What to look for when choosing a heat interface unit

Some reasons to replace an HIU are that the unit has been neglected without service and become uneconomical to repair, or that it has reached the end of its expected lifetime.

Types of HIU

The most common types of HIU are:

  • Indirect heating and instantaneous hot water;
  • Direct heating and instantaneous hot water.

An HIU for indirect heating and instantaneous hot water contains two heat exchangers. The first produces hot water. Mains cold water enters the heat exchanger and encounters heat from a communal or district heating water system at high temperature, thus emerging as hot water. The two sources of water do not mix; they are separated by a very thin stainless steel plate allowing heat to pass across. The second heat exchanger is to heat water which it pumps in a circuit around radiators or underfloor heating. The heating process is the same as for hot water – the water receives heat in an exchanger from a district or communal heating water system. Parts inside this type of HIU include a pump, expansion vessel, pressure gauge for the radiator system, a heat plate exchanger for heating system and a pressure relief valve for the secondary heating system.

An HIU for direct heating and instantaneous hot water has only one heat exchanger. Hot water is produced in the same way as in the indirect unit described above. For the heating circuit, hot water from the district or communal heating system flows through the HIU and is then circulated directly into radiators before looping back to the HIU which returns it to the district or communal system. Direct heating HIUs don’t usually need a circulating pump, expansion vessel or pressure relief valve.

There are many different HIU options to choose. Here are two.

Electronically controlled HIUs with mostly electric components to energise and control water flows.

Mechanically controlled HIUs have more mechanical parts for controlling water flows. They incorporate only one or two electronic components.

13/04/2022

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