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Bivalent heat supply

Bivalent heat supply heat pump gas boiler heat supply Heating load peak load electricity grid flexibility

Bivalent heat supply refers to a heating system in which two heat generators are used to cover the heat demand. The two generators usually use different energy sources. The term most often refers to a heat pump combined with a gas, oil or biomass boiler. The two generators can work alternately or at the same time. Which unit is supplying heat at any given moment depends on several factors: outdoor temperature, heating load, electricity and fuel prices, technical design and control strategy.

One common case needs a closer definition: the heat pump with an electric heating element. Because both generators use electricity, professional practice usually calls this monoenergetic operation rather than bivalent operation in the strict sense. For the power grid, the difference is decisive. A heating element does not reduce the electrical load in cold hours but increases it. The counterpart to bivalent supply is monovalent heat supply, in which a single generator covers the entire heating load on its own.

The term describes not only which devices are installed but also how they are operated. A bivalent system can be designed so that the heat pump supplies most of the annual heat and the second generator steps in only on a few very cold days. It can also run in such a way that the boiler regularly takes over a large share. For an assessment, the installed devices alone therefore do not count. What matters are the kilowatt hours of heat produced, the energy sources used, the emissions, and the power drawn from the electricity grid or fuel infrastructure in critical hours.

Energy and power: why bivalence works

The distinction between energy and power is important. A building's annual heat demand is a quantity of energy and is usually stated in kilowatt hours. The heating load, by contrast, is a power, measured in kilowatts. It indicates how much heat is needed at a given low outdoor temperature to maintain the desired room temperature. Very cold hours are rare. A generator that can deliver only part of the peak output can therefore still supply a much larger share of the annual heat. Bivalence exploits exactly this: it separates the question of who supplies most of the heat from the question of who covers the rare power peaks.

An example: an older building has a heating load of 15 kilowatts at the standard outdoor design temperature. At minus 7 degrees Celsius the heat pump delivers about 6 kilowatts. In the transitional seasons and on mild winter days it heats alone. Only in severe frost is its output no longer sufficient, and the boiler covers the difference. How large the heat pump's share of annual heat actually is depends on the climate at the site, the flow temperature and the controls. This has to be calculated or measured case by case.

Operating modes and the bivalence point

In practice, three operating modes are distinguished. In bivalent-alternative operation, the heat pump works down to a defined outdoor temperature, the bivalence point. Below that, the second heat generator takes over completely. This variant is often found in older buildings with high flow temperatures or with undersized heat pumps. In bivalent-parallel operation, the heat pump continues to run below the bivalence point, and the second generator provides additional heat. This is often more energy-efficient, because the heat pump still produces part of the heat efficiently even in cold weather. In bivalent partly parallel operation, there is first a range in which both work together. Below that lies a temperature range in which only the additional generator heats.

Which variant makes sense depends heavily on the building and the heat distribution. Underfloor heating and large heating surfaces allow low flow temperatures and improve heat pump operation. Small radiators, poor insulation and high hot water requirements mean that the system switches to additional operation more often. With air-to-water heat pumps, the coefficient of performance also falls as it gets colder outside. For the same amount of heat, the pump then needs more electricity, and at the same time the building's heating load rises.

The bivalence point is not a natural constant. It is planned, set and influenced by economic incentives. If it is low, the heat pump works alone for a long time even in severe cold. This raises the electrical peak load but saves fuel. If it is higher, the electrical peak load falls, but a larger share of the heat comes from the second energy source. Anyone wanting to understand the effect of a system must therefore look at the actual switching rule and not only at the name of the system.

Distinction from hybrid heating, backup and redundancy

Bivalent heat supply is often equated with hybrid heating. In everyday use this is understandable, but technically imprecise. Hybrid heating is a broad marketing and umbrella term for combinations of different heat generators, such as a heat pump and a gas boiler, solar thermal and a pellet boiler, or a combined heat and power unit and a peak-load boiler. Bivalence describes more precisely that two generators share the same supply task according to a defined operating logic. In German building law, now repealed, this logic was expressly tied to the three operating modes and to priority for the heat pump.

A pure backup is not the same thing either. An emergency heating element that only starts up during faults or in extreme cases makes a system multi-stage, but in the energy-economics sense it does not yet make it a meaningfully bivalent supply. For the power system, what counts is whether the second generator is reliably available when the heating load or grid load is high. Redundancy, by contrast, mainly means security against failure. A redundant system secures supply if one generator fails, without contributing to load control in normal operation.

Bivalence is also not the same as flexibility. A bivalent system can be operated flexibly, but does not have to be. Flexibility arises only when operation can be shifted in time, switched between energy sources or adapted to grid and market signals without unduly impairing comfort or security of supply. A poorly controlled bivalent system can even create rigid loads if it always switches over at the same temperature threshold and takes account of neither grid conditions nor prices.

Significance for the power grid

If heat is generated electrically, electricity demand rises. A single heat pump is usually unproblematic for the grid. But if many heat pumps run at the same time on a cold winter morning, a high load can arise, especially in the distribution grid with limited connection capacity. Bivalent systems can limit this simultaneity if the second generator takes over the electrical load in such hours. However, bivalence does not automatically relieve the grid. If the additional boiler is present only as an emergency reserve and barely runs, it changes the electrical load little. If, on the other hand, it systematically takes over at low temperatures or high electricity prices, electricity demand falls precisely in the critical hours.

In Germany, § 14a EnWG regulates how controllable consumption devices are integrated into the grid. According to the Bundesnetzagentur (Federal Network Agency), since 1 January 2024: new controllable consumption devices with a grid connection capacity of more than 4.2 kilowatts, which includes heat pumps, must be capable of being curtailed. The grid operator may reduce their electricity consumption only temporarily, and only if acute overload or damage to the grid is imminent. With direct control, a minimum output of 4.2 kilowatts per device is in principle maintained. In a bivalent system, the second generator can step in during such a phase. This works only if the controls address both generators together.

Costs and emissions

For households and building owners, the appeal often lies in lower conversion costs. If the existing boiler takes over the rare peaks, a smaller heat pump may suffice. In unrenovated buildings, this makes it easier to start electrification, because radiators, hydraulics and the building envelope do not have to be fully adapted immediately. This is understandable in business terms, but says little about long-term system costs.

For the energy system, bivalent systems can lower the costs of the transition if they avoid grid reinforcements, very high connection capacities or an inefficient design for full load. At the same time, they keep fuel networks, chimney sweep obligations, maintenance structures, supply chains and dependence on gas, oil or biomass in the system. These infrastructure costs do not disappear just because the second generator rarely runs. If many buildings keep their gas connection only for a few peak hours, the question arises who bears the fixed costs of the gas network. It is also unclear how long this infrastructure can be justified economically and in terms of climate policy.

The emissions balance also depends on operation. A heat pump with a high seasonal performance factor can achieve low emissions per kilowatt hour of heat even with an electricity mix that is not fully renewable. A gas boiler, by contrast, emits CO₂ directly at the building. If it runs only on a few very cold days, its annual share remains small. If it switches on regularly because of high electricity prices or unfavourable settings, the fossil share rises significantly. The installed combination therefore says less than the actual annual split of heat generation.

If biomass is the second energy source, the assessment is different. Biomass can be stored and serves peak loads well. However, its sustainable availability is limited. If it is widely used for building peaks, it competes with industry, district heating and other applications in which high temperatures are harder to generate electrically. Bivalence can thus solve local problems and at the same time shift resource issues to another level.

Legal framework in Germany (as of November 2026)

The Building Energy Act (Gebäudeenergiegesetz) in its 2024 version provided in § 71h for the heat pump hybrid heating system as a way to meet the 65 percent requirement for renewable energy. This meant an electric heat pump together with a gas, biomass or liquid fuel boiler. The requirements were: bivalent-parallel, partly parallel or alternative operation with priority for the heat pump, a joint control that can be addressed remotely, and, for gas or oil, a condensing boiler as peak-load generator. For the compliance option, the heat pump had to provide at least 30 percent of the heating load in parallel or partly parallel operation and at least 40 percent in alternative operation. Alternatively, a corresponding share of the output of the peak-load generator at part-load point A under DIN EN 14825 was sufficient.

These requirements are no longer relevant for new systems. The act was renamed the Building Modernisation Act (Gebäudemodernisierungsgesetz, GModG) and promulgated on 28 July 2026. With effect from 29 July 2026, it repealed the former §§ 71, 71b to 71p and 72, including the 65 percent rule and § 71h. Since then, owners replacing heating in existing buildings can choose in a technology-neutral way, including hybrid heating and, again, pure gas or oil boilers. Anyone installing a new gas, oil or LPG heating system in an existing building is subject to a progressively rising quota of climate-neutral fuels, the so-called biofuel ladder (Bio-Treppe). It starts at 10 percent from 2029 and rises to 60 percent by 2040. Under certain conditions, this obligation can also be met by hybrid heating systems with a heat pump, solar thermal or biomass. The federal states (Länder) may retain more far-reaching requirements. A constitutional complaint against the act has been announced.

Typical misconceptions

  • Bivalent is always climate-friendly: This is true only if the heat pump generates by far the largest share of the heat and the additional boiler rarely runs. If the fossil system frequently takes over because of high flow temperatures, poor controls or unfavourable prices, the balance looks different. The climate effect arises in operation, not in the brochure.

  • Bivalence automatically solves grid problems: A grid operator benefits only if the electrical load actually falls or is controllable in the relevant hours. This requires metering, controllability, suitable tariffs or grid-oriented controls. Without this integration, bivalence remains a property of building technology and not a reliable resource for grid operation.

  • Every bivalent solution delays the heat transition: This is also too simplistic. In buildings with a high heating load, limited connection capacity or difficult renovation, a bivalent system can quickly displace a large part of fossil heat. What matters then is whether it is designed for declining fossil running times or entrenches the existing fuel structure permanently.

Bivalent heat supply shows how closely heat planning, building condition, the power grid and market rules are interlinked. The term stands neither for a single technology nor for a fixed climate benefit. It denotes a supply architecture. Its effect depends on what power is needed in cold hours, which energy source supplies it, and by what rules the system is operated.

Sources

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Edited by eCMS on 11.11.2026 15:57

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