Hybrid heating system · Glossary

A hybrid heating system is a heating system that combines at least two different heat generators in a building or installation. Today this usually means an electric heat pump combined with a gas, oil or liquefied petroleum gas boiler. Combinations with biomass or solar thermal systems are also possible. Technically, it is generally a bivalent heat supply: both generators can serve the same heat demand. Depending on outdoor temperature, load, efficiency, energy price or control strategy, they take on different shares.

A hybrid heating system is first sized according to heating output in kilowatts, not according to the amount of heat in kilowatt-hours. The heating load indicates how much output a building needs to stay warm at a defined low outdoor temperature. Annual heat demand, by contrast, is stated in kilowatt-hours. This difference is central: in a few very cold hours a building can demand a lot of output, even though most of the annual heat is needed in milder weather. This is where the basic idea comes in. The heat pump supplies the bulk of the annual heat, while the boiler covers rare peaks or unfavourable operating conditions. One image for this: the heat pump is the workhorse for everyday use, the boiler the reinforcement for the coldest days.

Bivalence point and modes of operation

The bivalence point is the outdoor temperature or operating situation from which the second heat generator is switched on. Three modes of operation are distinguished:

Bivalent-alternative: Below the bivalence point, only the boiler operates; the heat pump switches off.
Bivalent-parallel: The heat pump keeps running, and the boiler supplements the missing output.
Bivalent-partially parallel: At first both generators run in parallel. Below a second, lower limit temperature the boiler takes over alone, for example because the heat pump can no longer efficiently reach the required flow temperature.

For assessment, these differences matter more than the mere statement of which devices are installed. If the heat pump delivers almost all of the annual heat and the boiler handles only a few peak hours, the result is entirely different emissions, electricity loads and fuel costs than in a system in which the boiler dominates large parts of the heating season. As long as operating shares, controls and design are unknown, the term hybrid heating system therefore says little.

Domestic hot water preparation also influences the mode of operation. Drinking water heating often requires higher temperatures than a well-designed surface heating system. If the heat pump has to deliver a high flow temperature, its efficiency drops. Some systems therefore use the boiler for hot water or for thermal disinfection against legionella. This can make technical sense, but it increases the fossil share if the boiler runs on natural gas or heating oil.

Distinction from heat pump, supplementary heating and combined heat and power

A hybrid heating system is not a separate heat generation technology but a combination with a control task. The heat pump remains a heat pump, the boiler remains a boiler. The benefit for the system depends on which generator is used when and why.

A hybrid heating system differs from a purely electric, monovalent heat pump system through the additional heat generator. This is usually a combustion appliance, but it can also be a solar thermal or biomass system. It differs from a simple supplementary heater in that both generators are integrated into the heating system in a planned and controlled manner. An electric immersion heater that serves only as an emergency reserve does not automatically make a system a hybrid heating system in the narrower sense, even though it is technically a second heat generator.

A hybrid heating system is not to be equated with combined heat and power. A combined heat and power plant produces heat and electricity at the same time. A heat pump with a gas boiler, by contrast, produces only heat and consumes electricity and fuel to do so. For the electricity system this is a considerable difference: the heat pump is an additional electrical load, whereas a CHP plant can, depending on its mode of operation, feed electricity into the grid.

Legal classification in Germany (as of November 2026)

On 29 July 2026, the main new provisions of the Building Modernisation Act (Gebäudemodernisierungsgesetz, GModG) entered into force. It is the renamed and amended Building Energy Act (Gebäudeenergiegesetz). The requirement that applied until then, that newly installed heating systems must in principle be operated with at least 65 percent renewable energy, has been dropped. Since then, owners can choose for themselves whether to install a heat pump, a district heating connection, a hybrid heating system, a biomass heating system or, as before, a gas or oil heating system. Under the old law, the heat pump hybrid heating system was regulated in § 71h GEG as one way of meeting the 65 percent requirement. This approach no longer applies to new installations.

In the draft of the new § 42, the heat pump hybrid heating system is expressly named as a separate option for replacing a heating system, alongside solar thermal hybrid heating systems and pure gas, oil or liquefied petroleum gas heating systems. Anyone who, after 29 July 2026, newly installs a heating system using gas, heating oil or liquefied petroleum gas in an existing building is subject under § 43 para. 1 GModG to the so-called bio staircase (Bio-Treppe). From 1 January 2029, at least 10 percent of the heat provided by this system must come from biomethane, bio-oil, biogenic liquefied petroleum gas or certain types of hydrogen and their derivatives. From 2030 the share is 15 percent, from 2035 30 percent and from 2040 60 percent.

Under § 43 para. 5 GModG, this obligation is deemed fulfilled if the boiler is part of a heat pump hybrid heating system with a sufficiently large heat pump. The benchmark is the heat pump's heating output at part-load point A according to DIN EN 14825. This point corresponds to an outdoor temperature of −7 °C in the average reference climate of the ecodesign specifications. In bivalent-parallel or partially parallel operation, the heat pump must achieve at least 30 percent of the output of the peak-load generator there, and in bivalent-alternative operation at least 40 percent. A worked example: with a 20 kW gas boiler, the heat pump would have to deliver around 6 kW at −7 °C, and in alternative operation around 8 kW. The reference value here is thus the boiler output and not the heating load. An oversized boiler therefore also raises the minimum size of the heat pump. Verification obligations also apply: in buildings with at least six dwellings and in non-residential buildings, after 31 December 2039 it must be demonstrated what share above 30 percent is to be credited to the heat pump. For other modes of operation, the verification obligation applies, according to the wording of the law, as early as 2029.

These rules apply only to Germany and concern new installations from the end of July 2026. Existing systems may continue to be operated. Funding conditions, rules for new buildings and state law may differ and should be checked in each individual case. The legal situation has changed several times since 2023, so older guides are often outdated.

Relevance for the electricity system and the heat transition

Hybrid heating systems link electrification and peak load. The more heat is generated with heat pumps, the more electricity the buildings sector consumes. What matters here is not only the annual amount of energy but the output drawn from the grid simultaneously on cold days. Particularly at low outdoor temperatures, many air-to-water heat pumps run at high output and declining efficiency. If the second generator supplies the heat in these hours, a hybrid heating system can limit the electrical peak load.

For distribution grids, this can be helpful. In streets with many unrenovated buildings and tight grid capacity, bivalent systems can temporarily delay or reduce grid reinforcements. However, this benefit arises only if the systems are actually controlled in a grid-serving way or if their design limits the electrical peak output. Two devices in the basement alone do not guarantee relief for the electricity grid.

At the same time, a hybrid heating system with a gas or oil boiler maintains a fuel infrastructure in the building. Maintenance, flue system, gas connection or oil tank, meters, base charges and safety requirements remain. This can partly eat up the cost advantage of a smaller heat pump. In addition, different responsibilities meet here: building owners decide on investments, electricity and gas network operators plan networks and calculate network costs, and the legislator sets requirements such as the bio staircase.

Typical misunderstandings

A common misunderstanding is to regard hybrid heating systems across the board as climate-friendly. The climate effect does not arise from the combination as such. It depends on the share of heat from the heat pump, its efficiency, the electricity mix and the fuel used. A gas boiler that regularly delivers large amounts of heat continues to cause CO₂ emissions. If the heat pump runs only as a symbolic add-on, little changes in fossil heating operation.

Equally imprecise is the opposite claim that hybrid heating systems are merely an extended fossil heating system. In buildings with high flow temperatures, an unclear renovation roadmap or limited electrical connection capacity, a well-designed hybrid solution can electrify a large part of the heat without having to solve all bottlenecks at once. Its practical value then depends on whether the fossil share can fall with later renovation, lower flow temperatures or changed energy prices.

Costs, too, are often oversimplified. Hybrid heating systems are sometimes seen as a cheap compromise because the heat pump can be smaller. Against this stand double investment and double maintenance. A control system that reacts only to short-term prices can also miss climate targets: if gas appears cheaper in certain hours, the boiler runs even though the heat pump would be the more sensible choice. The order of deployment depends, among other things, on network charges, CO₂ costs, dynamic electricity tariffs and, in future, also on the costs of biogenic fuel shares.

The heat pump's seasonal performance factor is also frequently misread. If it runs only in mild weather and the boiler takes over in the cold, its measured efficiency can appear high. But this does not mean that the overall system is particularly climate-friendly, because the cold operating hours show up in the boiler's fuel consumption. For a reliable assessment, electricity consumption, fuel consumption, the heat quantities produced by both generators and operating hours must be considered together.

Planning, control and long-term classification

Whether a hybrid heating system works well is decided in planning. Important parameters are the heating load according to a recognised calculation, flow temperatures, heat distribution and radiators, storage size, hot water demand, insulation condition and electrical connection capacity. A buffer tank can avoid frequent switching of the heat pump on and off (cycling) and shift operating times. However, oversized tanks or unfavourable hydraulics increase losses. The control system must clearly define what it prioritises by: temperature, cost, emissions, grid signal or comfort. If the system is installed under § 43 para. 5 GModG, the heat pump also needs the prescribed minimum output at part-load point A.

For the electricity system, a hybrid heating system can provide flexibility if it can shift electrical load in time or temporarily reduce it. This flexibility is limited by the thermal inertia of the building, storage size, comfort limits and the availability of the second generator. It is not an electricity storage device: the system does not store electrical energy but shifts heat generation or temporarily replaces electricity with fuel.

In the long term, it needs to be examined how adaptable a hybrid heating system is. It can serve as a transition if the fossil generator runs less often over time, is replaced or is dropped entirely after a renovation. Experts therefore often recommend sizing the heat pump so that it can heat the building on its own after a renovation. However, a hybrid heating system can also lock in investments in gas infrastructure even though heat demand is falling or is to be fully electrified. The term therefore describes not a target state of the heat transition but a specific technical configuration with an open development path.

A hybrid heating system is most precisely understood as a controlled interplay of several heat generators under particular building, grid, market and legal conditions. Its assessment depends not on the label but on design, operating shares, fuel and efficiency. And on the question of whether it prepares future electrification or secures fossil dependencies in the long term.

Sources

§ 43 GModG – Einbau einer Heizungsanlage, die mit Gas, Heizöl oder Flüssiggas beschickt wird — buzer.de (konsolidierter Gesetzestext; Fassung durch Gesetz vom 23.07.2026, BGBl. 2026 I Nr. 226); 2026-07-23. § 43 Abs. 1, 5 und 7 GModG
Neues Gebäudemodernisierungsgesetz — Presse- und Informationsamt der Bundesregierung; 2026-07 (genaues Datum unbekannt). Abschnitte „In Kraft getreten“ und „Anforderung von 65 Prozent erneuerbare Energien entfällt“
Deutscher Bundestag Drucksache 21/6278 (Gesetzentwurf zum Gebäudemodernisierungsgesetz) — Deutscher Bundestag; 2026-06-08. Entwurf § 42 Abs. 2 und § 43
Stellungnahme des Bundesverbands Wärmepumpe zum 65-Prozent-Konzeptpapier — Bundesverband Wärmepumpe e. V. (veröffentlicht durch das BMWK); 2022-08-22. Abschnitt zur Bemessung der Wärmepumpe an der Heizlast

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Edited by eCMS on 12.11.2026 05:10