S plan heating system with no HW. Occasionally motorized valve operates and boiler lights for approx 1 min.

[grahamhopper]

I wonder if anyone could offer any help with diagnosing the following problem.

I have an S plan heating system. A couple of days ago the hot water side stopped working. The central heating does work so there is no problem with the boiler. Currently, the only way I have of getting hot water is to run the central heating side with the HW motorized valve opened manually.

Additionally, very occasionally turning the hot water on at the controller, will start the boiler, but only for a short period of time.

After much research online it seems that the most likely fault would be a faulty motorized valve or less likely a faulty thermostat. Off to Screwfix for replacements for both. Took the cover off the existing motorised valve and tried the hot water again. Luckily this was one time that the system started. I was able to see the valve turn as expected and the boiler light. However, as before it turned off after a short time. So it looked to me as if the valve was working correctly so I decided to change the thermostat as I thought this might be working intermittently ie long enough to turn turn the valve and then turn off shortly afterwards.

Unfortunately, changing the thermostat didn't help.

Assuming that the valve is working correctly I think I am left with a couple of potential faults:

1) The cable connecting the thermostat to the connection block is damaged and moving it occasionally connects and then disconnects.
2) A possible fault with the controller that is occasionally powering the HW circuit for short periods.

So I have a number of questions:

1) What would have to happen for the valve to open and close over a short period. Do you think I am correct in assuming that the valve is working correctly.
2) Is there anything I might be missing?


Thanks,

Graham

 

[Johntheo5]

Follow the Mot.Valve wiring back to the nearest JB, connect your M.meter (set to 500V AC) between brown & N and switch in the HW, watch voltage, does it switch off after a short time?

If it switches off, go to the cylinder stat and test for 230V between ter.1 & N (or earth), keep moving back progressively, should be able to track down that fault.

 

[Murdochcat]

Motorised valves are notoriously unreliable, tank stats last for a very long time

 

[fixitflav]

Motorised valves are notoriously unreliable, tank stats last for a very long time

Sounds like the valve to me. Maybe the actuator has come adrift from the valve mechanism, so the microswitch closes, but the valve doesn't open. Then the boiler stops on no-flow.

 

[RandomGrinch]

Additionally, very occasionally turning the hot water on at the controller, will start the boiler, but only for a short period of time.

Although as has been said above, a faulty valve is likely, we have seen this specific behaviour before, from a faulty controller.
A controller fault is easy to rule out - either test with a multimeter as @Johntheo5 suggests, or bridge out the controller.
If you could post a picture of the wiring behind the controller, we can show you how to do this.

 

[grahamhopper]

Thanks for all the advice.

Took the simple option and tried changing the valve which seems to have solved the problem.