Boiler Replacement in an Old Building 2026: GEG Duty, Heat Pump, Costs and Funding
Many owners come under pressure when replacing a heating system in an existing building in 2026: the GEG's 65% renewables duty, transition periods tied to municipal heat planning and the question of whether a heat pump even works in an unrenovated old building. This guide organises the obligations, quantifies costs and funding realistically and shows how a heat pump runs efficiently and economically even in existing stock.
Replacing the heating system in an old building is, in 2026, the topic that puts many owners under pressure and unsettles them at the same time: the Building Energy Act (Gebäudeenergiegesetz) (GEG) generally requires newly installed heating systems to have a 65 percent share of renewable energy, the transition periods are tied to the municipal heat planning, and above everything stands the question: does a heat pump even work in my unrenovated existing building? This guide classifies the obligations, quantifies the costs and funding for 2026 realistically and explains how a heat pump also works efficiently in an old building. Anyone who builds new instead can calculate the prefab house costs for the total.
The GEG's 65 percent rule: what actually applies in 2026
Short answer: Since 1 January 2024, every newly installed heating system must cover at least 65 percent of its heat from renewable energy. In existing buildings, however, the obligation does not yet apply immediately to everyone in 2026: it is tied to the municipal heat planning. In large cities (over 100,000 inhabitants), the planning must be in place by 30 June 2026, in smaller municipalities by 30 June 2028. Only after that does the 65 percent obligation fully apply to replacing the heating system in existing buildings too. Until then, fossil-fuel heating systems may still be installed in many municipalities – but with a consultation obligation and rising renewable shares in the following years.
Important: if a functioning heating system keeps running, there is no replacement obligation. Only when the old system is irreparably broken (“heating breakdown”) or you switch voluntarily do the rules apply. For breakdown cases, there are transition periods of several years in which a used or rented fossil-fuel heating system may also be used temporarily. The 30-year replacement obligation for old constant-temperature boilers from the GEG remains unaffected by this.
How to meet the 65 percent obligation
You meet the GEG requirement in 2026 in several ways: with a heat pump, connection to a heat network, direct electric heating (only sensible in well-insulated buildings), a hybrid heating system (heat pump plus gas peak load), a biomass heating system (pellets, wood) or with a gas heating system that demonstrably burns 65 percent biomethane or hydrogen. For most existing buildings, the heat pump is the most economical and most heavily funded solution.
Heat pump in an old building: is that possible – and under what conditions?
Short answer: Yes, a heat pump works efficiently in most existing buildings in 2026 – contrary to the persistent myth that it necessarily requires underfloor heating and a passive house. Decisive is the required flow temperature: if it stays below about 55 degrees on the coldest days, a modern air-to-water heat pump runs economically. Many old buildings already achieve this through hydraulic balancing, larger radiators (low-temperature radiators) in the critical rooms and an insulated top-storey ceiling – that is, with manageable effort, without a complete core renovation.
The central parameter is the seasonal performance factor (Jahresarbeitszahl, JAZ): it shows how many kilowatt-hours of heat the heat pump generates per kilowatt-hour of electricity. A JAZ of 3.0 is considered solid in an old building, from 3.5 as very good. The lower the flow temperature, the higher the JAZ and the lower the electricity bill. A heat load check by a specialist firm or energy consultant reliably clarifies before purchase whether and with which accompanying measures the heat pump works.
- Have the heat load and flow temperature determined — the most important step before any purchase decision.
- Carry out a hydraulic balancing — lowers the flow temperature and is BEG-eligible for funding anyway.
- Replace critical radiators with low-temperature models (instead of complete underfloor heating).
- Insulate the top-storey ceiling or roof — the cheapest measure with a big effect.
- Plan in hot water preparation: hot water heat pump or fresh water station.
- Clarify the installation location for the outdoor unit (noise protection, distance to the neighbour).
Costs 2026: what to reckon with when replacing heating in existing buildings
Short answer: An air-to-water heat pump costs in an old building in 2026, including installation, storage and adjustments, usually €20,000 to €35,000, a more efficient brine-to-water heat pump with a borehole probe €30,000 to €45,000. A pellet heating system ranges at €25,000 to €38,000, a hybrid heating system (heat pump plus existing gas boiler) at €18,000 to €30,000. In addition come accompanying measures such as the hydraulic balancing (€500 to €1,500) and new radiators (€300 to €800 each). The BEG heating funding pushes the own contribution down by up to 70 percent.
Heating replacement in an old building 2026 — costs and suitability at a glance
| System | Costs incl. installation | JAZ / efficiency | Suitability old building |
|---|---|---|---|
| Air-to-water heat pump | €20,000–35,000 | JAZ 3.0–3.8 | very good (standard solution) |
| Brine-to-water heat pump | €30,000–45,000 | JAZ 4.0–4.8 | very good (with space for a probe) |
| Hybrid heating (HP + gas) | €18,000–30,000 | depending on share | good (transitional solution) |
| Pellet heating | €25,000–38,000 | high renewable share | good (with a storage room) |
| District heating connection | €8,000–18,000 | network-dependent | very good (if available) |
Funding 2026: up to 70 percent grant for the new heating system
Short answer: The heating funding is handled in 2026 via the KfW (no longer via the BAFA). It is made up of 30 percent basic funding, the climate speed bonus (20 percent for replacing an old fossil-fuel heating system that is at least 20 years old, or with a functioning device), the income bonus (30 percent with a taxable household income of up to €40,000) and the efficiency bonus (5 percent for heat pumps with a natural refrigerant or water/ground as the source). The bonuses are capped at a maximum of 70 percent, the eligible costs at €30,000 per residential unit.
For the own contribution after deducting the grant, there is a low-interest supplementary loan from the KfW (up to €120,000 per residential unit, with an interest reduction up to €90,000 household income). All funding pillars and the order of application are explained in the guide Renovation Funding 2026. A complete programme overview can be found under Funding.
KfW heating funding 2026 — bonuses and the maximum values
| Component | Funding rate | Requirement |
|---|---|---|
| Basic funding | 30% | for all eligible heating systems |
| Climate speed bonus | 20% | replacement of old fossil-fuel heating, owner-occupation |
| Income bonus | 30% | household income ≤ €40,000 (taxable) |
| Efficiency bonus | 5% | natural refrigerant or ground/water source |
| Overall cap | max. 70% | on max. €30,000 eligible costs |
Heat pump in an old building — with heat load check and funding handling included
We connect you with three specialist firms that determine the heat load in existing buildings, size the right heat pump and handle the KfW funding via an energy efficiency expert. This way you know your own contribution before you commit.
Transition periods and breakdown cases: what applies with a defective heating system
Short answer: If the old heating system fails irreparably in 2026, you do not have to install a GEG-compliant system immediately: the law grants a multi-year transition period for breakdown cases. During this time, you may temporarily operate a repaired, used or rented heating system – even fossil-fuel. In this case, you are obliged to seek advice to estimate the follow-up costs of fossil-fuel technology. Within the deadline, you then switch to a 65 percent renewable solution. This way the law prevents owners from having to make rushed and expensive decisions in winter in the event of a failure.
Despite the transition period: anyone who hastily installs a new pure gas or oil heating system in a breakdown case pays twice later – first for the transitional system, then for the renewable heating system. It is smarter to prevent the breakdown case early by planning the replacement in good time at the first signs of an impending defect (frequent repairs, age over 20 years) and applying for the funding calmly. How to steer the sequence of envelope and heating cost-optimally is shown in the guide Modernising a Prefab House.
Municipal heat planning: why it co-determines your decision
Short answer: Municipal heat planning determines where a heat network (district heating) will be created in the future and where owners are reliant on decentralised solutions such as the heat pump. Large cities must present their plans by 30 June 2026, smaller municipalities by 30 June 2028. For you this is decisive: if your house is in a planned district heating area, the connection can turn out cheaper and less complicated than your own heat pump. If no heat network is upcoming, the heat pump is usually the best choice. Before replacing the heating, ask the municipal utility about the state of the planning.
In practice, this means: anyone living in a large city in 2026 should check the district heating rollout status before acquiring a heat pump. In rural municipalities without a realistic district heating prospect, by contrast, waiting rarely pays off – here the funded heat pump is almost always the most economical route. Integrate the heat planning into your overall strategy so that you do not invest in a technology that is overtaken by the heat network a few years later.
Before the replacement: these three questions for the municipality
Before the purchase decision, clarify: first – is my plot designated in the heat plan as a district heating area? Second – by when would a connection realistically be available? Third – is there a connection obligation or a connection right? The answers decide whether it is worth waiting for district heating or whether you had better install a funded heat pump immediately.
Operating costs: is the heat pump worthwhile in ongoing operation?
Short answer: Whether the heat pump heats more cheaply than gas or oil depends in 2026 on the ratio of electricity to gas price and on the seasonal performance factor. With a JAZ of 3.5 and a heat pump electricity tariff of around 25 cents per kilowatt-hour, the heat pump in an old building is at about 7 cents per kilowatt-hour of heat – clearly below gas and oil including the CO2 price, which rises noticeably up to 2027. Anyone who couples the heat pump with their own photovoltaic system pushes the electricity costs further down, since part of the operating electricity is produced in-house.
Important for low operating costs: a special heat pump tariff (cheaper than household electricity), a high JAZ through a low flow temperature and a cleanly sized system. Oversized (“bought too big”), the heat pump cycles too often and wears out faster – another reason why the heat load check at the start is so important. Viewed over the years, the funded heat pump in an old building usually saves several hundred to over a thousand euros per year compared with a new fossil-fuel heating system.
Anyone who integrates the heating replacement into a more comprehensive modernisation will find the classification in the overall process in the guide House Building Process. Terms around GEG, JAZ and heat planning can be looked up in the Glossary.
Heating replacement at a fixed price and with maximum funding
We connect you with three specialist firms from your region that calculate your heating replacement at a fixed price, size the heat pump cleanly and make full use of the KfW funding. This way you heat economically and GEG-compliantly from day one.
What does your dream house really cost?
Request suitable quotes from vetted manufacturers for free and compare the prices per square metre for your construction project.
Compare house prices nowImportant questions briefly explained
The most common price questions around Boiler Replacement in an Old Building 2026 – answered concisely by the Prefabricated House editorial team (as of 2026).
