Heat pump in an older building: what the energy performance certificate reveals about suitability

If you own a house from the 1960s and are thinking about a heat pump, you have been hearing the same sentence for years: insulate first, then the heat pump. That is a rule of thumb about cost-effectiveness — and it is misunderstood as a statement about feasibility. Since November 2025 there have been measured values on this.

This article approaches the question of suitability through the quantity that matters: the flow temperature (Vorlauftemperatur). It shows how you can determine it with your existing heating, which details in the energy performance certificate help, what preparatory work and funding cost in 2026, and why replacing the heating moves the efficiency class more than any insulation.

#Does a heat pump suit an older building at all?

Yes, and the year of construction is not an exclusion criterion. The Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) measured 77 systems in one- to three-family houses built between 1826 and 2001 over four years. The seasonal performance factors ranged from 2.6 to 5.4, with an average of 3.4 for air-to-water systems. The finding comes down to one sentence: no correlation could be established between the year the buildings were constructed and the efficiency of the heat pump.1

The seasonal performance factor (Jahresarbeitszahl) is the heat delivered divided by the electricity consumed over a year. A second sentence does away with the most widespread condition: sufficiently dimensioned radiators ran on average at similarly low temperatures to surface heating.1 Underfloor heating is a convenience, not a prerequisite.

How it fits in with the other measures and funding pots is set out in the overview of energy-efficient renovation and funding; here we cover only the heating part.

#What flow temperature does your house need — and how do you find out?

The flow temperature is the temperature at which the heating water leaves the heat generator; the lower it is, the more efficiently the heat pump works. The consumer advice centre (Verbraucherzentrale) gives as its criterion a value that stays below 55 degrees Celsius as far as possible all year round.9 What counts is the coldest design day.

This threshold is not an opinion; it is in the law: § 559f (1) sentence 2 of the German Civil Code (BGB) exempts landlords from proving a seasonal performance factor of 2.5 if the building is heated at no more than 55 degrees Celsius at the local standard outdoor temperature.5 Funding measures at the same point too: it requires minimum efficiencies at 35 and at 55 degrees.6

A rough preliminary estimate can be made with the old heating: on a cold day, lower the flow temperature to 50 degrees and turn the thermostats up — if the heating output is sufficient, the radiators are suitable.15 If one room stays cold, that is the critical one. It becomes reliable with a room-by-room heat load calculation to DIN EN 12831, which funding requires anyway6 — the dena guide breaks it down like this:8

#What does the energy performance certificate say about suitability?

It does not answer the question on its own, but it provides three of the necessary details — and, with the demand certificate, the fourth.

First, the final energy indicator per square metre of usable floor area (Gebäudenutzfläche) and year: a first reference point for the heat demand. Second, the main energy source for heating and hot water, which § 85 (1) no. 14 of the Building Modernisation Act (Gebäudemodernisierungsgesetz, GModG) requires4 — it helps decide whether the climate speed bonus applies. Third, the energy efficiency class as the starting point for any before-and-after comparison.

The fourth is not on the paper but behind it: the room-by-room heat load calculation requires data on the heat losses of the building envelope, which has typically already been collected elsewhere — for example to prepare energy performance certificates.8 This is exactly where the certificate types part ways.

Anyone facing a heating replacement who has the choice is better served by the demand certificate (Bedarfsausweis); which type is permitted is clarified in consumption or demand certificate. What the certificate is not: a heat load calculation. It does not replace the calculation to DIN EN 12831 — but the data for it is then already available.

#Which preparatory work pays off first, and what does the state fund in 2026?

The order follows from the calculation: calculate first, then replace individual radiators, then balance hydraulically — and insulate where it reduces the heat load of the critical room. Balancing is mandatory; the funding guideline requires method B.6

Market observations, not prices of this portal.11

Funding is provided via KfW: 30 per cent basic funding, plus a 16 per cent climate speed bonus for owner-occupiers who replace a working oil, coal, gas storey or night storage heating system or a gas or biomass heating system at least 20 years old, and an income bonus of 40, 30 or 10 per cent. The upper limit is 70 per cent, or 80 for a taxable annual household income of up to 30,000 euros. Eligible costs are capped at 28,000 euros for the first residential unit.7 Whether you have to replace at all is clarified in heating law 2026.

#How does a heat pump change the efficiency class in the certificate?

More than expected, and the reason is a definition. Under § 86 (2) GModG, the classes follow directly from the final energy consumption or final energy demand.2 Final energy is what arrives at the building boundary: gas for a gas boiler, electricity for a heat pump. Because one kilowatt-hour of electricity becomes several kilowatt-hours of heat, precisely the figure on which the class depends goes down.

Two limitations belong with this. The class follows final energy, not primary energy — the law measures its requirements against the latter, a different figure on the same sheet. And a consumption certificate (Verbrauchsausweis) only shows the new class once bills with the heat pump are available; a demand certificate shows it immediately.

#Conclusion: the answer is in degrees Celsius, not in years of construction

Whether a heat pump suits your older building is decided by the temperature at which your rooms get warm — and you can measure that with the heating that is already there. The energy performance certificate provides the indicator, energy source and class, and a demand certificate the U-values on top. Anyone who proceeds this way does not buy a heat pump on spec.