Question 1

What is a thermostatic radiator valve (TRV)?

Accepted Answer

A thermostatic radiator valve (TRV) is a self-regulating valve fitted to the flow connection of a radiator. The valve head contains a wax capsule or liquid-filled bellows that expands when the room air temperature rises above the set point, pushing a pin that restricts or closes the valve and reduces hot water flow through the radiator. As the room cools below the set point, the capsule contracts, reopening the valve. This keeps the room at a roughly constant temperature without requiring any external power or connection - TRVs are entirely self-acting. They are numbered 1-5 (or 0-6 on some brands), typically corresponding to room temperatures from 10 degrees C to 25 degrees C approximately.

Question 2

Where should TRVs not be fitted?

Accepted Answer

TRVs should not be fitted in the same room as the main room thermostat. If a TRV and a room thermostat are in the same room, both devices will try to control the room temperature independently - the room thermostat may call for heat while the TRV has closed the radiator in that room, or vice versa. The standard installation is to fit a TRV on every radiator except the one in the room containing the room thermostat (typically the hallway or living room), where a lockshield valve (a non-thermostatic valve) is fitted on the inlet instead. TRVs should also not be fitted in bathrooms in some circumstances, as steam can confuse the temperature sensor.

Question 3

Are TRVs required by Building Regulations?

Accepted Answer

Yes. Approved Document L (Conservation of fuel and power) requires TRVs on all radiators in new dwellings, and on radiators in existing dwellings when a new boiler is installed (as part of the boiler replacement package). The exception is the radiator in the room containing the room thermostat, which has a lockshield valve instead. For new boiler installations under the Boiler Plus regulations (since 2018), the minimum controls package required is: a time and temperature zone control; a room thermostat or programmable room thermostat; and TRVs on all radiators except the one with the room thermostat. Higher efficiency controls (weather compensation, load compensation, smart controls) are strongly encouraged.

Question 4

What is a smart TRV?

Accepted Answer

A smart TRV is a motorised, programmable TRV controlled via a smartphone app or smart home system. Unlike a conventional self-acting TRV (which responds only to current room temperature), a smart TRV can be programmed with a heating schedule for each individual room and can be controlled remotely. Smart TRVs can also provide room-by-room temperature monitoring and heating schedules, allowing detailed optimisation of heating across all rooms. They are particularly useful in homes with variable room usage. However, smart TRVs do not remove the need for a separate room thermostat and programmer to control the boiler - they control the individual radiator, but the boiler must still be told when to fire. Some smart TRV systems include a central hub that handles boiler control.

Thermostatic Radiator Valve

Just so you know...