How to drain a single radiator without draining the whole system
⚠️ After refilling, repressurise a sealed system to 1–1.5 bar via the filling loop, and dose with inhibitor if system water was significantly diluted.
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Draining a single radiator is necessary to remove it for decorating, replace a leaking valve, or remove a cold radiator. You can isolate the radiator without draining the whole system.
Turn off both valves at the radiator — the TRV (thermostatic valve, usually one side) and the lockshield valve (the capped valve on the other side). The lockshield cap unscrews or lifts off to reveal a valve stem — close it with pliers, counting the turns so you can return it to the same position later.
Open the bleed valve at the top of the radiator to break the vacuum, using the bleed key. Keep a cloth ready as some water may drip.
Unscrew the union nut on the TRV end of the radiator (anti-clockwise, with a spanner on both the nut and the valve body to avoid twisting the pipework). Water will run out — have a large container ready or attach a hose to the tail.
Tilt the radiator to drain the remaining water from the opposite end, then disconnect the lockshield union nut. Carry the radiator out (they are heavy — have help for anything above 1200mm long).
On refitting, reverse the process. Reconnect both unions hand-tight first, then tighten with a spanner (no PTFE tape needed on compression-style unions). Open the lockshield the same number of turns as before. Open the TRV. Bleed the radiator, then check for leaks at both connections.
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Frequently asked questions
Do I need to drain the whole system to remove one radiator?
No — if the TRV and lockshield valves on the radiator are working, you can isolate and drain just that radiator. Close both valves, then crack open the bleed valve at the top and the union nut at the bottom to drain the radiator into a bucket. The rest of the system remains full.
How much water is in a typical radiator?
A standard single-panel 600×1400mm radiator holds about 5–8 litres. A large double-panel double-convector radiator may hold 12–15 litres. Have enough buckets ready before opening any connections, particularly for large radiators.
Do I need to repressurise the boiler after removing a single radiator?
Possibly — removing and refitting a radiator loses some water, which may drop your system pressure slightly. Check the boiler pressure gauge after refitting and repressurise to 1–1.5 bar via the filling loop if it has dropped below 1 bar.