This semi-gravity system uses a two-port valve and provides independent temperature control of both the heating and hot-water circuits in a pumped heating and gravity domestic hot-water heating system. The pump and the boiler are switched off when space and hot-water temperature requirements are met. Time control can be with either a time switch or a programmer. Thermostatic radiator valves (TRVs) could also be fitted to provide overriding temperature controls in individual rooms. The pump and circulation to the cylinder can be turned off independently by the thermostats, and there is an interlock to ensure that when the final circuit turns off so do the pump and boiler, thereby avoiding wasting energy.
This type of system is often used on small and micro-bore heating systems, which are covered in detail at Level 3.
Because the system feeds only one circuit at a time, you will find that this system tends not to be widely installed.
Be careful when working on existing systems: the three-port diverter valve and mid-position valve system look similar. However, the three-port valve and timing device are completely different for each system This system is not suitable for dwellings with a floor area greater than 150 m2