A faulty valve is one of the most common reasons a radiator heats poorly or will not switch off. Deposits on the pin narrow the waterway, the valve stops responding to the thermostatic head and the gland starts to weep.
Before replacing the valve, make sure it really is the cause: check whether the pin has seized and whether the thermostatic head works. If the valve leaks, is cracked or the spindle has snapped, it needs replacing, and with basic plumbing skills you can do it yourself.
What you will need
Important: Only replace a valve with the system depressurised and the water cold. In a block of flats the riser must be isolated and drained by the building management, so plan the job outside the heating season.
Step 1
Isolate and drain
In a house, switch off the boiler, let the system cool, close the radiator valves (if they still hold) and drain the radiator or the whole system from the drain cock at the lowest point. In a flat, arrange with the building management to isolate and drain the riser.
Lay down cloths and place a bowl underneath: even after draining, some dirty water stays in the radiator.
Step 2
Loosen seized joints
The union nuts and threads of an old valve have sat in moisture and limescale for years and are often rusted solid. Apply WD-40 Specialist Penetrant generously to the nut joining the valve to the radiator and to the thread on the pipe. Wait 10-15 minutes and repeat if needed.
Step 3
Remove the old valve
Undo the joints anticlockwise. Hold the valve body or pipe with one spanner and turn the nut with the other, so you do not twist the pipe or damage the radiator.
Step 4
Clean the threads and fit the new valve
Clean old sealant and rust off the threads on the pipe and radiator and wipe them dry. Wrap fresh hemp with paste or PTFE tape in the direction of the thread.
Screw in the new valve and tighten it with two spanners without excessive force, so the body does not crack. Make sure the valve sits straight and the arrow on the body points in the direction of water flow.
Step 5
Check the thermostatic head
Faulty thermostats are another common source of trouble. Undo the ring of the thermostatic head (with pliers if necessary) and check it for cracks or signs of leaking. Press the pin of the new valve: it should spring back freely. Refit a working head.
Step 6
Refill and test
Refill the system slowly, bleed the radiator at the bleed valve and bring the pressure up to working level. Wipe the new joint with a dry tissue: it should stay dry.
Once everything is dry, apply WD-40 Smart Straw to the spindle and threads. The protective film displaces moisture and stops the new valve from seizing.
Tip: Before you start, photograph the lockshield valve and count how many turns open it is. That way you keep the system balanced and the other radiators will heat as before.
What next
To keep the valve working for years, bleed the radiators every autumn, turn the handwheels slightly every few weeks and treat the valves with WD-40 Multi-Use once a season. Regular bleeding keeps water evenly distributed and reduces the risk of corrosion.
These uses come from the experience of WD-40 users in many countries. Always follow the instructions and warnings on the product label.
More guides in «How to use».