Heat Pump in Old Buildings — Does It Work?
Why the heat pump is a real alternative even in existing buildings — and what really matters.
The Myth: Only for New Buildings
One of the most persistent myths in heating technology: heat pumps only work in new buildings. This is simply not true. Modern heat pumps operate efficiently even at higher flow temperatures and are an economically viable solution for many existing buildings.
💡 Fact: According to the German Heat Pump Association, over 40% of all heat pumps in 2024 were installed in existing buildings — and the trend is rising.
The Key Factor: Flow Temperature
A heat pump's efficiency largely depends on the required flow temperature — the temperature to which the heating water must be heated. The lower, the more efficient.
- 30–35 °CIdeal — COP 4.0–5.0Typical with underfloor heating. Best efficiency.
- 45–55 °CGood — COP 3.0–4.0Works with larger radiators and partially insulated buildings.
- 60–75 °CPossible — COP 2.5–3.0High-temperature heat pumps can handle this. Still cheaper than gas from 2027.
COP = Coefficient of Performance. A COP of 4 means: 1 kWh of electricity produces 4 kWh of heat.
What Determines If It Works?
Three factors determine whether your old building is suitable for a heat pump:
Insulation Level
The building envelope determines heat demand. Even partial renovation (roof + top floor ceiling) can significantly lower the flow temperature.
Radiator Size
Larger heating surfaces = lower flow temperature. Old cast-iron radiators are often larger than necessary — that is actually an advantage.
Window Quality
Modern double or triple-glazed windows significantly reduce heat loss. Window replacement is often one of the most effective individual measures.
Common Scenarios
How we evaluate different existing buildings:
Old Building with Underfloor Heating
Underfloor heating systems work at low flow temperatures (30–40 °C). The heat pump achieves a COP of 4–5 here. No adjustments needed — simply switch over.
70s/80s House with Partial Insulation
Many houses from the 70s and 80s already have basic insulation and large radiators. With flow temperatures around 50 °C, the heat pump works efficiently. A hydraulic balancing may be worthwhile.
Uninsulated Pre-1960s with Small Radiators
Preparatory measures are recommended here: roof insulation, window replacement, or swapping individual radiators for larger models. Often 2–3 targeted measures are enough to lower the flow temperature to an efficient level.
Hybrid Solution: Best of Both Worlds
When the heat pump alone cannot cover the entire heat demand, there is a pragmatic solution: the heat pump handles the base load (about 80–90% of heating hours), while the existing gas or oil boiler only kicks in on the coldest days.
💡 Advantage: Immediate CO₂ reduction of 50–70%, without requiring a complete renovation first. The existing gas heating serves only as peak load coverage and can be gradually phased out.
Cost Comparison: Heat Pump vs. Gas Over 15 Years
A single-family home with 150 m² and consumption of 20,000 kWh/year:
Gas Heating
- Purchase~10.000 €
- Gas cost / year~2.400 €
- CO₂ levy / year (rising)~400–900 €
- 15-year cost~55.000–65.000 €
Heat Pump
- Purchase~28.000 €
- Subsidy (up to 70%)−12.000–19.000 €
- Electricity / year~1.500–1.700 €
- 15-year cost~30.000–42.000 €
* Calculation based on electricity price €0.30/kWh, gas price €0.12/kWh, CO₂ price from €55/t (2025) rising to ~€120/t (2030+). COP of 3.5 assumed.
Subsidies: Up to 80% Grant
The government generously subsidizes the switch to heat pumps. The subsidy consists of several components:
Base Subsidy
For every homeowner replacing an old fossil heating system with a heat pump. No income check required.
Climate Speed Bonus
Additional bonus for owners who proactively replace their functioning gas or oil heating (at least 20 years old or constant-temperature boiler). Since the July 2026 BEG reform: 16% instead of 20%, decreasing every six months from February 2027, expiring from August 2028.
Income Bonus
Three tiers since July 2026: 40% for taxable household income up to €30,000, 30% up to €40,000, 10% up to €50,000. Minor children reduce the applicable income by a one-time €10,000 (family allowance).
⚠️ Important: The total subsidy is capped at 70% — or 80% for an applicable income up to €30,000. The former efficiency bonus (5% for natural refrigerants or geothermal sources) has been dropped since the BEG reform of July 21, 2026. The application must be submitted to KfW before work begins. We handle this for you.
The Pre-Check: What an Energy Consultant Evaluates
Before we recommend a heat pump, we systematically evaluate:
- ✓Heat load calculation per DIN 12831 — how much heat does your house really need?
- ✓Flow temperature analysis — what temperature do your radiators need at −12 °C outside?
- ✓Building envelope assessment — roof, facade, windows, basement ceiling
- ✓Radiator inventory — type, size, location in every room
- ✓Power supply check — is the household connection sufficient? Separate heat pump meter possible?
- ✓Installation site — outdoor unit: distance to neighbors, noise protection
- ✓Subsidy options calculation — which combination yields the highest grant?
- ✓Profitability calculation — payback period compared to gas/oil
Heat Pump Types at a Glance
Air-to-Water Heat Pump
Uses outdoor air as heat source. Simple installation, no excavation needed. COP 3.0–4.5 depending on outside temperature. Cost: approx. €25,000–35,000 (before subsidies).
Ground Source Heat Pump
Uses geothermal energy via ground probes or surface collectors. Consistently high COP of 4.0–5.5 — independent of outside temperature. Cost: approx. €35,000–50,000 (before subsidies). Requires drilling or sufficient garden space.
Water-to-Water Heat Pump
Uses groundwater as heat source. Very high COP (5.0+), but requires complex permits and is not possible everywhere. Only viable if a suitable well exists or can be built.
Is Your House Ready for a Heat Pump?
We check free of charge and without obligation whether a heat pump is worthwhile for your building — including a subsidy check.