What is a motorised valve and why is my heating stuck on?
Heating that will not switch off, or hot water that never comes on, is very often one small valve. What it does, the three ways it fails, and the safe check that identifies which.
- Difficulty
- intermediate
- Time
- 25 min
- Read
- 5 min
- Safety
- warning
Short answer
A motorised valve decides whether heat goes to the radiators, the cylinder or both. When it sticks open the heating runs regardless of the thermostat; when it sticks shut that circuit never gets hot. Listen at the valve when the system calls, feel the head for warmth, and try the manual lever — those three checks separate a seized body from a dead motor from a failed switch.
Do this first
Heating that stays on when everything is switched off is alarming and expensive, and the cause is very often a component most householders have never heard of. A motorised valve is two things stacked together: a brass valve body in the pipe, and a detachable electrical head containing a small motor, a spring and a switch. The head can be changed without touching the water at all, which is why this is usually a modest repair rather than a big one.
Safety
Step by step
- Find the valve and read the label.On the pipework near the cylinder. Note the make and model from the head — replacement heads are model-specific and knowing it before you ring anyone saves a return visit.
- Check the manual lever position first.The lever on the side, marked AUTO and MAN. If somebody has left it in MAN, the valve is latched open and the heating will run whenever the boiler fires, regardless of every control in the house. This is a surprisingly common cause of heating that will not switch off and it costs nothing to check.
- Listen while the system calls.Turn the room thermostat up and stand by the valve. A healthy valve makes a soft whirring for a few seconds as the motor drives it open. Silence means the motor is not being told to run, or has failed. A continuous buzz or hum with no movement means the motor is running against a seized body.
- Feel the head.A head that is warm to the touch after the system has been calling for a while, with nothing having moved, is a motor stalled against a seized valve. This is the classic failure in an old system full of sludge.
- Feel the pipes either side of the valve body.Both hot means the valve is open and passing. One hot, one cold means it is shut. That tells you the mechanical state, which combined with what the controls are asking for tells you whether the valve is doing what it was told.
- Try the manual lever deliberately.Slide it to MAN. If heat now reaches the circuit, the valve body is free and the fault is electrical — a dead motor, or the head not being told to open. If nothing happens even with the lever latched, the body itself is seized. Either way, return it to AUTO before you finish.
- Test the microswitch case.If the valve opens correctly, both pipes go hot, but the boiler and pump never fire, the auxiliary switch inside the head has failed. The valve is moving but it is not telling the boiler it has moved. This produces heating that is 'on' but never actually gets hot.
- Hand over with a good description.Tell the engineer: whether the head is warm, whether the motor is audible, whether the manual lever produces heat, and whether the pipes either side go hot. That is enough for them to arrive with the right part.
Tips
- The head unclips from the body on nearly every make. That is why a failed motor is a cheap fix and a seized body is not — one needs no water drained and the other needs the whole system down.
- Sludge is the main killer of valve bodies. A system with a magnetic filter and inhibitor gets many more years out of its valves.
- Label the valves. In a two-zone S-plan house, knowing which head is upstairs and which is the cylinder saves a lot of guessing.
- If you use the manual lever as a stopgap over a cold weekend, put a note on the programmer so nobody wonders why the heating ignores it.
If it doesn't work
Heating runs constantly and the programmer makes no difference
Cause: The valve has stuck open, or the manual lever is latched in MAN — Fix: Check the lever first. If it is in AUTO and the valve is still open, the head or the body has failed. As a stopgap, switching the boiler off at its own switch stops the heat, though it stops the hot water too.
Hot water works, radiators never get hot
Cause: On S-plan, the heating valve has failed shut. On Y-plan, the valve is stuck in the hot water position — Fix: The manual lever will confirm — if latching it open produces hot radiators, the body is fine and the head is at fault.
Head hums or buzzes but nothing happens
Cause: The motor is running against a valve body seized with sludge and scale — Fix: The body needs replacing, which means draining the system down. Worth doing a system clean at the same time, since sludge is what seized it.
Valve opens, pipes go hot, boiler never fires
Cause: The auxiliary microswitch in the head has failed, so nothing tells the boiler to start — Fix: A head replacement. The body is fine and the water never has to be drained.
Radiators go lukewarm every time the hot water heats
Cause: Normal Y-plan or W-plan behaviour, where hot water takes priority — Fix: Not a fault. If it is a real nuisance, converting to S-plan is the answer, and that is a rewire and repipe rather than an adjustment.
Valve clicks repeatedly on and off
Cause: A control calling intermittently, or a failing switch chattering — Fix: Note which control is calling when it happens. Repeated cycling wears the motor quickly, so it is worth attending to rather than tolerating.
Questions people ask
Can I replace the head myself?
No. It is a fixed mains-wired component and the connections are made in the wiring centre. Replacing it is electrical work. The good news is that it is quick work for someone qualified, because the water side is untouched.
Is it safe to leave the lever in MAN over a weekend?
Mechanically yes, and it is a normal stopgap. But that circuit is now permanently open, so the boiler heats it whenever it fires and the thermostat has no control over it. Use it to get heat, not as a long-term arrangement.
Why do these fail so often?
They are small motors driving a valve through water that in an untreated system carries abrasive black magnetite sludge. Keeping the system water clean is the single biggest thing that extends their life.