Bivalence point · Glossary

The bivalence point is the outdoor temperature at which a second heat generator is switched on in a bivalent heating system. From this temperature on, the heat pump can no longer provide the required heating output on its own. The bivalence point is therefore not a property of the weather. It describes the intersection between a building's power demand and the output that the heat pump can deliver or can deliver economically.

The reference variable is the outdoor air temperature in degrees Celsius. As it falls, the building's heating load rises. At the same time, the usable heating output of many heat pumps decreases, especially air-to-water heat pumps. The reason is the larger temperature lift between the heat source and the heating system. If both curves are plotted against outdoor temperature, the bivalence point lies at the intersection of the heat pump's falling output curve with the rising heating load curve. In practice, the switch-on point can also be set by a controller that does not lie exactly at this technical intersection.

Heating load means the thermal output a building needs at a given outdoor temperature to maintain the desired room temperature. It is stated in kilowatts. This must be distinguished from the annual amount of heat consumed, in kilowatt-hours. A building can have a high peak heating load even though it does not consume particularly much energy over the year. For the bivalence point, what matters above all is the output during cold hours, not the annual consumption.

A simplified calculation example

Suppose a house needs 10 kilowatts of heating output at minus 12 degrees Celsius and no space heating at all from 15 degrees Celsius upward. In simplified terms, the heating load rises linearly in between, i.e. by around 0.37 kilowatts per degree of cooling. At minus 3 degrees Celsius it is then about 6.7 kilowatts. If the heat pump delivers at most 6.7 kilowatts at this temperature and the required flow temperature, the bivalence point is at about minus 3 degrees Celsius. Below that, output is lacking: at minus 12 degrees Celsius in the example, a good 3 kilowatts, in fact somewhat more, because the heat pump loses further output at even lower temperatures. Because very cold hours are rare in large parts of Central Europe, a heat pump with such a bivalence point can still supply the largest part of the annual heat in parallel operation. Real planning does not use a straight line but a heating load calculation and manufacturer performance maps.

Bivalent operation and its variants

A heating system is called bivalent if two heat generators using different energy sources contribute to the heat supply. Often this is an electric heat pump combined with a gas or oil boiler. Such combinations are also called hybrid heating. Biomass boilers, district heating or solar thermal are also possible. If only an electric immersion heater is added as a supplement, technical language usually speaks of monoenergetic operation, because both generators use electricity. There, too, there is a switch-on point, which is often likewise called the bivalence point. A purely monovalent heat supply, by contrast, covers the entire heating load down to the design temperature and has no bivalence point within the design range.

Bivalent-alternative: Below the bivalence point the second heat generator takes over completely and the heat pump is switched off.
Bivalent-parallel: The heat pump keeps running and the second heat generator only supplements the missing output.
Bivalent-partially parallel: At first both work together. Below a second, lower limit point the additional generator takes over alone, for example because the required flow temperature becomes too high for the heat pump.

This distinction has practical consequences, because the same bivalence point has quite different effects depending on the operating mode. A gas boiler that supplements only on a few very cold days is something different from a system that switches over completely to gas at moderate cold. In alternative operation, the heat pump's share of the annual heat is lower than in parallel operation at the same bivalence point. The temperature limit alone is therefore not enough to assess operating costs, electricity demand or emissions. More on the system variants can be found under bivalent heat supply.

Distinction from design temperature, heating limit and coefficient of performance

The bivalence point is often confused with other temperature terms. The standard outdoor temperature, also called design temperature, is a location-dependent low outdoor temperature for which the heating load is calculated. In Germany this is the heating load calculation according to DIN EN 12831-1 with the national values of DIN/TS 12831-1. Depending on the region, it is usually between about minus 10 and minus 16 degrees Celsius, in some cases lower at high altitudes. It is a planning assumption for the maximum heating load, not a switch-on point. In bivalent systems, the bivalence point is generally above it.

The heating limit is the outdoor temperature from which a building no longer needs space heating. For older buildings, around 15 degrees Celsius is often assumed, while well-insulated buildings are in some cases significantly below that. It marks the transition between heating and not heating, not the switch between two heat generators.

The coefficient of performance, the COP, is also something different. It states the ratio of heat delivered to electricity used at a particular operating point. At low outdoor temperatures and high flow temperatures it usually falls. This can shift the economically sensible switchover point, but technically the two quantities are not the same. A heat pump can still have a usable COP below the bivalence point and simply no longer deliver enough output. Likewise, an additional system can be switched on for grid, cost or control reasons. Over the whole year, the seasonal performance factor describes efficiency.

Why the bivalence point matters for the power system

The bivalence point links building technology with the power system. A heat pump shifts part of the heat supply from fuel to electricity. How strong this shift is in cold hours depends not only on the number of heat pumps. It also depends on their sizing and on the bivalence points of many individual systems.

If heat pumps are sized so that they run alone even at very low temperatures, their electrical power consumption rises in cold periods. For well-insulated buildings with a low flow temperature and sufficient connection capacity, this can make sense. For the distribution grid it becomes relevant when many systems draw high power at the same time. The bivalence point thus influences not only annual electricity consumption but also the peak load in certain hours.

A higher-set bivalence point can reduce electrical load peaks if a fuel boiler takes over below this temperature. However, this does not automatically relieve the energy system as a whole. If the additional demand is covered with fossil gas or heating oil, the electrical load does fall, but fuel consumption and CO₂ emissions rise. With an immersion heater, the supply remains fully electric but requires considerably more electricity: one kilowatt-hour of electricity becomes roughly one kilowatt-hour of heat. A heat pump turns it into several kilowatt-hours, depending on conditions. Precisely in the coldest hours, an immersion heater can therefore even increase grid load.

In Germany, new heat pumps in the low-voltage grid with more than 4.2 kilowatts of electrical power that enter operation from 1 January 2024 onward count as a controllable consumption device under § 14a EnWG. If overload threatens, the grid operator may temporarily throttle their electricity draw. However, at least 4.2 kilowatts must be maintained, and higher minimum values apply to large heat pumps. For sizing, this means that such throttling can, in rare cases, briefly limit the available output. Heat storage and the thermal inertia of the building can bridge this.

Typical misunderstandings

A common misunderstanding is that a heat pump no longer works below the bivalence point. In most cases it can technically keep running until it reaches its operating limit. For many air-to-water units this is considerably lower. The bivalence point only says that a second heat generator is added or that the heat pump no longer covers the heating load alone. Whether it is switched off, supplemented or supported by an immersion heater depends on the control and the operating concept.

Equally imprecise is the assumption that the lowest possible bivalence point is always better. A low bivalence point often means that the heat pump has to be sized larger. This can raise the investment and, with unfavorable sizing, lead to frequent cycling in milder periods. Cycling means that the unit switches on and off often instead of running stably for longer periods. This can impair efficiency and service life. Output-controlled units with a wide modulation range mitigate this problem.

Conversely, a high bivalence point is not automatically bad. In existing buildings, a bivalent solution can be a transition if radiators, insulation or electrical connection capacity make a complete changeover difficult. What matters then is how often the second heat generator actually runs, which fuel it uses, and whether the building is later improved step by step. A high bivalence point becomes problematic if the heat pump supplies only a small part of the annual heat and the fossil boiler takes over most of the winter. Then a heat pump is installed, but it achieves little in the system.

The bivalence point and the economic limit are also not the same thing. Some controllers switch based on costs and take into account, for example, the electricity price, the fuel price or time-variable tariffs. The technical bivalence point can then differ from an economic switchover point. In practice, technical output limit, flow temperature, comfort requirements, tariff rules and control logic overlap.

Relationship with building standard and flow temperature

The bivalence point depends strongly on the building. A poorly insulated building loses a lot of heat in cold weather, and its heating load curve rises steeply. A heat pump must then either be very large or receive support earlier. A well-insulated building has a flatter heating load curve. There, the same heat pump can cover a much larger temperature range on its own. If a building is renovated later, the bivalence point of an existing system often falls by itself.

The flow temperature is particularly important. The warmer the water for radiators or underfloor heating has to be, the less output and efficiency the heat pump achieves. Low flow temperatures enlarge the range in which it suffices alone. For this reason, the bivalence point, hydraulic balancing, radiator size, insulation and control are closely interrelated.

A reliable bivalence point is therefore not derived from a rule of thumb. It results from the heating load calculation, the heat pump's performance data at the real flow temperature, the desired operating mode and the properties of the additional system. For bivalent air-to-water systems, technical sources often cite values between about 0 and minus 7 degrees Celsius. This is, however, only a rough orientation. Anyone who plans only on rough assumptions risks systems that are unnecessarily large and expensive or that fall back too heavily on the second generator in cold weather.

Legal framework in Germany (status: November 2026)

Since 29 July 2026, the main rules of the Gebäudemodernisierungsgesetz (GModG, Building Modernisation Act) have applied. It replaces the Gebäudeenergiegesetz (Building Energy Act). The earlier obligation to operate new heating systems with at least 65 percent renewable energy has been dropped. As a result, the previous detailed requirements for heat pump hybrid heating systems also no longer apply in that form. Hybrid heating systems remain expressly permitted.

For newly installed gas and oil heating systems, the GModG provides from 2029 for a stepwise blending of renewable fuels, the so-called Biotreppe (bio-staircase). For the stages from 2029 to 2034, the technical information of the Deutsche Energie-Agentur names as an alternative, among others, a heat pump hybrid heating system in bivalent-parallel operation with priority for the heat pump. Operating mode and control are thus also legally significant, not just the temperature limit. For a specific system, the statutory text with its exact minimum requirements should be checked.

Economic and systemic consequences

For households, the bivalence point is a question of cost. It influences investment, electricity consumption, fuel demand and maintenance. A larger heat pump covers more hours on its own but costs more and may need a stronger grid connection. A smaller unit with an additional boiler appears cheaper but shifts part of the operating costs to fuels. These include their price risks, for example through rising CO₂ pricing.

For grid operators, what counts is not the individual bivalence point but the simultaneity of many systems. If many heat pumps in a grid area need high output at the same time in cold weather, the grid must carry this load. Otherwise, control, grid expansion and suitable tariffs help. This also concerns flexibility: heat storage, the storage mass of the building and intelligent controls can shift short load peaks.

For the climate balance, what matters is which heat generator operates below the bivalence point and how often this happens. A few operating hours of a gas peak-load boiler have a different effect than a system that heats fossil for large parts of the heating season. Seasonal performance factor, electricity mix, fuel emissions and the distribution of operating hours must be considered together.

The bivalence point is thus more than a device setting. It shows how a building is supplied in cold hours, what loads arise in the power grid and what role a second heat generator actually plays. Used precisely, the term describes the temperature limit of an operating strategy. It only becomes meaningful, however, when output, operating mode, building characteristics and additional energy are also stated.

Sources

Neues Gebäudemodernisierungsgesetz — Presse- und Informationsamt der Bundesregierung; 2026-07 (genaues Datum unbekannt). Abschnitte „In Kraft getreten“ und „Anforderung von 65 Prozent erneuerbare Energien entfällt“
GModG 2026 – Neuerungen und Fristen: Heizungen, Energieausweise und Nullemissionsgebäude — Gebäudeforum klimaneutral (Deutsche Energie-Agentur, dena); unbekannt (Stand der Lesefassung: Juli 2026). Abschnitt „Änderungen Artikel 1“ (im Suchergebnis wiedergegeben)
Informationen zum §14a EnWG — Energieversorgung Mittelrhein (evm); unbekannt. Abschnitte zum Anwendungsbereich und zur Steuerung
Wärmepumpen-Heizung — Systemkonzepte und Auslegung nach VDI 4645 — mepbau; unbekannt. Abschnitt „Systemtypen – Bivalente Anlage“

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Edited by eCMS on 11.11.2026 15:59