How to change a radiator valve or TRV

Head or body: which part has failed?

A thermostatic radiator valve (TRV) is two parts. The head is the numbered dial you turn. The body is the brass part that carries the water and screws onto the radiator and the pipe. Many problems only involve the head or the small pin underneath it, and those do not need the water drained.

If the radiator will not warm up or will not turn off, take the head off and look at the pin. If it is stuck down, it can often be freed by gently working it up and down with pliers. If the head is cracked, loose or no longer moves the pin, a replacement head that fits that make of body is a two minute job. It is the body that needs replacing when it leaks from the spindle or the joints, when the pin is seized solid, or when you want a different style of valve.

Can you change a radiator valve without draining the system?

Only in limited cases. The valve you are changing is the thing that would normally hold the water back, so once it is off, water will pour from the pipe. On a sealed system you cannot close anything else near enough to stop that.

Engineers get round it in two ways. On a radiator high in the house on some systems, they drain down just enough for the water level to fall below that valve. Or they use a pipe freezing kit, which freezes a plug of ice in the pipe below the valve so it can be changed with the system still full. Freezing needs the right equipment and some experience, because a freeze that fails halfway through a job floods the room. For most people, draining down is the dependable method.

What you need

Buy the valve before you drain anything, and check it matches the pipe. Most radiators run on 15mm copper pipe, but some older homes have microbore pipes of 8mm or 10mm, which need a valve or reducer to suit.

• The new valve, which should come with its own radiator tail
• Two adjustable spanners, and a hex key or radiator tail tool for the tail
• PTFE tape or a thread sealant approved for heating systems
• A spare 15mm olive in case the old one has to come off
• Towels, a dust sheet and a shallow tray
• A hose that reaches from the drain-off valve to outside, and a radiator key
• Central heating inhibitor to redose the system after refilling

Step 1: drain the system down

Switch the boiler off at the controls and let the system cool. On an open-vented system with a small tank in the loft, turn off the supply to that tank first so it does not refill while you drain. On a sealed system, the filling loop should already be closed.

Fit the hose to the drain-off valve, which is usually at the lowest point of the pipework, often on a pipe near a ground floor radiator or by the boiler. Run the hose outside to a drain and open the drain-off with a spanner. Then open the bleed valves on the upstairs radiators to let air in, and the water will run out much faster. Keep going until the radiator you are working on is empty.

Draining lets air in, removes the inhibitor that protects the system, and can disturb sludge. It is a good moment to change any other failing valves at the same time, so the system only has to be drained once.

Step 2: take off the old valve

Put towels and the tray under the valve. Hold the valve body steady with one spanner and undo the union nut that joins it to the radiator with the other. Then undo the compression nut on the pipe below. Holding the body while you turn stops you twisting the pipe, which can crack a joint under the floorboards.

Lift the old valve off. Now unscrew the old radiator tail from the radiator with a hex key or tail tool. The new valve comes with a matching tail, and mixing an old tail with a new valve is a common reason the union weeps later.

Step 3: fit the new valve

Wrap the thread of the new tail with PTFE tape or apply a suitable sealant, and screw it firmly into the radiator. Slide the compression nut and olive onto the pipe. If the old olive will not come off, it can be left in place and reused if it is undamaged, or cut off carefully with a junior hacksaw without scoring the pipe.

Offer the new valve up to the tail and the pipe, check it sits square without forcing the pipe sideways, and tighten the compression nut and the union nut while holding the valve body. Tighten firmly but do not overdo it; you can always nip a nut up once the water is back in. Leave the head off a new TRV for now, or fit the decorator's cap supplied with it.

Step 4: refill, bleed and check

Close the drain-off valve and every bleed valve. On an open-vented system, turn the supply to the loft tank back on. On a sealed system, open the filling loop and bring the pressure up to around 1 to 1.5 bar, or to the figure in your boiler manual.

Bleed each radiator in turn, starting downstairs, and top the pressure up again as it drops. Add inhibitor as the manufacturer directs, then run the heating and check every joint you touched while the system is hot. A slow drip often stops with a small extra turn on the nut. Once the radiators are all hot and there are no leaks, fit the TRV head. After changing several valves, the radiators may need balancing again.

Choosing the right new valve

Most modern TRVs can be fitted on either the flow or the return pipe, but some older designs have a direction arrow, so check the instructions. Angled, straight and corner valves suit different pipe layouts, and the one you buy should match the way your pipe comes up to the radiator.

The room with the main wall thermostat is usually left without a TRV, or with its TRV fully open, so the two controls do not fight each other. If your boiler is being replaced, current building regulations guidance expects TRVs on the radiators in the other rooms, so it can be worth planning valve changes at the same time.

When to call a plumber

Call for help if the drain-off valve is missing or seized, the pipe is microbore or corroded where it leaves the floor, a nut will not move without twisting the pipe, or the system will not hold pressure afterwards. Those are the points where a quick valve swap turns into a leak under the floor.

Loyal Plumbers replaces radiator valves across London and everywhere inside the M25, 24 hours a day, with 10 vans on the road. Our call-out charge is £60. Tell us how many valves need changing, whether you want TRVs or manual valves, and the type of boiler, and the engineer can bring the right valves and drain, refill and treat the system in one visit.