A thermostatic radiator valve works by sensing the air temperature in the room, not the water in the pipe. Inside the head is a capsule of wax or liquid that expands as the room warms. That expansion pushes a small metal pin down into the brass valve body, throttling the flow of hot water into the radiator. As the room cools, the capsule contracts, a spring lifts the pin, and the flow returns. There is no electricity and no wiring involved. It is a purely mechanical loop that repeats all day.
That simplicity is why TRVs last so long, and it is also why they fail in only a handful of ways. Four faults account for almost every dead radiator valve we see: a seized pin, a spent capsule, a leak at the gland, and a valve that no longer regulates at all. Two of those need nothing more than a new head. Two mean the whole valve comes off. This guide shows you how to tell which is which in about a minute, without draining your system.
In this article
- How does a thermostatic radiator valve actually work?
- What do the numbers on a TRV mean?
- The 4 faults that mean you need a new TRV
- How do I test whether it is the head or the valve body?
- Which replacement should I buy?
- FAQ
How does a thermostatic radiator valve actually work?
A TRV is two parts that join together. The head is the plastic or metal cap you turn. The body is the brass valve plumbed into your pipework. Understanding that split is the single most useful thing you can know about radiator valves, because it decides whether your repair costs five pounds or twenty.
Inside the head sits a sealed capsule filled with wax or, on better valves, a liquid. Both expand predictably when heated. When the room air around the head warms up, the capsule grows and drives a plunger against the pin protruding from the valve body. Push that pin far enough and it seats against a brass shoulder inside the body, cutting the flow of hot water. The number you set on the dial simply changes how far the plunger has to travel before it closes, which is the same as setting a target room temperature.
Liquid filled sensors respond faster and more accurately than wax, which is why they appear on premium valves such as the Drayton TRV4. Wax capsules are slower and tend to overshoot slightly, but they are cheap, robust and perfectly adequate in a bedroom or hallway.
Two things follow from all this. First, a TRV does not know or care what your boiler is doing, so it cannot switch the boiler off. That is your room thermostat's job. Second, because the head reads the air immediately around it, anything that traps heat there will fool it. A long curtain draped over the valve, a radiator cover, or a sofa pushed against it will all make the head think the room is warmer than it is and shut the radiator down early.
What do the numbers on a TRV mean?
The numbers are target room temperatures, not flow rates. Turning a valve to 5 does not make the radiator hotter, it just means the room has to reach a higher temperature before the valve closes. Exact figures vary between manufacturers, but a typical UK valve maps roughly like this.
| Setting | Approximate room temperature | Best for |
|---|---|---|
| ✻ (frost) | About 7°C | Empty rooms, holidays, frost protection |
| 1 | About 10°C | Garages, utility rooms |
| 2 | About 15°C | Hallways, landings |
| 3 | About 18 to 20°C | Bedrooms, most living space |
| 4 | About 22°C | Living rooms in cold weather |
| 5 | About 25°C and above | Bathrooms |
Used properly, this room by room control matters. Research from the University of Salford cited in the BEAMA householder factsheet found a typical gas boiler used around 18 percent less energy over an average UK heating season with TRVs fitted and set correctly. It is also why, since June 2022, Part L of the Building Regulations requires thermostatic room controls in every room when a boiler is replaced in England.
The 4 faults that mean you need a new TRV
Here is the short version. Two of these four are head faults and cost very little to fix. Two are body faults and mean replacing the valve.
| Fault | What you notice | What you replace |
|---|---|---|
| 1. Seized pin | Radiator stone cold or permanently scalding, whatever the setting | Free it first. If it stays stuck, the whole valve |
| 2. Spent capsule | Dial turns with no resistance and nothing changes | The head only |
| 3. Weeping gland or union | Damp patch, white crust or rust staining under the valve | The whole valve |
| 4. No regulation | Room overheats or never warms, and the dial does nothing useful | Usually the head, sometimes the valve |
Fault 1: the pin has seized in the valve body
This is the most common failure by a wide margin, and it is almost always caused by summer. Leave a valve closed from April to October and limescale and sludge bind the pin in place. When the heating comes back on the pin cannot lift, so the radiator stays cold, or it cannot drop, so the radiator runs flat out and the room cooks.
Take the head off and try to free the pin with penetrating oil and gentle taps, or grip it with pliers and work it up and down. Plenty come back to life. If it will not move, or it moves but does not spring back, the body is worn out and the valve needs replacing. To avoid this next year, open every TRV fully for a few minutes once a month through the summer.
Fault 2: the wax or liquid capsule has lost its charge
Capsules do not last forever. Ten to fifteen years is a fair working life, less in a bathroom where the head takes constant humidity. When one fails, the giveaway is that the dial spins between 1 and 5 with almost no resistance and the radiator behaves identically at every setting, usually stuck fully open.
This is the cheapest fix in heating. The brass body in your wall is fine and the head simply unscrews. As long as the thread matches, a replacement head takes two minutes and needs no tools, no draining and no plumber.
M30 x 1.5 thread, which fits most UK valve bodies. Swap it by hand in two minutes with no draining down. Not for Danfoss click-fit bodies.
Fault 3: it is weeping from the gland or the union
Look under the valve. A damp patch, a chalky white crust, rust marks on the pipe or a stained carpet all point to a leak. If water is escaping around the spindle where the head sits, the gland packing has failed. Sometimes a quarter turn on the gland nut buys you time. More often the seal is done, and on a valve that is already fifteen years old it is not worth the second callout.
A leak at the union nut where the valve meets the radiator tail is different and usually just needs the joint remaking with fresh PTFE or a new olive. Water coming from the valve body itself is terminal.
Fault 4: the valve no longer regulates at all
The pin moves, nothing leaks, but the room still overheats or never gets warm. Either the internal threads in the head have stripped so the plunger no longer travels the right distance, or the capsule has partially failed and now closes far too early. Start by replacing the head, since that fixes it the majority of the time and costs very little. If a brand new head behaves the same way, the seat inside the body is worn and the valve needs replacing.
Before you condemn anything, rule out the two impostors. A radiator that is cold at the top needs bleeding. A radiator that is cold at the bottom is full of sludge and needs flushing. Neither is a valve fault.
How do I test whether it is the head or the valve body?
You do not need to drain the system to find out. The head is a sensor, not a seal, so removing it does not release any water.
- Turn the head to its highest setting. This retracts the plunger and takes the pressure off the pin.
- Release the collar. Most modern heads sit on an M30 x 1.5 thread and unscrew by hand. Danfoss heads are the exception: they clip on, so they release with a catch or a squeeze of the collar rather than unscrewing. Older Drayton types may need a gentle turn with grips or have a small grub screw.
- Lift the head off. You will see a small metal pin, roughly the size of a matchstick head, sitting proud of the brass body.
- Press the pin. It should move a couple of millimetres and spring straight back when released.
One warning. With the head removed, the pin is normally in its fully open position, so that radiator will heat up. That is expected, and it also confirms the body is passing water correctly.
Which replacement should I buy?
If it is the head, the only thing that matters is the fitting. M30 x 1.5 is the standard on the vast majority of UK valve bodies, including Pegler, Myson and the generic valves fitted in most new builds, and it is what our LavaTherm heads use. A few older Drayton TRV4 bodies and some continental valves use their own fitting, so check yours before ordering.
Danfoss is the one big exception
Danfoss TRV bodies do not have an M30 x 1.5 collar. They use a click release system, so the head clips on and springs off instead of unscrewing, and a standard M30 head will not fit one. Check before you order: a knurled ring that turns is M30, a smooth collar with a release catch is Danfoss.
If you have a Danfoss body you have two options. Fit the ENGO Smart TRV, which is supplied with a Danfoss adaptor, or replace the whole valve with an M30 x 1.5 body and use any standard head from then on.
If it is the whole valve, you need to match the shape to your pipework. Angled valves suit pipes rising from the floor into the side of the radiator and cover most UK homes. Straight valves suit pipes running along the skirting. Corner valves turn the flow through 90 degrees and keep everything tight to the wall. Buy the valve and its matching lockshield together so the pair looks right and balances properly.
Complete twin pack: one TRV and one matching lockshield, ready for a standard 15mm angled installation. Also in chrome, black, anthracite, brushed brass and nickel.
Liquid filled sensor for faster, more accurate response than wax, plus the bi-directional body that made the TRV4 an installer standard.
Is it worth going smart while the head is off?
If you are already unscrewing a dead head, a smart TRV is the cheapest moment you will ever get to upgrade. A smart head fits the same M30 x 1.5 thread, so the brass body stays where it is, and the ENGO ships with a Danfoss adaptor so it goes onto a click-fit Danfoss body too, but it adds a schedule, app control and the ability to heat the bedrooms only in the morning and the living room only in the evening. In a house with rooms that sit empty half the day, that is where the real savings come from.
Zigbee and RF 868 room by room control on the standard M30 x 1.5 thread, and it includes a Danfoss adaptor so it fits click-fit Danfoss bodies as well. Needs an ENGO E25 or E40 thermostat and an E-Gate Pro gateway, so check compatibility before ordering.
Frequently asked questions
How long should a thermostatic radiator valve last?
Expect ten to fifteen years from a decent TRV in normal use. Heads in bathrooms and kitchens fail sooner because of humidity and heat, and any valve left closed all summer is far more likely to seize. Exercising every valve once a month in warm weather is the single best thing you can do to extend their life.
Can I change a TRV without draining the system?
You can always change the head without draining, because it is only a sensor sitting on top of the valve. Changing the body is different and does require draining down, or at least isolating and locally freezing the pipe. If you are not confident isolating your system, this is the point to call a plumber.
Will a new TRV head fit a Danfoss valve?
Not a standard one. Danfoss radiator valves use a click release fitting rather than the M30 x 1.5 thread found on most UK bodies, so an M30 head will not screw on. Your options are to fit a head supplied with a Danfoss adaptor, such as the ENGO Smart TRV, or to change the valve body itself for an M30 x 1.5 one, after which any standard head will fit.
Which radiator should not have a TRV fitted?
The radiator in the same room as your room thermostat, normally the hall or living room. If a TRV there closes down, the thermostat reads a cold room, calls for heat that has nowhere to go, and the boiler short cycles. Fit a manual or lockshield valve on that radiator instead. Traditionally at least one radiator is left without a TRV to act as a bypass, though on systems with an automatic bypass valve this is no longer strictly needed.
Why is my radiator hot when the TRV is turned off?
Almost always a seized pin holding the valve open. Take the head off and check whether the pin springs back. If it does move freely and the radiator is still hot, check you have not mixed up flow and return, and that the lockshield at the other end is not wide open on a system with poor balancing.
Are TRVs a legal requirement in the UK?
Not on their own. Since June 2022, Part L of the Building Regulations in England requires thermostatic room controls in every room when a boiler is replaced. TRVs are the usual way of meeting that, paired with a room thermostat, but smart controls or an underfloor system with per room thermostats also qualify.
Does a TRV control the boiler?
No. A TRV only throttles the water going into one radiator. Nothing about it tells the boiler to fire or stop. That is your room thermostat and programmer, which is why Part L asks for both a room thermostat and thermostatic control in the other rooms.
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Heads from £5.99, complete TRV and lockshield sets from £7.49, in six finishes and every shape. Trade prices, no account needed.
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