How do I replace a TRV head without draining the system?
The head unscrews from the valve body without touching the water, provided the fittings match — how to identify yours and what to buy.
- Difficulty
- beginner
- Time
- 20 min
- Read
- 4 min
Short answer
The plastic head is a separate part that screws onto the valve body, and swapping it involves no water at all. Turn the head to its maximum setting to release the pressure on the pin, unscrew the collar, lift it off and fit the new one. The only thing that matters is that the new head fits your valve body's collar.
People replace whole radiator valves, and drain systems to do it, when the part that has actually failed is a plastic head containing a wax capsule. Heads are cheap, universal patterns cover most valves, and the swap takes two minutes.
What you'll need
- A replacement head of the correct fitting
- Possibly a pair of grips for a stiff collar
- A cloth
Step by step
- Confirm the head is the problem.If the radiator is cold with the head on and gets hot with it removed, the head has failed or the pin is stuck. If it is cold either way, the pin or the flow is the problem, not the head.
- Turn the head to maximum first.Setting it to 5 or fully open retracts the internal plunger, so the collar is not under load and comes off easily.
- Identify the collar fitting.The commonest by far is M30 x 1.5, a threaded collar. Others include clip-on types used by particular manufacturers. Measure across the thread or take a photo to the merchant.
- Unscrew the collar by hand.It should turn by hand. If it is stiff, wrap a cloth round it and use grips gently — do not lever against the valve body or the pipework.
- Lift the head straight off.No water comes out. The valve body underneath stays sealed and the system stays full.
- Check the pin while it is exposed.Press the pin with a finger. It should move a few millimetres and spring back freely. Free it now if it is stiff — a new head on a seized pin achieves nothing.
- Note the orientation you want.Heads can usually be fitted at various angles. Avoid one that ends up upside down or buried against a wall, where its own sensor reads the radiator rather than the room.
- Fit the new head at maximum.Set the new head to its highest number before fitting, so its plunger is retracted, then screw the collar up hand tight. No tools needed and no sealant.
- Test it through its range.Turn it to minimum and check the radiator cools, then back up and check it heats. Give it ten minutes at each setting; TRVs respond slowly by design.
- Consider a remote sensor head.Where the valve sits behind a curtain, in a cabinet or very close to the floor, a head with a remote capillary sensor reads the room properly instead of its own microclimate.
Tips
- M30 x 1.5 covers a large majority of UK valve bodies. Photograph the collar and take the picture with you.
- Free the pin every time you have a head off. It is the maintenance that prevents most cold radiators.
- Set both old and new heads to maximum before removing and fitting — it makes both operations effortless.
Common mistakes
- Unscrewing the collar with the head set to a low number — The plunger is pressed hard against the pin, so the collar is under load and stiff. Set it to maximum first and it turns by hand.
- Undoing the valve body instead of the head — That opens the water system and will empty the radiator and drop the system pressure. The head is the part above the collar; nothing below it should be touched.
If it doesn't work
New head fitted, radiator still cold
Cause: Pin seized under the head — Fix: Remove the head, free the pin, refit.
Collar will not undo
Cause: Corroded, or head not at maximum — Fix: Set to maximum, then grips with a cloth; penetrating spray on a corroded collar.
New head does not fit
Cause: Non-standard collar — Fix: Identify the valve manufacturer; adaptor rings exist for most common mismatches.
Room overheats regardless of setting
Cause: Head sited in a hot or enclosed spot — Fix: Refit at a different angle, or use a remote sensor head.
Questions people ask
Do I need to drain the system to change a TRV head?
No. The head is a mechanical part that screws onto the outside of the valve body. Changing it does not open the water circuit at all.
What if I need to change the whole valve body?
That does open the system. It can be done with a valve-changing tool that plugs the radiator tail, but on a sealed system it usually means draining down at least partly. That is a bigger job than a head swap.