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Glossary · Energy industry & energy transition

Building heat interface unit (Hausübergabestation)

Building heat interface unit district heating heat network heat exchanger building heat supply

A building heat interface unit (Hausübergabestation) is the technical interface between a heat network and a building's heat distribution system. It draws heat from the network pipe and passes it on to the building's heating and domestic hot water preparation. In district heating and local heating, this is where the heat supplied by the network becomes usable building heat. Unlike a gas boiler or a heat pump, the unit does not generate heat itself. It takes over the heat from the network, measures it and adapts it to the building's demand.

Terms: transfer station, building central unit, building substation

In everyday usage, "Hausübergabestation" usually refers to all of the equipment in the building connection room. The technical connection conditions (TAB) of many district heating suppliers make a more precise distinction. The transfer station in the narrower sense connects the service connection pipe with the building central unit (Hauszentrale). The building central unit adapts heat, volume flows and temperatures to the building's heating circuits and domestic water heating. Together, the two form the building substation (Hausstation). They can be physically separate or housed in one device as a prefabricated compact unit. Some suppliers even mandate the compact design. Anyone comparing offers, contracts or responsibilities should therefore check which components the term refers to in the specific case. The term heat transfer station is also used in everyday language.

Indirect and direct connection

In an indirect connection, a heat exchanger, i.e. a heat exchanger, separates the network's heating water from the building's heating water. The network side is called the primary side, the building side the secondary side. The two circuits are hydraulically decoupled. As a result, pressure, water quality and faults in the building do not directly affect the network. In a direct connection, the network water itself flows through the radiators. This places higher demands on pressure maintenance, water quality and operational safety. Direct connections are mainly still found in older existing systems. For new systems, the TAB of many suppliers permit only the indirect connection, and some expressly exclude direct connections.

Components and metering

A building substation typically includes shut-off valves, strainers or filters, a differential pressure or volume flow controller, control valves with actuators, temperature sensors, safety devices, the heat exchanger, an electronic controller and a heat meter. The heat meter measures the volume flow and the temperature difference between flow and return and uses them to calculate the delivered amount of heat in kilowatt hours or megawatt hours. The transferable thermal output, by contrast, is stated in kilowatts (see kilowatt, megawatt and gigawatt). An analogy: kilowatts describe how far the tap can be opened. Kilowatt hours describe how much water has flowed through in total. Many TAB also require a return temperature limitation. Its sensor is to be located in or as close as possible to the heat exchanger so that temperature changes are detected quickly.

Distinction from the service connection and the customer installation

In Germany, the AVBFernwärmeV governs the general conditions for the supply of district heating. It does not apply to the connection and supply of industrial undertakings. Under § 10, the service connection (Hausanschluss) links the distribution network with the customer installation. It begins at the branch-off point of the network and ends at the handover point, unless agreed otherwise. Service connections generally belong to the district heating supplier and are established and maintained only by it. Under § 12, the connection holder is responsible for the installation downstream of the service connection. The supplier's metering and control equipment is excepted. Under § 11, the supplier may require the connection holder to provide, free of charge, a suitable room or space for metering, control and shut-off equipment, converters and other technical equipment.

Who owns which parts of the unit therefore depends on the contract and the TAB. Often the transfer section and the heat meter belong to the supplier and the building central unit to the building owner. There are also contracting and rental models in which a supplier or service provider supplies the entire unit. Under § 18, the supplier determines the type, number, size and installation location of the metering and control equipment and bears the costs of the metering equipment. Under § 17, it may set further technical requirements in the TAB insofar as a safe and trouble-free supply requires this. These requirements must not contradict the generally recognised rules of technology.

Return temperature as a key variable

The unit translates the building's heat demand into volume flows and temperatures on the network side. Particularly important is the primary-side return temperature, i.e. the temperature at which the network water flows back after the heat exchanger. The lower it is, the greater the spread relative to the flow temperature. For the same output, less water then has to be circulated. This saves pump power, relieves bottlenecks and reduces heat losses in the return. TAB therefore set maximum values for the return temperature, which differ from network to network. Examples from networks using heating water are 50 °C or 60 °C. For domestic water heating, separate limits partly apply.

A building substation is therefore not a passive box in the basement but a control and metering point. Typical faults are heating curves set too high, oversized or poorly designed control valves, a missing hydraulic balancing in the building, fouled heat exchangers and return admixing in the building's installation. They drive the return temperature up. This increases consumption in the building and impairs network operation, even if the network and the building are in themselves appropriately dimensioned. In § 15, the AVBFernwärmeV requires that customer installations be operated so that other customers are not disturbed and no disruptive effects on the supplier's equipment arise.

Output, contract and dimensioning

In addition to the energy price per kilowatt hour, many heat supply contracts include a base or capacity charge. It is based on the agreed connected load. From this output and the temperature spread, many TAB derive a maximum volume flow, which the supplier limits in the transfer station. If the output is set too high, the customer permanently pays for unused capacity. If it is too low, things can become tight on very cold days or with high hot water demand. A good design therefore takes into account building standard, heating surfaces, hot water profile, simultaneity and control strategy, and not just the living area. Under § 3 AVBFernwärmeV, customers can adjust the contractually agreed heat output once a year with four weeks' notice to the end of a month. A reduction of up to 50 percent requires no proof. A greater reduction, or termination with two months' notice, is possible if the customer demonstrably intends to replace the output with renewable energy.

Significance for low-temperature networks and sector coupling

In future, heat networks are to operate more often at lower temperatures. Waste heat, solar thermal energy, geothermal energy and large-scale heat pumps can then be integrated more easily and more efficiently. This works only if buildings can manage with lower flow temperatures: this requires sufficiently large heating surfaces, a correctly set heating curve and a unit with a suitably designed heat exchanger. An unrenovated building with small radiators often needs high flow temperatures, while a renovated one with surface heating manages with considerably less. The building substation does not decide on the use of large-scale heat pumps, electrode boilers or power-to-heat. But it shapes the temperatures and load profiles that the network has to serve. Low returns, suitable storage and well-regulated units increase the scope for flexible operating modes oriented to the electricity market.

Distinction from apartment interface units

A building heat interface unit generally supplies an entire building or part of a building. Apartment interface units (Wohnungsstationen), by contrast, are located in individual apartments and prepare space heating and usually also domestic hot water there in a decentralised manner. Larger buildings often have both levels: the central unit takes over the heat from the network, and the apartment units distribute and regulate it close to the consumer. This gives rise to specific requirements for hygiene, metering and billing.

Legal status

The rules cited here correspond to the AVBFernwärmeV, which was last amended on 13 July 2022. On 26 August 2026, the Federal Cabinet adopted key points for a heat network package. Accordingly, the AVBFernwärmeV and the Ordinance on Consumption Recording and Billing for District Heating (FFVAV) are to be revised and merged in a new Heat Network Act. Until that act enters into force, the existing ordinances apply. Technical details such as return limit values or the permissible type of connection are in any case set out in the TAB of the respective network operator.

Sources

Related glossary entries

Edited by eCMS on 12.11.2026 05:05

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