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Key takeaways
- A TRV has two parts: a head with the numbers, which senses room temperature, and a valve body with a spring-loaded pin. The pin is what sticks.
- Test first: take the head off. If the radiator starts to heat within a few minutes, the head or pin was the problem.
- A working pin stands proud and springs back. Radiator maker Stelrad says it should travel about 3mm cleanly once freed.
- Ease the pin with pliers using small movements. Never hit it or force it, and never use a hammer on the valve.
- Leave TRVs fully open over summer to stop pins seizing. If several stick at once, suspect sludge.
If a radiator with a thermostatic valve stays cold while the others heat up, the TRV pin is probably stuck. The small metal pin under the TRV head opens and closes the valve, and after months unused, often over summer, it can seize in the closed position. Turning the dial does nothing because the head can no longer move it. You can usually free a TRV pin stuck down by removing the head and gently working the pin up and down, without draining the system.
How a thermostatic radiator valve works
A TRV controls how much hot water enters one radiator based on the temperature of the room. Inside the head is a capsule filled with wax, liquid or gas that expands as the room warms. As it expands, it presses down on a small pin in the valve body below. The pin pushes a valve disc towards its seat, closing off the flow. When the room cools, the capsule shrinks, a spring pushes the pin back up and the valve opens.
The numbers on the head do not show a temperature. As Smart Energy GB explains, each number maps to a temperature range, and the valve regulates water flow to hold the room around that point.
The weak point is the pin. It slides through a small seal, and the disc it moves sits in the heating water. Limescale, corrosion and sludge can glue it in place, especially if it has been held down for months.
Why TRV pins stick after summer
Over summer the heating is off, rooms are warm, and the TRV capsule is expanded. The head holds the pin pushed down, closing the valve, for weeks or months. With no water moving and the disc pressed against its seat, deposits build up around it. When the heating comes back on in autumn, the head relaxes but the spring is not strong enough to pull the pin back up.
The result is a radiator that stays cold on every setting. Many people only notice in a cold January when a bedroom that is rarely used fails to warm up.
How to tell it is a stuck TRV pin
A cold radiator has several possible causes. This quick test singles out the TRV.
- Check other radiators are hot. If several are cold, look at the boiler, pump or air first.
- Turn the TRV to its highest number.
- Remove the TRV head. Most unscrew by hand with a knurled collar; some use a nut or a clip. The valve body and pin stay in place and no water comes out.
- Wait a few minutes with the heating on. If the pipe into the radiator and then the radiator itself start warming, the head was holding the valve shut or the pin has freed itself.
- If it stays cold, look at the pin. A free pin stands up a few millimetres. A stuck pin sits low and does not spring back when pressed lightly.
| What you find | Likely problem |
|---|---|
| Radiator heats with the head off | Faulty or sticking TRV head |
| Pin sits low and does not move | TRV pin stuck down |
| Pin moves freely, radiator still cold | Lockshield closed, air, sludge or blockage |
| Radiator hot at the bottom, cold at the top | Trapped air; bleed it |
| Radiator stays hot even on the lowest setting | Pin stuck up (open) |
If the pin is not the problem, our guide to one radiator not heating up works through the other causes.
How to free a stuck TRV pin
This is a user-level job that does not involve draining the system. The steps follow the method set out by radiator manufacturer Stelrad in its guide to fixing a stuck radiator valve.
- Turn the heating off so the valve is cool enough to handle.
- Remove the TRV head as above, and put a cloth round the valve body.
- Find the pin. It is about 3 to 4mm across, in the centre of the valve body.
- Grip it gently with pliers, ideally with a bit of cloth between the jaws to avoid scoring it.
- Work it up and down a few millimetres with small, repeated movements. Do not pull hard. A light tap on the valve body, not the pin, can help break the grip of scale.
- Check it springs back. When freed, the pin should rise on its own and travel about 3mm cleanly when pressed.
- Refit the head by hand, set it to your normal number and turn the heating on.
- Check the radiator warms. If it is warm at the bottom but cold at the top, bleed the radiator and check your boiler pressure afterwards.
What not to do
- Do not force a seized pin. It can snap, or the valve body can crack, and then you have a leak and a drain-down repair.
- Do not hit the pin with a hammer. Tap the valve body lightly if needed.
- Do not soak the valve in spray lubricant. It can damage the seal round the pin. A small weep from the pin seal after freeing it means the valve needs attention.
- Do not leave the head off for good. Without it the radiator runs flat out with no room control.
When the TRV needs replacing
Freeing the pin is often a lasting fix, but not always. Stelrad notes that a pin that seizes again within weeks usually means the valve is worn. Replace the valve, or have it replaced, if:
- the pin will not move without heavy force
- it moves but sticks again soon after
- water weeps from around the pin
- the head is cracked, or turning it has no effect even with a free pin
Replacing a TRV head alone is simple if the new one matches the valve body thread. Replacing the valve body means draining or freezing the pipe and remaking the joints, which is a job for a heating engineer or plumber. Our guide to a leaking radiator valve covers what to do if you see drips.
Smart TRV heads and stuck pins
Smart radiator thermostats replace the manual head with a motorised one that presses the same pin. They are only as good as the valve body underneath. If the pin is stiff, a smart head may not have the force to open or close it fully, and some report a calibration error or a radiator that never matches its setting. Free the pin first, then fit the smart head.
If you have had stuck pins before, check whether your smart head has a valve protection or anti-seize setting in its app that moves the valve periodically over summer. Our guide on whether smart radiator valves are worth it looks at the wider pros and cons.
How to stop TRV pins sticking
- Leave every TRV fully open over summer. The pin then sits up, away from the seat.
- Exercise the valves by turning each TRV fully up and down a couple of times a year.
- Keep the system water clean. If several pins stick at once, sludge is likely. A magnetic filter and correct inhibitor levels help. Our guide to thermostat and TRV settings in winter also covers sensible daily settings.
- Run the heating briefly before winter so any problem shows up in autumn, not on the first freezing night.
Still cold after freeing the pin?
Most stuck TRV pins can be freed in a few minutes with the head off and a pair of pliers. If the pin will not move, keeps seizing or starts to weep, or the radiator stays cold with a free pin, there is another fault to find, such as a closed lockshield, sludge or a balancing problem.
Gasmaster’s engineers fix cold radiators, replace valves and clean heating systems across Stoke-on-Trent, Staffordshire, Cheshire and Derbyshire. See our central heating services or contact us to book an engineer.
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Frequently asked questions
Can I remove a TRV head without draining the radiator?
Yes. The head sits on the outside of the valve and only presses on the pin. Unscrewing it does not open the water circuit, so nothing leaks out. Turn the head to its maximum setting first, which makes it easier to remove and refit. The radiator will run fully open while the head is off.
What if my TRV pin is stuck open?
A pin stuck in the up position leaves the valve open, so the radiator stays hot even on the lowest setting. Free it the same way by gently working it with pliers. If it still will not close when you press it, the valve body or head needs replacing. Our guide to radiators hot when the heating is off covers related causes.
Should the radiator with the room thermostat have a TRV?
Generally no. If a TRV closes the radiator in the same room as the room thermostat, that room may never reach the thermostat setting, so the boiler keeps running. That radiator usually has a manual valve or a TRV left fully open. Your installer will advise based on your controls.
Why is my TRV radiator cold at the top only?
That pattern usually means trapped air rather than a stuck pin, because water is reaching the radiator but cannot fill it. Bleed the radiator with the heating off, then check the boiler pressure gauge and top up if it has dropped below its normal range. A stuck pin usually leaves the whole radiator cold.
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Abdul runs Gasmaster Plumbing & Heating and has worked on boilers, heating and gas appliances across Stoke-on-Trent since 2005. He is Gas Safe registered (reg. 220932), and these articles come from his day to day work in homes and businesses across Staffordshire, Cheshire and Derbyshire.
