What a TRV does
A TRV senses the room temperature through a wax or liquid capsule inside the head. When the room reaches the set temperature, the capsule expands and closes a pin, cutting flow to the radiator. When the room cools, the capsule contracts, flow returns, the room warms.
It's a local thermostat for each radiator. Your boiler thermostat controls the whole house; the TRV controls each room individually.
How to set them
Numbered 1–5 (or *, 1, 2, 3, 4, 5). Rough guide for most domestic systems:
- *, frost protection only, around 7°C. Unused rooms.
- 1, 10°C. Sparingly used rooms.
- 2, 15°C. Bedrooms, landings, halls.
- 3, 20°C. Most living spaces.
- 4, 22°C. Bathrooms, young children's rooms.
- 5, 25°C+. Rarely needed.
Why TRVs matter
- 01 Zone control, each room set independently.
- 02 Reduced gas bills, typically 5–15% saving on a balanced system.
- 03 No overheating spare bedrooms or unused rooms.
- 04 Protects against freeze-up in rooms that would otherwise be turned off.
- 05 Required for Building Regs Part L compliance on new installs.
The room thermostat rule
One radiator in the house, usually in the hall or main living room, should NOT have a TRV (or should be fully open). This is the room that holds the main room thermostat. If every radiator has a TRV, they can all close simultaneously, leaving nothing for the boiler to heat into. The main room thermostat needs a guaranteed-open radiator to work against.
When TRVs stick
TRV pins stick closed after summer (the most common call installers get in October). Symptom: radiator stays cold when heating is on, even with the TRV fully open.
The fix: unscrew the plastic head from the valve body (spins off, no tools needed). The exposed pin should spring up and down when pressed. If it's stuck down, tap the end gently with a spanner. If it springs back up, the TRV works fine, reseat the head. If it stays stuck, the TRV needs replacing (30-minute job, £45–£65 per valve).
Upgrading a whole system
Fitting TRVs to a full system that doesn't have them is a common upgrade. Typically £420 for 8–10 radiators including valves, labour, system drain-and-refill and fresh inhibitor. Payback period is usually 1–2 winters through gas savings.
TRV upgrade?
Radiator installation and TRV fitting.
New TRVs from £45 per valve. Full-system TRV upgrade from £420. Rebalancing included.
See radiator installation