What is Boiler Interlock?

You should not fit a TRV to the radiator nearest to the room thermostat
(if you do already have a TRV here, then leave it on its maximum setting).
The room thermostat (sometimes called a roomstat) is usually found in the hall.
The boiler is turned off when the temperature set by the room thermostat is reached.
This is called a boiler interlock. It prevents the boiler from firing when there
is no demand for heat from the central heating system.

If all of the radiators in the house are fitted with TRVs, including the radiator nearest to the
room thermostat, then all the TRVs can close at the same time and turn off all the radiators
while the temperature in the house is still below the temperature set by the room thermostat.
In this situation the room thermostat has not turned off the boiler but all the radiators are turned off.
The boiler will keep running and waste energy. This arrangement is not allowed by part L of the
current building regulations concerning the conservation of fuel and power.

An automatic bypass valve must also be fitted.

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