How do thermostatic radiator valves work?
What the numbers on a TRV mean, why one valve is different from the other, and how to fix a stuck one.
- Difficulty
- beginner
- Time
- 20 min
- Read
- 2 min
Short answer
The numbers are relative settings, not temperatures — roughly 1 is about 15°C and 5 is about 25°C on most valves. The valve at the other end of the radiator is the lockshield, which sets the balance and should not be adjusted casually.
Every radiator has two valves doing different jobs. The thermostatic valve controls that room's temperature; the lockshield, usually under a plain cap, controls how much flow the radiator gets relative to others. Confusing the two is behind a lot of unevenly heated houses.
What you'll need
- Adjustable spanner (optional)
Step by step
- Identify which valve is which.The TRV has numbers and turns by hand. The lockshield has a plain push-on cap and needs a spanner or pliers to move.
- Understand the numbers as target room temperatures.Approximately: 1 is 15°C, 2 is 17°C, 3 is 20°C, 4 is 22°C, 5 is 25°C. Frost or star is around 5°C.
- Set each room to what it actually needs.Bedrooms 2-3, living rooms 3-4, hallways and unused rooms 1-2. There is no benefit in heating rooms nobody uses.
- Leave the radiator in the room with the main thermostat fully open.Two thermostats fighting over one room gives poor control of the entire house.
- Do not cover a TRV.It senses the air around it. Behind a curtain or a sofa it reads the wrong temperature and shuts off while the room is still cold.
- Free a stuck pin if a radiator stays cold.Remove the TRV head to expose a small pin. If it does not move freely, tap it gently and work it with pliers, or apply penetrating oil. Seized pins are very common after summer.
- Exercise the valves at the end of summer.Turn every TRV fully open and closed a few times before the heating season. It prevents most seizing.
- Leave lockshield valves alone unless balancing deliberately.They are set to distribute flow correctly across the system. Random adjustment unbalances every other radiator.
Tips
- A radiator cold at the top needs bleeding; cold at the bottom means sludge; cold all over with a warm pipe means a stuck TRV pin.
- Smart TRVs allow per-room scheduling and can be worthwhile in houses with very different room usage patterns.
- Never fit a TRV to the radiator in the room with the main thermostat — the two controls will work against each other.
Common mistakes
- Treating the numbers as heat output levels — They are target temperatures. Setting 5 does not make the room heat faster, it just aims higher.
- Adjusting the lockshield to warm a cold room — It steals flow from other radiators and unbalances the system. Balancing is a deliberate whole-house exercise.
- Covering a TRV with curtains or furniture — It senses trapped warm air and closes while the room is still cold.
Questions people ask
What do the numbers on a radiator valve mean?
They are approximate target room temperatures, not heat levels — roughly 1 is 15°C up to 5 at 25°C. The valve closes once the room reaches that temperature.
Why is one radiator always cold?
Most often a seized TRV pin. Remove the valve head and check the pin moves freely; they commonly stick after a summer of not being used.