Many district heating schemes operate at temperatures and pressures that are compatible with typical building services. In this case, consumers’ heating systems can be directly connected to the district scheme, with the same water that flows in the district network flowing through consumers’ heating circuits.
However, sometimes it is necessary to operate district heating systems at higher temperatures than consumers require and higher pressures than consumers’ systems are designed to withstand. For example, the network must operate at a flow temperature at least that of the highest individual consumer’s distribution flow temperature. Additionally, the variations in building height and ground levels, across a district-heating system, can result in a need to operate the district-heating network at higher pressures than the consumers’ own heating systems.
Where the temperatures and pressures of consumers’ heating systems differ from those of the district heating network, it is usually necessary to utilise a thermal substation. A thermal substation typically consists of a plate heat exchanger which acts as a pressure break between the district network and consumer system. The heat exchanger often forms part of a metering station (with flow control valves, isolation vales and a heat meter) which acts as the demarcation point between the district scheme and consumer systems.
The size of such a metering station varies depending on the size of the energy loads it is designed to meet. A district heating connection will typically offer a substantial space saving when compared to the space that would be required for conventional boiler plant to meet the equivalent heat load.
Domestic connections are typically achieved with Heat Interface Units (HIUs, sometimes referred to as “heat boards”). As with commercial connections, HIUs also contain flow controls, isolation valves and heat metering equipment. Instantaneous domestic hot water supplies are generated in a plate heat exchanger and space heating circuits can be either directly connected to the residence’s systems or also connected via a plate heat exchanger.