How to fit thermostatic radiator valves

Video by Plumberparts. This walk-through is based on the video "Thermostatic Radiator Valves - Plumbing Tips" from the Plumberparts YouTube channel. It covers how TRVs work, which rooms to fit them in, and the installation process — useful background to watch before you start, especially the section on understanding which radiator should be left without a TRV.

1. Understand what a TRV does and where to fit them

A thermostatic radiator valve senses the air temperature in the room and throttles the flow of hot water to the radiator accordingly. When the room reaches the set temperature, the valve closes; when it cools down, it opens again. The result is that each room heats to roughly the right level rather than cooking until your boiler thermostat cuts in. Over a winter, that adds up to a noticeable difference on the gas bill.

The one room where you should not fit a TRV is wherever your main room thermostat lives — usually the hallway or living room. If a TRV closes that radiator before the thermostat is satisfied, the boiler will keep firing unnecessarily. Leave that radiator on a standard manual valve. Every other room in the house is a fair candidate for a TRV, though bedrooms often benefit most because people prefer them cooler overnight.

2. Gather the right tools and check the valve size

You will need a pair of adjustable spanners or grips, some PTFE tape, a bowl and towels for catching water, and the new TRV itself. Most UK radiators use a 15 mm compression fitting at the valve end, so a standard TRV will normally fit straight onto the existing tail in the radiator. That said, older properties sometimes have imperial-sized tails — if in doubt, take the old valve head to a plumber's merchant and match it up before buying.

Check which side of the radiator the flow pipe enters on. TRV valves are directional — there is usually an arrow on the body indicating flow direction. Fitting it the wrong way round will not necessarily stop the radiator working, but it can cause noise and slightly reduce efficiency. The flow pipe is typically the one connected to the smaller-bodied valve (the manual valve on the opposite end is the lockshield return).

3. Isolate or drain the radiator

If the radiator has an isolator valve on the flow pipe already, close it and open a bleed valve slightly at the top of the radiator to release the water pressure. This lets you swap the TRV body without losing water from the rest of the system. Have your bowl ready underneath the old valve — there will be some residual water in the radiator that comes out when you undo the nut.

If there is no isolator, you have two options: fit one while the system is drained, or use the freeze kit method (a spray that freezes the pipe temporarily, giving you a few minutes to swap the valve). Freeze kits are available from most plumber's merchants and are genuinely handy for a quick single-valve swap. To be fair, if you are not confident using one, draining the system down is the safer route, even if it takes longer.

4. Remove the old valve

Use one spanner to hold the radiator tail firm and another to undo the compression nut on the valve body. Work slowly to begin with — an old valve that has been in place for years can be stiff, and jerking it can stress the copper pipe. Once the nut is loose, the last few threads will release quickly, so have your bowl in place. Pull the old valve body away and inspect the olive (the small brass or copper ring that compresses onto the pipe to make the seal). If it looks deformed or corroded, it is worth cutting it off and fitting a new one.

Remove the old valve tail from the radiator as well — it unscrews anticlockwise using a radiator tail key or a large flat-bladed screwdriver in the slot. Wind several layers of PTFE tape around the thread of the new tail before fitting it. Wrap the tape clockwise as you look at the thread end, so tightening does not unwrap it.

5. Fit the new TRV body

Offer the new TRV body up to the pipe and the radiator tail. Thread the compression nut on finger-tight first, then tighten with the spanners — roughly a quarter to half a turn past hand-tight is usually enough. Do not go mad; over-tightening can split the olive or crack the valve body. Tighten the radiator tail end at the same time, holding the TRV body steady so it does not twist.

Open the isolator valve slowly (or repressurise the system if you drained it) and check for drips at both ends of the valve. A small weep will often stop once the system is up to pressure, but a steady drip needs tightening or re-doing with fresh PTFE. Give the joints a wipe dry and check again after 30 minutes before declaring the job done.

6. Fit the thermostatic head and set the temperature

The thermostatic head simply clicks or screws onto the valve body — most modern TRV heads have a universal fitting that works across several brands. Line up the pin on the valve body with the sensor in the head, press or twist to lock, and you are done with the physical fitting. The numbered dial on the head usually goes from 1 to 5, where 1 is around 10 °C and 5 is fully open. Position 3 is roughly 20 °C, which suits most living rooms.

Set each room to a sensible starting point, run the heating for a day, and adjust from there. Bedrooms are often comfortable at 2 (around 16 °C); a bathroom with a heated towel rail can often sit at 1 or 2 once the towel rail is taking the edge off. It depends on the room, the insulation, and how your household likes the temperature — there is no single right answer, just trial and adjustment over a couple of weeks.

When to call a handyman

Call Richard if the existing valves are corroded or stuck and will not undo without risking pipe damage, if the pipework layout means a full drain-down is unavoidable, or if you want TRVs fitted across several radiators in one visit. It is also worth getting help if the boiler loses pressure repeatedly after the work — that usually points to an air lock or a drip somewhere that needs finding before it causes damage.