Oil central heating issue, Radiator actuator keeps switching off!!

Eoincruiser · 2026-02-03T17:13:44+0000

Hi,
I have an oil central heating system. The burner fires and heats the water no problem, but when i want to heat the radiators the actuator that controls the radiator automatically switches to the off position, If I manually turn the wheel to the on position it will stay on for a few seconds then again it will switch off. If i take the actuator off and turn the value on manually and turn the hot water on the burner will go and it will heat all radiators as long as the water needs heated. Wondering could anyone point me in the direction of what the issue could be? I have attached pictures of the actuators and also the thermostat which is in the hall of the house.

Thanks in advance

ericmark · 2026-02-03T17:48:29+0000

It is called a motorised valve, it has a micro switch inside, so the boiler will only run after it has opened.

I have tried reading the description, but can't work out if valve at fault, or what supplies the valve (thermostat and programmer) what tools do you have to test with?

Eoincruiser · 2026-02-03T18:04:01+0000

Thank you for your reply, sorry i wasn't to clear in my description. I am competent with electrics and have a multimeter there and most tools if there is anyway you can suggest to find the fault? I should say i have already replaced the programmer and have the same issue.

oilhead · 2026-02-03T19:52:38+0000

As I see it, the top connedtion is a switched from your room thermostat the 2nd one is neutral, the third is a switched to boiler and pump whilsthe bottom is a permanent live. If that is how it is wired, then I would be inclined to suspect the actuator. The only other complication that may affect it is if the permanent live is commoned with the switch live. Assuming the hot water valve is the same model, could you swap them over and see if the fault follows it.

Eoincruiser · 2026-02-03T20:20:01+0000

oilhead said:
As I see it, the top connedtion is a switched from your room thermostat the 2nd one is neutral, the third is a switched to boiler and pump whilsthe bottom is a permanent live. If that is how it is wired, then I would be inclined to suspect the actuator. The only other complication that may affect it is if the permanent live is commoned with the switch live. Assuming the hot water valve is the same model, could you swap them over and see if the fault follows it.

Hi, thank you for the suggestion. The hot water and radiator values are the exact same model so let me swap them around and ill get back to you shortly. Appreciate the reply

Eoincruiser · 2026-02-03T20:47:07+0000

Ok, I have swapped over the actuators. So the situation is still the same were the water is working as it should but the heating value keeps turning itself off even after i manually move the wheel to open. Any other suggestions. Thanks again

Eoincruiser · 2026-02-03T21:00:46+0000

Another update. I tested the switched live for the radiator value with the heating turned on, putting the thermostat up full I am still getting zero volts. This leads me to think the thermostat is faulty?? Can anyone recommend a direct replacement for the sunvic TLX 2356 thermostat as they are no longer made?

denso13 · 2026-02-03T23:26:12+0000

Can you link the live and switched live in the stat and then check if the actuator works. That would rule in/out the room stat.

Eoincruiser · 2026-02-03T23:33:17+0000

denso13 said:
Can you link the live and switched live in the stat and then check if the actuator works. That would rule in/out the room stat.

Thanks for the reply, I will try this in the morning and update here. Thank you!

terryplumb · 2026-02-03T23:37:57+0000

Did the system work ok previously ,and your current problem is a recent occurrence?

Eoincruiser · 2026-02-04T00:38:17+0000

terryplumb said:
Did the system work ok previously ,and your current problem is a recent occurrence?

Yes it was working, then working sometimes, now not working at all

Harry Bloomfield · 2026-02-04T07:30:13+0000

ericmark said:
It is called a motorised valve, it has a micro switch inside, so the boiler will only run after it has opened.

It's a MOMO actuator, Eric. The OP's description, sounds a little confused, but I would suspect the actuator....

If both of those actuators are identical models, I would try swapping one actuator, for the other one, as a quick way to eliminate that. Failing that, then the actuator is simply not getting the signal to open.

terryplumb · 2026-02-04T07:51:01+0000

The OP has already tried swapping the valves,see post #6 Harry.
Sounds like a wiring issue or dodgy stat.

polesapart · 2026-02-04T08:01:12+0000

https://www.mytub.co.uk/product_images/Sunvic_SZMV23205_Installation.pdf

may help in fault-finding

Motor on/motor off valve FWIW.

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Oil central heating issue, Radiator actuator keeps switching off!!

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