apartment building cannot, for hygiene reasons. This single point decides the economics more than any insulation does.

Does a heat pump work in an existing apartment building at all?

The common assumption that heat pumps are for insulated new builds with underfloor heating is out of date by the measured data. Fraunhofer ISE spent four years measuring 77 heat pumps in one- to three-family houses dating from 1826 to 2001. Air-to-water units averaged a seasonal performance factor of 3.4 (range 2.6 to 4.9), ground-coupled systems 4.3. A correlation between year of construction and efficiency could expressly not be established.

Two caveats belong with this, and they matter for apartment buildings. First, small buildings were measured, not the twelve-unit block. Second, the seasonal performance factor is an average over the heating period, not a promise for the individual case. The figure shows what is possible when sizing and operation are right – and in the weakest installations the team regularly found planning and installation errors, not unsuitable buildings.

Why is the flow temperature the decisive figure?

A heat pump becomes more efficient the smaller the temperature lift between the heat source and the heating loop. As a rule of thumb the Verbraucherzentrale states: a building is suitable if the flow temperature stays below 55 degrees all year round; lower is better. Every degree less cuts electricity use by the order of two to three per cent.

For space heating this is achievable in most existing buildings. Old radiators are often oversized, because they were designed for the full load of earlier decades; once the required output drops through partial refurbishment, a low flow temperature is enough. Where individual rooms will not warm up, replacing a few undersized radiators is usually sufficient rather than a full renovation. The practical test for this is simple: on a cold day, turn the heating curve down far enough that the flow reaches 50 to at most 55 degrees, and check whether the building stays warm.

What makes hot water the real problem in an apartment building?

This is where the apartment building parts ways with the single-family house. Central drinking-water heating installations count as large installations under DVGW worksheet W 551 as soon as the store holds more than 400 litres or the pipe volume between the heater and the most distant tap exceeds three litres – in an apartment building with central hot water that is the norm. For large installations the rules require 60 degrees at the store outlet and at least 55 degrees in the circulation loop, to protect against Legionella. The Drinking Water Ordinance refers, for this duty, to the generally recognised rules of technology.

Those 60 degrees are expensive for a heat pump. While the space heating runs at 40 degrees, part of the annual work has to be delivered at a markedly higher temperature, often with a poorer performance factor or an electric booster heater. That is why I budget more conservatively in an apartment building than the Fraunhofer averages suggest. The usual ways out – a high-temperature heat pump, a separate hot-water loop, decentralised fresh-water stations on each floor or a small reheating stage – solve the hygiene issue but cost either efficiency or capital. Anyone planning a heat pump in an apartment building has to settle the hot-water question first, not last.

What does this mean for electricity costs?

The worked example is deliberately limited to energy costs and fully disclosed. Building: 12 flats, around 900 square metres, 120,000 kilowatt hours of heat demand a year. By way of comparison, the gas boiler: at an efficiency of 93 per cent it needs 129,032 kilowatt hours of gas, which at 11.17 cents per kilowatt hour (BDEW, data as of August 2026) comes to about 14,413 euros in energy costs a year. There is no official average for heat-pump electricity – the BDEW takes heating-current tariffs out of its analysis – so I work with two assumptions, 22 and 27 cents.

Seasonal performance factor	Electricity demand/year	Electricity cost at 22 ct/kWh	Electricity cost at 27 ct/kWh
2.7 (unfavourable: high flow and hot-water temperature)	44,444 kWh	€9,778	€12,000
3.0 (realistic in stock with central hot water)	40,000 kWh	€8,800	€10,800
3.4 (Fraunhofer mean, well sized and turned down)	35,294 kWh	€7,765	€9,529

Even in the unfavourable case the heat pump’s pure energy costs are below those of the gas boiler. But that is only half the sum: the heat pump shifts cost from energy to capital, it is markedly more expensive to buy. Whether it is cheaper on balance is decided by investment, subsidy and term – I work that through in the full-cost comparison of gas, oil, district heating and heat pumps. For landlords there is the added point that the BEG subsidy does not pay out the income- and owner-occupier-related bonus; the basic grant remains. Which rates apply after the revised subsidy guidelines belongs in a guide of its own.

What owners and landlords should check now

1. Settle the hot water first: Check whether there is a large installation under W 551 and how the 60-degree requirement can be met with as little high-temperature operation as possible – a high-temperature heat pump, decentralised fresh-water stations or a separate reheating stage.
2. Run the 55-degree test: On a cold day, turn the heating curve down and see whether the space heating manages on a flow below 55 degrees. If it passes the test, the heating side is no obstacle.
3. Look at the heat load and individual radiators: Do not renovate the whole building; identify the few rooms with too little surface area and upgrade there.
4. Set your own performance factor conservatively: For the economics, work with 2.7 to 3.0, not the data-sheet value at 35 degrees, and use your own heat-pump electricity tariff.
5. Check noise and the installation site early: The immission limits of the TA Lärm at the nearest neighbouring building determine how and where the outdoor unit may stand – in dense stock often the harder bottleneck than the technology itself.
6. Get two engineering designs: At least two sizings stating the expected seasonal performance factor and the hot-water concept, not just a unit price.