What is the best heating system to install when renovating an old building?

When is it worth replacing the heating system in an old building?

If you want to completely renovate your old building, you should also check your heating system. Especially when renewing or improving the insulation, it is worth taking a critical look at the old heating system. If the old building's consumption is clearly too high, it is usually worth buying a new one. After all, you can turn your gas or oil heating system into an energy saver simply by switching from a constant-temperature boiler to the more up-to-date condensing boiler technology. Depending on the fabric of the old building, this alone can save up to 30 percent of annual heating costs. If the oil heating system has been in service for at least 30 years, replacement is even mandatory. Even if the heating system is still functional, the heating system must be replaced in accordance with the Building Energy Act (Gebäudeenergiegesetz, GEG). 

It can even be worthwhile to replace more recent heating systems in old buildings, for example if the repairs are excessive. A defective boiler or expensive spare parts certainly justify the purchase of a modern and energy-efficient new heating system. Living comfort is also increased by modernising the heating in old buildings. In combination with a facade and window renovation, an innovative new heating system provides a significantly better indoor climate. Another important aspect is the increase in the value of the old building due to the new heating technology. At the same time, the new heating system is much more environmentally friendly. Here are good reasons to replace the heating system or to install a supplementary heating system with renewable energies in an old building:

• the heating is older than 30 years and there is a legal obligation to replace it

• the heating system is often in need of repair due to its age (15 years or more)

• reduced heating capacity of the heating system

• the energy costs of fossil fuels are too high

What type of heating is suitable for an old building?

The current plans for the energy transition also extend to the heating systems in old buildings. It is expected that from 2024 onwards, only heating systems that use at least 65 percent renewable energies will be permitted. This means that conventional gas or oil heating systems will be obsolete in the future, as they only use fossil fuels. Actually, even the sale of oil heating systems will be completely banned from 2026 at the latest. Since these two heating systems are still relatively inexpensive, policymakers are focusing on financial incentives for new heating systems with renewable energies for the renovation of old buildings. But it is not practical to only use wood heating, condensing boiler heating, solar thermal energy or heat pumps in all cases. This is where a hybrid solution of gas heating and supplementary heating with renewable energies usually makes sense. Sometimes it is enough to supplement the existing heating system in an old building with a modern heating system. A solar thermal system or heat pump can be used to supplement the heating system without having to replace it entirely. The energy costs for natural gas heating, for example, are also noticeably reduced by using a new heating system with additional renewable energies. These types of heating or combinations thereof could be suitable for an old building:                           

• condensing boiler with gas or oil

• heat pump (air, water, geothermal)

• heating with wood or pellets

• gas heating with solar thermal

• pellet heating with heat pump

• RE heating to supplement an existing heating system

What kind of heating makes sense in an old building?

Heat pumps are one of the heating systems of the future. They use air, ground or water as a free heat source. With an electricity share of around 20 to 30 percent, the heat pump is one of the cheapest heating systems to use. Unlike oil heating, there is no need for large storage areas for the fuels. As part of the energy transition, property owners receive attractive subsidies for installing a heat pump. This makes the heat pump the best heating system for old buildings, at least in theory. But this is not always true. The heat pump proves to be particularly efficient when used in combination with a comprehensive underfloor heating system. In contrast to a radiator, the heat is distributed evenly over the entire living area. A radiator, on the other hand, radiates heat into the room from one particular point. 

This is why heating systems with radiators require a higher flow temperature than underfloor heating. In order for this gentle heat to keep the rooms continuously warm, the old building needs airtight windows and good thermal insulation. If these prerequisites are met, heat pumps are a future-proof investment despite higher acquisition costs, and at the same time offer an appealing level of living comfort. If the building fabric is defective, however, other heating systems are clearly the better choice. If a renovator only wants to use renewable energies in an old building, wood or pellet heating systems are a good alternative to heat pumps. If there is a lack of space for fuel storage, a gas condensing boiler in combination with solar thermal can be a suitable heating option for old buildings.

Heat load calculation as a basis for choosing the heating system for old buildings

More is not always better, and this also applies to heating systems. Because while a heating system that is too small quickly reaches its limits and therefore does not work energy-efficiently, a heating system that is too large is also problematic. This is because the sensors constantly switch the heating on and off when it is providing too much heating power. This has a visible impact on energy consumption. Every old building is different, so there is no one-size-fits-all solution for the size or type of new heating. Homeowners should seek extensive advice from heating professionals or energy experts before making a purchase. Their job includes assessing the energy performance of the old building. Because the assessment of the structural condition of windows and insulation is just as important as the selection of the appropriate heating system. Calculating the required heat load correctly can save you money. Excessive heating systems not only consume more energy, but also cost more.

What subsidies are available for new heating systems in old buildings?

Different subsidies are available depending on the choice of heating system for the old building. Generally speaking, the more future-proof and energy-efficient the heating system, the higher the subsidy. Heating systems using renewable energies are clearly in the lead. With any new heating system, it is important that renovators apply for funding from BAFA or KfW before starting the measures in their old building. Some federal states offer additional subsidies such as the HeiztauschPlus programme in Berlin, on which regional energy advisors can provide detailed information. The BAFA also subsidises the costs of the consultation with the energy efficiency expert, which the latter conveniently applies for himself.

Oil heating or oil condensing heating

Oil heating is being phased out, as evidenced not only by the sales ban from 2026. There are no longer any subsidies even for replacing a conventional heating boiler with an energy-efficient condensing boiler. If you want to replace your old boiler due to statutory requirements or a defect, you should do so quickly. The planned ban on oil heating systems of all kinds is currently being debated once again and may well come into force before 2026. However, oil hybrid heating systems are not affected by this.

The gas condensing boiler or gas hybrid heating system

The situation is somewhat different for gas condensing boilers. If it is used as the sole heating system, it also no longer qualifies for any subsidies. BAFA or KfW subsidy programmes are available for gas hybrid heating, i.e. in combination with an additional solar thermal system or heat pump. This can involve funding for individual measures or within the framework of an individual renovation roadmap. In the case of gas hybrid heating, a solar thermal system or heat pump supplement the new heating system with a gas burner. Up to 30 percent funding can be applied for. In the context of an individual renovation roadmap, you are eligible for another five percent. If property owners do not want to commit to hybrid heating at this time, they can have a renewable-ready gas heating system installed and subsidised. For the maximum 20 percent subsidy, the new gas condensing boiler must already be hybrid-capable and have the necessary buffer storage tank. Renovators then have two years to retrofit the solar thermal system or heat pump.

RE hybrid heating

Renewable energy hybrid heating goes one step further. It consists of at least two heating systems in one heating installation that exclusively use renewable energies. This can be any combination of solar thermal, heat pump and biomass heating such as pellet heating. Unlike gas hybrid heating, homeowners receive up to 35 per cent subsidy for all purchase costs of a RE hybrid heating system, up to a maximum of 60,000 euros. If the hybrid heating replaces an old oil heating system, a further ten percent will be paid. A biomass system such as a pellet heating system is an innovative heating system for old buildings that can qualify for a 5 percent innovation bonus from BAFA. However, maximum limits for particulate matter emissions of 2.5 milligrams per cubic metre must be observed. This means that if an oil-fired heating system is replaced by an RE hybrid heating system and the relevant requirements are met, 50 percent of the costs can be covered by the BAFA subsidies.

The wood heating or pellet heating system

Biomass heating systems are carbon-neutral and are therefore among the preferred heating systems of the future. This includes heating systems that use wood in any form as fuel. These, in turn, include classic log boilers as well as those for wood chips. Since both heating systems have to be manually loaded, many renovators prefer the automated pellet heating system. The latter must demonstrate a boiler efficiency of at least 90 per cent to be fully eligible. Even for the buffer storage tank, regulations stipulate a minimum size. It must have a capacity of 55 litres per kilowatt if it is a log boiler or a combi boiler. 30 litres per kilowatt are required for wood chips. Furthermore, the new heating system must comply with emission limits for carbon monoxide and particulate matter. At 15 milligrams of particulate matter per cubic metre and if all other requirements are met, BAFA grants a 35 percent subsidy. For low-emission heating systems with less than 2.5 milligrams per cubic metre, the Federal Office of Economic Affairs and Export Control grants a further five percent. If the biomass heating system replaces an oil heating system, homeowners of an old building receive an extra subsidy of ten percent. If the heating system modernisation is part of an individual renovation roadmap, these extra five percent can lead to a total subsidy of 55 percent.

The heat pump

The respective heat pumps draw free environmental heat from the air, water or earth to heat old buildings. Air-to-water heat pumps are reasonably priced, as there is no need to drill extensive boreholes. However, their efficiency is lower than that of water or brine/water heat pumps. Especially in winter, the outside air is cool and the heat pump therefore needs more electricity. Using geothermal or groundwater heat is more efficient, but also more expensive to purchase. If you use a surface heating system such as underfloor heating in a well-insulated old building, these heat pumps work all year round. BAFA grants the same subsidy for heat pump systems as for biomass heating systems, which means up to 55 percent in the best case. 

Solar thermal energy

The sun is an inexhaustible source of free renewable energy. At the same time, solar thermal energy is cheaper to purchase than heat pumps. But their handicap is the low solar radiation in winter or if the roof area faces north. But if your roof is sunny and optimally aligned, solar thermal energy is certainly one of the ideal heating systems of the future for old buildings. Therefore, subsidies of up to 30 percent are available for a solar thermal system. These can increase to up to 45 percent when combined with other programmes such as the renovation roadmap and the replacement of an oil-fired heating system. In old buildings, the solar system is easy to retrofit and can be used as a hybrid heating system with almost all types of heating.

The right heating system for old buildings: an individual decision

While policymakers would prefer to see only renewable energies in use in order to achieve the goals of the energy transition, the energy efficiency of a new heating system depends on the structural condition of the old building. The optimum is certainly the heating system that offers the lowest energy costs and the highest energy efficiency for the respective old buildings. Higher purchase prices for the future-oriented heating systems are often offset by subsidy programmes with correspondingly extensive support. As tempting as the low purchase price of a gas heating system may be: Anyone who wants to make their property fit for the future will have no choice but to install a renewable energy heating system, at least as a supplement. After all, the increase in living comfort associated with a heating system upgrade, the reduction in energy costs and the simultaneous increase in the value of the property quickly compensate homeowners for the costs incurred.