Motorised valve failure or Boiler problem?

[robin7]

Yesterday, my boiler shut down, and the house went cold.

I have an open-vented system configured as a Y-plan; the boiler is a Potterton Netaheat 16-22 Electronic; the 3-way valve is a Honeywell V4073; there is a Honeywell cylinder stat and there is no room stat; TRVs are fitted on all radiators except those in the bathroom and ensuite, which act as a bypass.

After the boiler had shut down and the house had become cold, I checked the f/e tank which was full enough; and checked the pump which seemed to be spinning ok.

I moved the lever on the Motorised Valve, and heard the boiler ignition click as if the boiler was lighting. I went downstairs to check that the boiler had fired up but it had not; I could hear the fan whirring but the burner had not lit.

After another 10 minutes or so, the burner did light, without any further intervention on my part.

My question is – does this sound like a motorised valve failure which I could probably sort out myself; or a boiler component failure for which I would need to call an RGI?

 

[stem]

Inside the motorised valve is a microswitch that turns on the boiler when central heating only is running. If this switch is faulty the the boiler won't run.

However, what happens with the hot water? When the hot water cylinder is heating, the cylinder thermostat controls the boiler so it would run then, even if the switch in the valve was faulty.

If you are competent with a multimeter, the best way to check is to test the wiring at the boiler. There will be a control signal back from the controls to operate the boiler, this is the one connected to the switch on the motorised valve and the cylinder thermostat. When it’s live, the boiler runs, if you are getting this live signal and the boiler isn’t running, then the fault will be with the boiler, if not it's the external wiring.

Your boiler manual will identify the correct terminal to test (sometimes called SL for switched live) but only do this if you are competent to do so safely.

 

[robin7]

Thanks for that, stem. Still a little confused, however - if the motorised valve is faulty and not sending a switched live to the boiler, how come the electronic ignition and fan are working?

Conversely, if the m/v is working ok and is sending switched live to the boiler, how come the burner isn't firing?

 

[stem]

Hmm... yes good points you make. Maybe when you moved the valve it made contact for a while and then switched off again as you went downstairs, many boilers also have a permanent live to keep some functions running to allow a phased shutdown, perhaps that's what you heard. I don't know the detailed working of the Netaheat sequence, perhaps someone else can advise on that.

Possibly when it did light up later the hot water had just switched on. Unlikely coincidence though I have to admit.

If you or someone else were able to make the checks with a multimeter, if the main supply L&N are OK and the switched live was 'on' and the boiler still isn't working, you would know the boiler is at fault.