Thermostatic Radiator Valve Replacement

A valve rarely fails overnight. It usually gives one of these signs first:

• The radiator stays fully hot even with the head turned to the lowest setting
• The radiator stays cold while the others in the home heat up
• Turning the head makes no difference to the radiator temperature
• Damp, drips or green or rusty deposits appear around the valve
• The radiator still has an old manual valve with no thermostatic control

Why does a radiator sometimes refuse to heat after the summer? Quite often the small pin inside the valve, which the head presses on, has simply stuck. With the head removed, you can gently press the pin in and let it spring back a few times. If it will not move, or water seeps past it, the valve insert is worn. If a different, working head fixes the problem, the valve body is fine and only the head needs replacing.

On a single radiator this is a short job. A heating engineer usually works like this:

1. Check the radiator, pipework and threads, and decide whether the head, the valve insert or the whole valve body needs replacing.
2. Isolate the radiator. In a house this often means closing the TRV and the lockshield valve on the other side; if that is not possible, the system is partly drained. In a block of flats with shared heating, this must be agreed with the building management or owners' association first.
3. Remove the old valve without damaging the radiator thread or the pipe. On older systems, clean off corrosion and dirt.
4. Fit the new valve with a properly sealed thread. If the body has an arrow, it must point in the direction the water flows.
5. Set the presetting on the valve, if it has one, and fit a compatible head.
6. Refill and repressurise the system, bleed the radiator and check every joint for leaks. If the system was drained, top up the corrosion inhibitor as the boiler or inhibitor manufacturer recommends.
7. Check that the radiator responds when the head is turned.

In blocks of flats with communal heating, the work is usually done outside the heating season, when the riser pipes are empty or easy to isolate. In winter it can mean draining part of the building, which the management is often reluctant to allow and which affects the neighbours too.

Choose the valve according to how the radiator is connected, not by looks.

• Angled valve: suits pipes coming out of the wall or floor that turn into the radiator
• Straight valve: used when the pipe runs straight into the side of the radiator
• Axial or three-way adjustable valve: lets you position the head where it has space and is not hidden behind furniture
• Valve insert: radiators with a bottom connection often have the valve built into the radiator itself, so only the insert is changed, not a separate valve

In blocks of flats, the presetting matters. This is a small adjustment on the valve that limits the maximum flow. After the heating in a building has been balanced, each radiator has its own setting. If the new valve gets a different one, the radiator may take heat from the neighbours or, the other way round, heat poorly. Ask the building management for the original value or check the balancing records. In a house, the heating engineer balances the radiators using the lockshield valves and presettings.

Heads are either mechanical, filled with liquid or wax, or electronic. An electronic or smart head can follow a time programme, for example warmer in the morning and evening and cooler during the day. Most common valves take heads with an M30 x 1.5 thread, and adapters are sold for other types. If you are planning smart controls, check compatibility before buying the valve, so that later you only need to swap the head.

How much work the job takes depends mostly on the condition of the system:

• A working lockshield valve lets one radiator be isolated without draining anything
• Good access to the valve and clean, undamaged threads also help
• An old valve that is seized or corroded is harder, because it may not loosen without risking damage to the pipe
• Having to drain part or all of the system adds work, especially in a block of flats
• A non-standard connection size or spacing means the pipework has to be adjusted
• If several valves are being replaced, it makes sense to combine the job with bleeding and a check of the whole heating system

Leave replacing the valve body to a heating engineer: the system is under pressure, and a mistake can flood your home and the flat below. The head on its own, however, you can change yourself, and no water escapes while you do it.

1. Turn the old head to the highest number so it no longer presses on the pin.
2. Undo the nut or release the clip and lift the head off.
3. Check that the valve pin moves freely and that no water seeps around it.
4. Make sure the new head fits the valve, or fit an adapter.
5. Fit the new head in the fully open position and tighten the nut by hand.
6. Set the level you want and check after a few hours that the radiator responds.

If the pin does not move or water leaks past it, call a heating engineer.

• Valve fitted against the flow direction, so the radiator hums or regulates poorly
• Presetting forgotten or set wrongly in a block of flats
• Joint overtightened, damaging the radiator thread
• Head fitted vertically just above the radiator, where rising heat warms it
• Head covered by a curtain or furniture, so it measures the wrong temperature
• Incompatible head that cannot close the valve fully

• Valve and lockshield joints are dry even after several hours of heating
• Any arrow on the valve points in the direction of water flow
• The radiator warms up when the head is turned up and gradually cools at the minimum
• The head sits horizontally, is easy to reach and is not covered
• The presetting matches the value from the building management or the balancing records
• The radiator is bled and the system pressure is within the range given by the boiler manufacturer
• The heating engineer has told you which valve and head were fitted, for future servicing