How to Replace a Thermostatic Radiator Valve (TRV)
Modern TRVs cut heating bills by 8–12% by throttling individual rooms. Replacing an old lockshield or seized TRV is a one-hour DIY job — no full drain-down required if you use freeze-plugs or drain locally.
Step-by-step method
1. Choose the right TRV
Bidirectional TRVs (Drayton TRV4, Honeywell VT117, Pegler Terrier) fit either flow or return. Get 15mm for standard UK copper, or 22mm for larger runs. Angled body for horizontal pipe entry, straight for vertical.
2. Isolate the radiator
Close both existing valves. Note the number of turns on the lockshield — you'll match this on the new one to preserve balancing. Use a drain-off point or catch water in a shallow tray.
3. Undo the old valve
Two spanners: one on the union nut, one holding the valve body still. Loosen slowly — old brass shears easily. Have a bowl ready for the 200–500ml of trapped water.
4. Fit the new TRV
Wrap the tail thread with 8–10 turns PTFE plus Loctite 55. Screw the tail into the radiator (28Nm — snug but not gorilla). Slide the union onto the pipe, tighten to the valve body with two spanners.
5. Refill, bleed, test
Reopen both valves, bleed the radiator, top up boiler pressure to 1.2 bar, and fire up the heating. Set the TRV head to setting 3 (~20°C) as a baseline.
Frequently asked questions
Do I need to drain the whole system to fit a TRV?+
No. Closing the flow valve at the boiler and both radiator valves, plus a freeze plug or towel, is enough for a single-rad swap.
Which room should NOT have a TRV?+
The room with your main thermostat — usually the hall or living room. TRVs there fight the thermostat and short-cycle the boiler.