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Glossary · Energy industry & energy transition

Booster heat pump

Booster heat pump heat pump district heating low-temperature network building heat transition flexibility

Definition and operating principle

A booster heat pump is a heat pump that raises heat from an existing medium or low temperature level to a higher usable temperature level. It does not obtain its heat from outdoor air, the ground or groundwater. Instead, it uses an upstream heat source, for example a low-temperature district heating network, the return line of an underfloor heating system, a central buffer storage tank or waste heat. It deliberately "boosts" this heat for a specific purpose. In practice this is mostly domestic hot water heating, and less often space heating.

Technically, the key issue is the temperature lift. Depending on the network concept, a heating network supplies water at around 20, 30 or 45 degrees Celsius. At about 40 to 45 degrees Celsius, a well-designed underfloor heating system in a renovated building can often be supplied directly. At about 20 degrees Celsius, however, this is not sufficient even for space heating. Such so-called cold networks therefore require a heat pump in every building that provides both space heating and hot water. For domestic hot water, around 60 degrees Celsius is usually required for hygienic reasons. The booster heat pump closes this gap locally: in the boiler room, in the transfer station or apartment by apartment. The greater the difference between source and usable temperature in kelvin, the more electricity the heat pump needs per kilowatt hour of heat.

The booster heat pump is often part of a system. A typical setup: a central heat source charges a low-temperature storage tank, and the booster heat pump draws heat from this tank and uses it to charge a smaller high-temperature domestic hot water tank. In apartment-level units, the heat pump and the domestic hot water tank are often combined in a compact unit. Manufacturers state source temperatures of about 10 to 45 degrees Celsius and hot water temperatures of up to around 60 to 65 degrees Celsius for such units. These are product specifications, not generally applicable limits.

Distinction from neighbouring technologies

  • Heat exchanger: A heat exchanger transfers heat between two circuits but cannot actively raise the temperature level. The secondary side is always somewhat cooler than the primary side.

  • Immersion heater and instantaneous water heater: They convert electricity directly into heat; one kilowatt hour of electricity yields at most about one kilowatt hour of heat. A booster heat pump additionally uses the available heat source and therefore delivers a multiple of useful heat per kilowatt hour of electricity. An immersion heater, by contrast, increases the electricity consumption of the overall system.

  • Large-scale heat pump: A large-scale heat pump raises large quantities of heat centrally to network temperature. The booster heat pump, by contrast, performs the decentralised or building-level temperature raising.

  • Conventional building heat pump and high-temperature heat pump: A conventional building heat pump uses outdoor air, the ground or groundwater as its source. Booster units, by contrast, use an already heated, water-based source. Some technical sources also call booster heat pumps high-temperature heat pumps, because they operate in a higher temperature range and often with different refrigerants.

Role in low-temperature and cold networks

Booster heat pumps are becoming more important with so-called fourth- and fifth-generation networks. According to the technical literature, fourth-generation networks operate with flow temperatures of about 50 to 60 degrees Celsius. Fifth-generation networks, often referred to as cold local heating, are considerably lower still and need additional temperature raising for hot water, for example by boosters. Conventional district heating networks, by contrast, are often designed for considerably higher temperatures. Higher network temperatures mean greater heat losses and make it harder to use low-temperature heat sources.

Booster heat pumps make it possible to operate the network at a lower temperature. According to technical studies, this reduces distribution losses, because reheating takes place locally only where high temperatures are actually needed. At the same time, more heat sources become usable, for example industrial waste heat, data centres, wastewater, borehole heat exchanger fields, solar thermal energy or ambient heat. The underlying idea is not to design the infrastructure for the highest temperature demand, which is limited in space and time. Space heating in well-renovated buildings manages with low temperatures. Domestic hot water requires high temperatures, but only at certain times and in limited quantities.

Hygiene requirements for domestic hot water (Germany)

The required hot water temperature largely determines the size of the temperature lift. In Germany, the Drinking Water Ordinance (Trinkwasserverordnung) refers to the generally recognised rules of technology, and for protection against legionella mainly to DVGW worksheet W 551. For large-scale systems, W 551 requires at least 60 degrees Celsius at the outlet of the domestic hot water heater. Nowhere in the installation may the hot water fall by more than 5 kelvin below this outlet temperature, so in the circulation system not below 55 degrees Celsius. Large-scale systems are systems with more than 400 litres of storage volume or with more than 3 litres of pipe content between the domestic hot water heater and the draw-off point. For private single-family and two-family houses there are no fixed temperature requirements. The German Environment Agency (Umweltbundesamt) recommends consistently more than 55 degrees Celsius.

This matters for the system concept. A central booster heat pump with circulation in a multi-family building generally has to maintain the 60/55 degree regime permanently. Apartment-level solutions with a small storage tank and short pipes, by contrast, can qualify as a small-scale system if both limits are met. Lower temperatures are then possible in principle. The German Environment Agency does not consider a "legionella cycle", in which the storage tank is only briefly heated to above 60 degrees Celsius from time to time, to be a reliable variant of legionella protection. The exact classification depends on the individual case and must be technically assessed.

Efficiency: coefficient of performance and seasonal performance factor

Efficiency is described by the coefficient of performance (COP) or the seasonal performance factor. The COP indicates the ratio of heat output to electrical power under fixed operating conditions. The seasonal performance factor captures real operation over a year, including part load, storage and circulation losses, control and fluctuating temperatures. In practice it is more meaningful.

Measurements show how much the source temperature matters. In a laboratory test at the Technical University of Denmark, a booster heat pump for domestic hot water achieved a COP of up to 4.95 at a network flow temperature of 45 degrees Celsius. A further study found that, at a fixed hot water temperature of about 60 degrees Celsius, the temperature at which the network water leaves the heat pump has a significant influence on the COP. Energy-efficient operation therefore cools the network water only as far as is permissible for the network return temperature. A calculation example from the German Environment Agency for an ordinary building heat pump in a single-family house shows how large the hygiene effect can be. If the hot water temperature rises from 48 to 60 degrees Celsius and a circulation system also runs, the seasonal performance factor of domestic hot water heating falls there from 3.22 to 2.19. Such laboratory values and calculation examples cannot be transferred directly to every system.

Overall balance instead of isolated assessment

A common misunderstanding: low network temperatures do not automatically mean high overall efficiency. A low-temperature network does reduce heat losses and opens up more heat sources. However, if poorly designed boosters, oversized storage tanks or frequently activated immersion heaters operate in many buildings, electricity consumption, peak load and operating costs may rise. Technical studies also cite the investment costs of many small units as an economic obstacle. One option that has been studied is booster heat pumps at neighbourhood level instead of in every building. The assessment must therefore cover the whole chain: heat source, network temperature, distribution losses, booster operation, storage, control and the actual load profile.

Significance for the electricity system

Booster heat pumps turn heat demand into electricity demand. They usually need considerably less electricity than direct electric heating, but the demand does not arise evenly. Hot water is drawn mainly in the morning and evening. If many units run at the same time, additional electrical peak load arises. This affects service connections, distribution networks and tariffs. With domestic hot water or buffer storage tanks, intelligent control and suitable price signals, part of the demand can be shifted in time; this is load shifting. Without such incentives, this flexibility remains technically possible but is not necessarily used in operation.

Organisational integration

A booster heat pump is also a question of responsibilities. The parties involved include heating network operators, building owners, possibly contracting providers, metering point operation and, for rented apartments, the tenancy. Who invests in the unit, and who bears maintenance and the risk of failure? Does the electricity come via the building connection or the apartment, or is it part of a heat supply contract? How are network heat, electricity and storage losses billed? The answers determine whether a technically sensible concept remains economically viable and transparent in everyday use. In planning, the heat transfer station often has to be included as well, because it forms the interface between the network and the building.

Limits of use and classification

In the heat transition, the booster heat pump can form a bridge between central and decentralised supply. Heating networks then do not have to meet all temperature requirements centrally. At the same time, it can avoid equipping every building with its own air-source heat pump. This is relevant in densely built-up neighbourhoods, where noise, space requirements and outdoor installation set limits. It is not a blanket solution. With very high flow temperatures, poor hydraulics or extensive circulation systems, the temperature lift becomes so large that the advantages shrink. In other cases, central temperature raising is more efficient or more economical, because operation, maintenance and control can be bundled. Which solution fits depends on heat density, building standard, hot water demand, hygiene concept, available heat source, network length, electrical connection and billing model.

In short: the booster heat pump provides high temperatures specifically where they are needed, instead of maintaining them throughout the whole heating network. Its benefit depends on how carefully the temperature lift, hygiene requirements, electrical peak load and organisational integration are planned.

Sources

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Edited by eCMS on 11.11.2026 16:17

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