Many homeowners believe a heat pump only works in well-insulated new builds. That used to be true, but not anymore. Modern air-water heat pumps now achieve flow temperatures up to 70°C, enough to comfortably heat old buildings with classic panel radiators. In this guide, we explain what to watch out for and how to use the KfW funding of up to 70% even in old buildings.
The Most Important Points at a Glance
- Heat pumps with 70°C flow temperature are suitable for old buildings without full renovation
- Hydraulic balancing and large radiators are often the cheapest optimization
- Up to 70% funding via KfW 261, also in old buildings
- Annual performance factor (JAZ) of 3.0+ is realistic in old buildings
- Cheapest variant: Air-water heat pump from approx. €15,000–25,000 before funding
Why a Heat Pump in Old Buildings is Often Cheaper Than Thought
The fear of cold feet in winter is understandable: Old houses have poor insulation, single glazing, and often small radiators. But precisely for this reason, legislators have made funding so attractive. Those who switch their old building to a heat pump not only save heating costs, but also massively reduce CO₂ emissions.
A typical single-family home (built 1970–1990, approx. 150 m²) consumes about 150–200 kWh/m² per year with old gas heating. That's heating costs of €2,400–3,200 at today's gas prices. With a well-adjusted heat pump, energy costs drop to about €1,200–1,800, a savings of €1,000–1,500 per year.
Requirements: What Your Old Building Needs
Not every old building is the same. The decisive factor is not the year of construction, but the heating load of your house. An energy consultant calculates this using the DIN EN 12831 procedure. Important influencing factors:
| Factor | Good for Heat Pump | Problematic |
|---|---|---|
| Insulation standard | Partially renovated or better | Completely unrenovated (built before 1960) |
| Radiators | Large Type-22 or Type-30 radiators | Small Type-10–11 panels |
| Windows | Double or triple glazing | Single glazing and old windows |
| Heating curve | Flow temperature under 55°C feasible | Only 70°C+ flow temperature sufficient |
The Three Renovation Levels in Old Buildings
You don't have to insulate the whole house immediately. In practice, three levels prove successful:
Level 1: Minimal: Heat Pump with Existing Radiators
If your radiators are large enough, just replace the heating system. Additional measures: Hydraulic balancing (approx. €500–1,500) and optimization of the heating curve. This is often sufficient if the old building is at least partially insulated or has new windows. Costs: from €15,000 for an air-water heat pump including installation.
Level 2: Medium: Replace Radiators + Insulation Optimization
Replace small radiators with larger models (Type 22 or 30), insulate the basement ceiling and the top floor ceiling. Both are also funded via KfW. This significantly lowers the required flow temperature and increases JAZ by 0.3–0.5 points.
Level 3: Full: Complete Renovation
Facade insulation, new windows, underfloor heating, this is the most expensive but most efficient variant. This achieves a JAZ of 4.0+ and minimal operating costs. Often sensible as a step-by-step approach if renovation is needed anyway.
Costs: Heat Pump in Old Buildings 2026
Costs for a heat pump in old buildings are typically higher than in new builds because additional adjustments are needed:
| Position | Costs (approx.) |
|---|---|
| Air-water heat pump (10–12 kW) | €8,000–14,000 |
| Installation & commissioning | €3,000–5,000 |
| Hydraulic balancing | €500–1,500 |
| New radiators (if needed) | €2,000–6,000 |
| Hot water storage tank | €800–2,000 |
| Total (before funding) | €14,300–28,500 |
With KfW funding of up to 70% (see below), net costs drop to €4,300–8,500. Most old building owners effectively pay less than for a new gas heating system.
Example Calculation: Old Building 150 m², Built 1980
Gas consumption: 180 kWh/m² × 150 m² = 27,000 kWh/year
Gas costs (€0.12/kWh): €3,240/year
Heat pump JAZ 3.2: 27,000 / 3.2 = 8,440 kWh electricity
Electricity costs (€0.30/kWh): €2,530/year
PV surplus (50% self-consumption): ~€1,800/year
Savings: approx. €1,440/year, Amortization after funding: 4–6 years
KfW Funding: Up to 70% Grant Also in Old Buildings
Federal funding for efficient buildings (KfW 261) explicitly applies to old buildings as well. Installing a heat pump as a heating replacement is funded with at least 30%:
- 30% basic funding, for every new heat pump
- +20% speed bonus, when replacing an old oil, gas, or night storage heater
- +20% income bonus, for household income up to €40,000 taxable income
- +5% iSFP bonus, if the measure is in the individual renovation roadmap
Eligible costs are capped at €30,000 per residential unit. This means a maximum grant of €21,000. Important: The application must be submitted BEFORE construction begins! An energy efficiency expert (EEE) submits the application via the KfW portal.
Calculate Your Funding and Costs Now:
Frequently Asked Questions About Heat Pumps in Old Buildings
Does a heat pump pay off in an unrenovated old building?
Yes, a heat pump can be economical even in unrenovated old buildings. The key is that the radiators are large enough for low flow temperatures or can be replaced accordingly. With a high-temperature heat pump (flow up to 70°C), the heating works even with existing radiators. The high KfW funding makes the investment lucrative even without full renovation.
How long is the service life of a heat pump?
Air-water heat pumps have a service life of 15–20 years, ground-source heat pumps even 20–25 years. This is comparable to modern gas boilers. Plan maintenance costs of about €150–300 per year.
Do I need a permit for an air-water heat pump?
In most federal states, no building permit is required. In Bavaria, depending on the municipality, a permit may be necessary, especially for outdoor installation. Ground-source heat pumps always require a permit from the lower water authority due to drilling.
What if it gets very cold below -15°C?
Modern air-water heat pumps still operate at -20°C. On extremely cold days, the electric heating rod (emergency heating) switches on optionally. This is acceptable in the short term but shouldn't happen throughout the entire winter. In very cold regions, a ground-source heat pump is the better choice since ground temperature remains constant year-round.
Can I combine heat pump with photovoltaics?
Absolutely, this is one of the most effective combinations. Solar power reduces the operating costs of the heat pump significantly. With a properly sized PV system (8–12 kWp), you can generate 40–60% of the heat pump's electricity demand yourself. Also read our article on combining photovoltaics + heat pump.