Boiler & heating breakdowns
How a Thermostatic Radiator Valve Works
Published
A thermostatic radiator valve has a sensor in its head filled with wax, liquid or gas. As the room warms, the sensor expands and pushes a pin down into the valve, cutting the hot water going into the radiator. As the room cools, it contracts, the spring-loaded pin rises and water flows again. The number sets the room temperature it aims for.
The parts of a TRV
A TRV is two parts that screw or clip together. The valve body is the brass piece joined to the pipe and the radiator. Inside it is a small valve with a spring-loaded pin sticking up out of the top. When the pin is up, water flows. When it is pushed down, the valve closes.
The head is the part with numbers on it. It holds the sensor, a small sealed capsule, and a plunger that sits on top of the pin. Turning the head does not open or close the valve directly, the way a tap does. It changes how far the sensor has to expand before the plunger meets the pin, which is how you choose the temperature.
That separation is why a valve can fail in two different ways. The head can stop responding, or the pin in the body can seize. It is also why you can take the head off without any water escaping.
What happens as the room heats up
Say the head is on 3 and the room is cold. The sensor is contracted, the pin is fully up and the radiator gets full flow from the boiler. As the room warms, the air around the head warms the sensor. Wax heads use a wax that expands as it warms; liquid and gas heads work the same way with a fluid or vapour. The capsule pushes the plunger down, and the pin gradually closes the valve.
It does not snap shut. As the room nears the set temperature the valve throttles back, so the radiator gets less water and cools a little. If the room drops again, the pin rises and flow picks up. A working TRV spends most of its time partly open, holding the room steady rather than switching on and off.
Liquid and gas sensors respond faster than wax, so they track the room more closely. Wax heads are slower and cheaper and are still very common.
What do the numbers mean?
The numbers are not degrees. They mark points on a scale, and the temperature each one targets varies between makes. As a rough guide, on many heads 3 is around 20°C and each number is a few degrees apart, with the snowflake or star setting keeping the room just warm enough to stop pipes freezing. Check the maker’s leaflet for the exact figures on yours.
Turning a TRV up to 5 does not heat the room faster. The radiator is already at full flow while the room is cold. The higher setting only means it keeps going until the room is hotter. Setting it to the temperature you actually want, and leaving it there, is how it is meant to be used.
- The head senses the air right around it, so curtains, furniture or a radiator cover over it will make it shut off early
- A TRV in the same room as the main room thermostat can fight it. Set the TRV there fully open, or high, and let the room thermostat do the work
- Heads should be horizontal where possible. A vertical head sits in the rising heat of the pipe and can read the room as warmer than it is
Why every radiator but one has a TRV
If every TRV in the house closed at once, the pump would be pushing water round a system with nowhere to go. That is why systems need a bypass: an automatic bypass valve near the boiler, or one radiator with ordinary valves, usually the one in the room with the main thermostat. Many modern boilers have a bypass built in, but the installer still has to make sure the system meets the boiler maker’s minimum flow.
Building Regulations Approved Document L also sets out controls. Under the current edition, when a boiler is replaced in an existing home, each room should have its own temperature control where that is technically and economically feasible. In most homes that means TRVs on all radiators except the one in the room with the room thermostat.
Radiator thermostat not working? What to check
When a TRV stops working, work out which half has failed.
- Radiator cold on every setting, pipe into the valve hot: the pin has stuck down. Take the head off and see our guide to a stuck thermostatic radiator valve
- Radiator hot even on the lowest setting: the pin has stuck up, or the head sensor has failed. Press the pin with the head off. If it moves freely, replace the head
- Radiator only warm at the valve end: the valve may be fine and the problem is air, sludge or flow. See one radiator not working
- No radiator responds to its TRV: look at the controls and flow temperature instead. Our boiler thermostat not working guide covers that side
- The valve hums, bangs or chatters as it closes: a one-way valve fitted the wrong way round, or the pump speed set too high
If the valve body or gland nut is weeping water, do not tighten it hard. Put a cloth under it and get the valve replaced.
Replacing a TRV head or valve
A new head is a quick swap if it matches the valve body, and many makers sell adaptors to fit their heads onto other bodies. Smart heads that connect to an app fit the same way and let you set times room by room.
Replacing the valve body means isolating the radiator and draining part of the system, then refilling, bleeding and on a sealed system restoring the pressure. As typical UK market prices, a head costs from about £10 to £30 and a plumber fitting a whole new valve usually charges around £100 to £200. Our radiator repair visit covers this, with the £60 call-out at any hour and the repair price agreed with you on site before any work starts.
Questions people ask
Does a thermostatic radiator valve need electricity?
No. A standard TRV works purely on the expansion of the sensor in its head. Only smart heads use batteries or power, to drive the pin with a small motor.
What number should my radiator valve be on?
Many people find 3 comfortable for living rooms and 2 for bedrooms. On many makes 3 is around 20°C, but the scale varies, so adjust by one step at a time and give the room a few hours to settle.
Does a TRV save energy?
It stops rooms overheating once they are warm, which saves on heating that would be wasted. It only works well if the head can sense the room, so keep it clear of curtains and covers.
Why is my radiator cold with the TRV on 5?
The pin in the valve body has probably stuck down. Unscrew the head and check whether the pin springs up when pressed. If the boiler is off or every radiator is cold, the problem is elsewhere.