Why is it wrong to put a TRV in the same room as the room thermostat?

Question from John:
Why is it wrong to put a TRV in the same room as the room thermostat?

Answer from Martyn Bridges, Director of Marketing and Technical Support at Worcester, Bosch Group:
The installation of a TRV is really to provide individual room comfort. All rooms with a TRV on the radiator can be operated at different temperatures, for example, the bedrooms at 18ºC while the lounge is at 21ºC.

The requirement of the Building Regulations is that all heating systems in domestic dwellings have a minimum set of controls. For a combi boiler system, this is a timer or programmer room thermostat and the ability to operate the sleeping accommodation at different temperatures to the living area is generally achieved with TRV’s.

The other requirement is that the room thermostat is sited in such a position where it is unaffected by any secondary heating, generally this is the landing or hall as the lounge is likely to have a fire and the kitchen will of course have heat from the cooking activities. The room thermostat is the boiler interlock, if the room thermostat shuts off, the boiler and heating system will also turn off. Consequently it is important that the room thermostat picks up the average temperature of the house rather than the extreme temperature otherwise the heating system will either turn off when various rooms in the house are still cold or stay running and possibly allow excessive heat to be provided to rooms.

Research found that if the radiator in the hallway or landing had a TRV fitted then quite often the room thermostat was not sensing the average temperature of the house and didn’t turn off with a consequential increase in room temperatures and boiler on time. An uncontrolled radiator provided a more average temperature of the house and more accurate assessment of the actual temperature required.

Neither arrangement is faultless and ideally every room could do with a room thermostat however, this isn’t practical and the suggested method has the better results.

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