Heat pump in an old building: What works, what doesn't?
"A heat pump in an old building is not possible" – this blanket statement is incorrect. It stems from a time when heat pumps truly only operated efficiently in well-insulated new builds with underfloor heating. Modern devices now achieve significantly higher flow temperatures and can therefore also cope with conventional radiators – in most old buildings, a heat pump works if you consider a few points.
Crucial is an honest assessment: What is the building's heating load, what flow temperature do the existing heating surfaces require, and what are the structural limitations? This guide shows you the possibilities and limitations – including costs and subsidies. The key data upfront: 60–80% of old buildings are suitable for a heat pump, the maximum BEG subsidy is 70%, and the typical amortization period is 15+ years.
Prerequisites: When is an old building suitable?
Whether a heat pump in an old building runs economically essentially depends on a single question: Can the building get warm with moderate flow temperatures? Heat pumps work more efficiently the smaller the temperature difference between the heat source and the heating water. A house that manages with a maximum flow of 55 °C is therefore a good candidate – a house that doesn't get properly warm even at 70 °C, on the other hand, is not without preliminary work.
The flow temperature can, by the way, be practically tested before you invest: On a cold winter day, set the existing heating to a 55 °C flow and observe whether all rooms become sufficiently warm. If the house remains comfortable, the most important hurdle has been overcome. In addition, practical factors play a role – from space for the outdoor unit to the capacity of the power connection and the neighborhood's sensitivity to noise.
Suitable if
- Heating load under 8 kW
- Maximum flow temperature of 55 °C possible
- Radiators large enough or underfloor heating present
- Electricity house connection at least 24 kW
- Space for the outdoor unit (3 m²)
Difficult if
- Heating load over 12 kW (very poorly insulated)
- Only small panel radiators
- Non-expandable electricity connection
- Listed building with design requirements
- Noise-sensitive neighborhood
Which heat pump for old buildings?
Not all heat pumps are created equal – the systems primarily differ in the heat source from which they draw their energy. The air-to-water heat pump uses outside air and is the easiest to install, but has to work hardest when it's coldest outside. Ground and groundwater, on the other hand, offer stable temperatures all year round and thus better annual performance factors – but this comes at the cost of drilling, approval procedures, and higher investment costs.
For old buildings, the air-to-water heat pump is practically the most common choice because it requires no excavation work and can also be implemented on smaller plots. Those who have the option of a ground probe and plan long-term often find the brine variant more efficient. The overview shows the strengths and weaknesses of the systems:
| Type | Advantages | Disadvantages |
|---|---|---|
| Air-to-water HP | Inexpensive, simple | Noisier, efficiency decreases in winter |
| Brine-to-water (ground probe) | Highly efficient, quiet | More expensive (drilling), permits |
| Water-to-water (well) | Highest COP | Permits, not possible everywhere |
| Ventilation with active HRG (Nilan) | Combination: ventilation, heating, hot water | Ducting is difficult in old buildings |
Read more about the combination of ventilation and heat pump in the guide to comfort ventilation with heat recovery. Suitable devices can be found in the heat pumps category.
Preparation in old buildings: Step by step
The most common mistake in a heat pump project in an old building is impatience: those who order the device directly without first having the heating load and heating surfaces checked risk an incorrectly sized system – and this either works inefficiently or will not be able to maintain living comfort. An oversized heat pump cycles frequently and wears out faster; an undersized one cannot warm the house on cold days.
A solid sequence therefore always begins with calculation: only when it is in black and white how much heat the building actually needs and whether the existing radiators can emit this heat at low flow temperatures should specific devices be discussed. In parallel, it is worth looking at the infrastructure – power connection, installation location, and sound insulation are best clarified early on, while alternatives are still open.
- Heating load calculation according to DIN EN 12831 – by an energy consultant (mandatory for BEG funding)
- Heating surface check: Are the existing radiators sufficient at 55 °C flow? This includes the hydraulic balancing with calculation
- Check insulation status: Is the roof insulated? Are windows at least double-glazed? Otherwise, insulate first
- Power connection: 24–32 kW are often necessary; expand the house connection if necessary (chargeable)
- Outdoor installation: Minimum distance of 3 m to the neighbor's boundary, comply with sound levels according to TA-Lärm
Note: For very old houses (built before 1980, uninsulated), you should prioritize insulation first – then the heat pump suddenly makes sense.
Costs and Subsidies 2026
The investment in a heat pump in an old building is noticeably higher than installing a new gas or oil boiler – no one should sugarcoat that. In addition to the device itself, there are often ancillary costs in existing buildings that do not arise in new construction: the expansion of the power connection, the replacement of individual, too-small radiators, or the mandatory hydraulic balancing. Those who plan for these items from the outset will not experience unpleasant surprises later.
On the other hand, there is generous government funding that cushions a significant portion of the additional costs – and the ongoing savings compared to fossil fuels over the entire lifespan of the system. The typical cost blocks at a glance:
| Item | Costs |
|---|---|
| Air-to-water HP complete | 20,000–30,000 € |
| Brine HP with drilling | 30,000–50,000 € |
| Hydraulic balancing | 500–1,500 € |
| Expand power connection | 1,500–5,000 € |
| Heating surface replacement (if necessary) | 3,000–8,000 € |
| Total investment | 25,000–65,000 € |
With 70% BEG funding, the effective out-of-pocket costs are 7,500–19,500 €. You can read all the details in the Funding Guide 2026. Accessories for integration into the heating system can be found under heating pipes and press fittings.
Frequently Asked Questions
Is a heat pump really possible in an old building?
Yes, in 60–80% of old buildings. Prerequisite: heating load under 8 kW and a flow of maximum 55 °C possible. A practical test with the existing heating in winter quickly provides clarity.
Do I need new radiators?
Not necessarily. If the existing radiators have sufficient output at 55 °C flow (calculation!), it works with the existing ones. Often, it's enough to replace only individual critical radiators with larger models.
How much electricity does a heat pump in an old building need?
A COP of 3.0–4.0 is typical for old buildings. With a heating energy demand of 20,000 kWh, that's 5,000–6,700 kWh of electricity.
Can I combine the heat pump with the old heating system?
Yes, a "hybrid heating system" is allowed. The heat pump takes over the base load, and the old heating system kicks in during peak load. This can be a sensible temporary solution if the building is only renovated gradually.
Conclusion
A heat pump in an old building is feasible in most cases – the decisive factors are heating load, flow temperature, and adequately sized heating surfaces. Those who have a proper heating load calculation done before investing and limit the existing heating to a test flow of 55 °C in winter will quickly know where they stand. With up to 70% BEG funding, the out-of-pocket costs are significantly reduced, so the project can also be economically viable. And even if the building is not yet ready: With targeted insulation and hydraulic balancing, almost any old building can be gradually made suitable for a heat pump.
Heat pumps and accessories can be found in the heating assortment of wohnungstechnik.de.
