Thermostatic Radiator Valve (TRV) Not Working: How to Fix It

Updated July 2026 · Emergency Plumber Chelsea

A thermostatic radiator valve, or TRV, is the valve with a numbered head, usually marked from a frost symbol or 0 up to 5 or 6. Inside the head is a wax or liquid capsule that expands as the room warms up. As it expands it pushes down on a small pin in the valve body, which gradually closes the valve and cuts the flow of hot water into the radiator. As the room cools the capsule contracts, a spring lifts the pin, and the valve opens again. The numbers are not temperatures and they are not a speed control. They set the room temperature at which the valve starts to close, so turning a TRV to 5 does not heat the room faster, it simply lets it get warmer before the valve shuts.

The valve at the other end of the radiator is the lockshield. It usually has a plain plastic cap and no numbers, and it sets how much water that radiator gets compared with the others in the house. It is adjusted once when the system is balanced and then left alone. If you are not sure which end is which, the one you can turn by hand with numbers on it is the TRV. Do not start adjusting the lockshield to fix a TRV problem, because you will change how the whole system is balanced.

The most common TRV fault shows up at the start of the heating season. The radiator was fine last spring, and now it stays cold even with the head turned up to maximum while the rest of the house warms up. Over the summer the pin sat in one position and seized, usually stuck in the closed position. Before you do anything else, check the boiler is firing and the other radiators are hot, so you know the problem really is this one valve. Our guide to one radiator not heating up covers the other single-radiator causes, such as air and sludge, if the TRV turns out to be fine.

To check the pin, turn the head to its highest setting and then take it off. Most heads either unscrew with a knurled collar or come off by pressing a release button or ring, and no tools are needed on the common makes. Underneath you will see a small metal pin sticking up from the valve body. With the head off, the valve should be fully open and the pin should be standing proud. Press it down gently with your thumb or the flat side of a small spanner. It should go down with light pressure and spring back up when you let go. If it is stuck down, or it moves stiffly and does not spring back, that is your fault.

A stuck pin can usually be freed by hand. Tap it lightly from the side with the handle of a screwdriver, or grip it gently with pliers and work it up and down a few times until it moves freely and springs back on its own. Do not pull hard or bend it, and do not try to pull it out of the valve. A drop of penetrating spray around the base can help if it is very stiff, but wipe off any excess. Once it moves freely, refit the head at its highest setting, give the heating half an hour, and then set the head back to the number you want. If the pin will not free, or it frees and sticks again within days, the valve body is worn and should be replaced.

The opposite fault is a radiator that stays hot whatever setting you choose. Here the pin is stuck open, or the head is no longer pushing it down. Test it by taking the head off and pressing the pin: if the pin moves freely, the problem is the head. Heads fail when the sensing capsule leaks or wears out, and they also fail when they have been fitted wrongly or knocked out of alignment. Many heads can be replaced on their own without draining anything, because the head just screws or clips onto the valve body. The replacement needs to be compatible with the valve body, which is why it helps to note the make stamped on the valve before you buy one.

Some TRV problems are not faults at all. A TRV reads the air around its head, so if the head is behind a long curtain, inside a radiator cover or boxed in by furniture, it warms up quickly and shuts the radiator off while the room is still cold. A TRV fitted in the same room as the main room thermostat can also fight it, with one turning the heating off as the other asks for more. Most systems have one radiator, often in the hall or wherever the room thermostat sits, that is meant to have no TRV at all. If every radiator in the house has a TRV and they all close at once, the boiler can end up with nowhere to send its water, which can cause noise and short cycling. Our guide on which heating controls you need explains how TRVs fit alongside the thermostat and programmer.

A TRV that weeps water is a different job. A slow drip from under the head usually comes from the gland seal around the pin, and a drip from the nut where the valve joins the pipe or the radiator is a joint that has loosened or corroded. Put a bowl or towel underneath and do not overtighten the nut, because old brass fittings in period properties can crack. Our guide to why a radiator is leaking explains how to tell the leak points apart and what can be done without draining the system.

Replacing a whole TRV valve body means isolating that radiator and letting water out, because the body is part of the pipework. On many systems you can close the lockshield and a manual valve or cap on the TRV to isolate the radiator, drain it down and swap the valve without draining the whole house. If the old valves will not close fully, which is common on older systems in Chelsea's terraces and mansion blocks, the system may need draining. Our guide to how to drain a central heating system covers that. Either way, the system needs refilling, bleeding and repressurising afterwards, and a fresh dose of inhibitor is worth adding at the same time.

Autumn is when these faults show up, because it is the first time the valves have moved in months. Before the cold weather arrives, turn every TRV up to maximum and back down a few times, and do it again once or twice over the summer. It takes a few minutes and stops most pins seizing. If several TRVs stick, radiators are cold at the bottom, or the water you let out when bleeding is black, the system is carrying sludge and a power flush may be worth considering.

If a TRV will not free, a radiator stays cold or hot whatever you do, or a valve is leaking, call Emergency Plumber Chelsea on 020 3002 6667. We replace TRV heads and valve bodies, rebalance radiators and find the underlying cause as part of our central heating repairs across SW3, SW10, Knightsbridge, South Kensington and Fulham. There is no call-out fee on jobs over 3 hours.

Quick tips

• Turn a stuck TRV to maximum, take the head off, and check the pin moves down and springs back up.
• Free a sticking pin by tapping it gently or working it up and down with pliers; never pull it out.
• A radiator that stays hot on every setting with a free-moving pin usually means a failed TRV head.
• Keep TRV heads clear of curtains, covers and furniture so they read the room, not a pocket of warm air.
• Exercise every TRV from minimum to maximum a few times before the heating season starts.

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