Last edited 27 Oct 2020

Heat interface unit

The BSRIA guide to heat interface units BG 62/2105 defines heat interface units (HIU sometimes referred to as ‘heat boxes’) as ‘…the means of connection of the consumer’s heating and hot water systems to a heat network and may include isolation and control valves, heat exchangers, pumps and metering devices.’

Traditionally in the UK, individual properties have been provided with their own heating systems, typically a boiler, providing domestic hot water (DHW) and central heating (CH). District heating provides heating to a group of buildings from central thermal energy generation plant via a network of fluid distribution pipes. It is widely used for residential, commercial, local authority, government, and industrial buildings as well as for universities and hospitals where there are a variety of discrete buildings located around a campus.

As greater attention has been paid to energy consumption and carbon emissions, so there is increasing interest in the use of district heat networks as a means of supplying heat more efficiently, particularly in multi-storey developments, where alternative, individual low-carbon technologies may be more difficult to install.

Modern district heating systems generally use pressurised hot water for transferring heat from the central plant. Combined heat and power systems (CHP and sometimes called cogeneration), or other low-carbon technologies can be used for the central heat supply plant. CHP involves the local generation of electricity with the waste heat from the generation process being recovered and used in building heating systems or industrial processes. CHP is very energy efficient compared with traditional local generation of heat and import of electricity from large centrally-located electricity generation stations.

Heat interface units allow connection to a centralised heat network whilst still giving individual occupants control over the temperature in their property and allows individual metering and billing. They can provide heating only, or can provide heating and domestic hot water. BSRIA Guide BG 62/2105 suggests that installing a HIU linked to a heat network rather than a conventional boiler can have significant economic and environmental benefits, but that successful operation depends on appropriate system design and specification, and competent installation and maintenance.

There are a number of potential advantages to heat interface units:

Central heat generation can be more efficient than local generation.
It can be easier to integrate low carbon technologies.
As local boilers are not required, there is no gas distribution network or flue requirement. This can result in both capital an operational cost savings.
HIU’s can be installed outside of individual properties, giving easy access for landlords.
They can allow remote monitoring and metering of consumption.
They may allow a higher flow rate than domestic combination boilers.

Multi-zone systems may be provided for larger properties, and some systems may include a hot water cylinder. Typically, domestic how water supply is provided at around 75°C, and so if lower temperature water is required for a heating system such as underfloor heating , a mixing valve may be required. There may be hydraulic separation between the distribution network and the property, where distribution pressures may be high (such as in tall buildings) or to reduce the potential impact of leaks.