Glossary · Central heating, radiators and hot water
What is a thermostatic radiator valve (TRV)?
A thermostatic radiator valve, or TRV, is a self-regulating valve that fits directly onto a radiator. It controls the temperature of that radiator by changing how much hot water flows into it. It usually has a numbered dial, so each radiator can be set separately from the boiler's own controls.
Key points
- A TRV is a self-regulating valve on the radiator that changes the flow of hot water into it, according to Vaillant.
- Glow-worm says it usually sits at the bottom of one side of the radiator and often has a numbered dial.
- The valve on the other end is a lockshield valve, which balances the system and rarely needs adjusting.
- Baxi says the pin inside a TRV can stick, especially on a radiator that has not been used for a while.
- A TRV controls its own radiator. The boiler is controlled separately.
How a thermostatic radiator valve works
A TRV is a self-regulating valve that fits directly onto the radiator. It controls the temperature of that radiator by changing the flow of hot water into it.[1]Vaillant, Thermostatic radiator valve (TRV) Vaillant describes a thermostatic valve as one that monitors the temperature and adjusts automatically to hold a regular room temperature.[3]Vaillant, Why is my radiator not working?
Glow-worm says a TRV is usually on one side of the radiator at the bottom and often has a numbered dial that sets the temperature.[2]Glow-worm, Why is my radiator cold at the bottom? It only controls its own radiator. The boiler and the room thermostat are separate controls.
The valves on a radiator
A radiator normally has three valves. Glow-worm describes them like this.[2]Glow-worm, Why is my radiator cold at the bottom?
| Valve | Where | What it does |
|---|---|---|
| Thermostatic radiator valve | Bottom, one side | Controls hot water flow into the radiator |
| Lockshield valve | Bottom, opposite side | Balances the system by controlling how much water leaves the radiator. It rarely needs adjusting because the installer sets it. |
| Bleed valve | Top, one side | Releases trapped air |
Not every radiator has a TRV. Vaillant says not all systems have them, and a radiator with a manual valve shows its setting so you can adjust it by hand.[4]Vaillant, How do I balance radiators[3]Vaillant, Why is my radiator not working?
What to check if the radiator stays cold
Start with the dial. Vaillant says to check the valve is on the right setting, and that a thermostatic valve may be set to an incorrect temperature.[3]Vaillant, Why is my radiator not working?
If it is still cold, Baxi says the pin in the valve can stick, especially on a radiator that has not been in use for a while. If the valve has a twist knob with a moulded cover, you can remove the cover to see whether the pin moves up and down. Baxi says to call a heating engineer if you are not confident.[5]Baxi, Why are my radiators cold?
Vaillant says a valve can also become blocked by sludge or limescale. In that case it recommends a professional heating engineer.[3]Vaillant, Why is my radiator not working? See radiator not heating up.
A radiator valve that leaks is a different problem. See leaking radiator valve.
Example
Dev in Leeds finds the radiator in his spare room has been cold since the spring. The dial on the TRV is on a middle number. He removes the cover and sees the pin is not moving. He is not sure how to free it safely, so he asks his heating engineer to look at it at the next visit.
What people get wrong
Many people assume a TRV is a heating control that works on the boiler. It is not. It only controls the radiator it sits on.
The other mistake is to adjust the valve at the other end. That is the lockshield valve, which Glow-worm says is set by the installer to balance the system and rarely needs adjusting.[2]Glow-worm, Why is my radiator cold at the bottom?
The bottom line
A TRV is the valve on a radiator that controls how much hot water enters it. It lets you set each room separately. If a radiator stays cold, check the dial and whether the pin moves, then call an engineer if the valve is stuck or blocked.
Common questions
What does a TRV do?
Vaillant says a TRV is a self-regulating valve fitted directly on the radiator that controls its temperature by changing the flow of hot water to it. It is how you set one room warmer or cooler than another.
Where is the TRV on a radiator?
Glow-worm says it is usually on one side of the radiator at the bottom, with a numbered dial. The lockshield valve is at the bottom on the opposite side, and the bleed valve is at the top.
Why is my radiator cold when the TRV is on?
Baxi says that if one radiator will not heat, the cause is likely air in it or a stuck valve. Vaillant says to check the valve setting, and that a valve can be blocked by sludge or limescale.
Do all radiators have a TRV?
No. Vaillant says not all systems have them. Some radiators have a manual valve instead, whose setting you can see and adjust by hand.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Vaillant. Thermostatic radiator valve (TRV). Checked 11 November 2026. ↩
- Glow-worm. Why is my radiator cold at the bottom?. Checked 11 November 2026. ↩
- Vaillant. Why is my radiator not working?. Checked 11 November 2026. ↩
- Vaillant. How do I balance radiators. Checked 11 November 2026. ↩
- Baxi. Why are my radiators cold?. Checked 11 November 2026. ↩