Glow worm fuelsaver 2 - cut out switch reset help

[MalcH]

Had an engineer out last week when pilot light wouldn't stay on. He found the overheat cut out switch had activated but couldn't find anything that might cause this.

All ok until yesterday when heating & water didn't come on. Pilot light was off so re-lit. Stays on but boiler won't fire up. Any suggestions please?

 

[ChrisR]

No obvious connection between the overheatstat/thermocouple bit - which would stop the pilot , and the bits which are failing now.
External controls (timer/stat) provide the "go" signal, then to a transformer (16~25V on load, 25~40V off load) through the boiler stat to the solenoid on the gas valve (orange wires, usually). So with a voltmeter it's easy to diagnose..

 

[MalcH]

Many thanks for the comments. My knowledge is limited but if I understand you correctly there should be a residual voltage when the boiler is off which rises when the timer is activated. There is no voltage across the orange wires in either instance. The timer activates the pump so does this mean the boiler thermostat is the problem or the solenoid? If so is it DIY-able or an engineer call out?

 

[ChrisR]

NOt quite, no!
There should be no voltage on the orange wires when there's no call for heat. If you disconnect them and put your meter across them , you should see between 25 and 40 volts AC when there is a call for heat. With the wires on the solenoid coil that voltage should be between 16 & 25VAC.
If the voltage is "killed" by the solenoid then it has shorted. If the volts aren't getting there look on the transformer.

Having looked it up it's pin C on the transformer , which is also connected to pin 3 on the combined thermostat and pump overrun switch, and pin D in the transformer which goes straight to the solenoid. The other orange wire (they're all orange) goes from pin 6 on the switch to the other pin on the solenoid. When the thermostat clicks it connects pins 3 and 6. Draw it - it'll make sense then!

If you're not seeing any volts at all look for mains across the transformer inputs, terminal block 10(Neutral, blue) and 12(Switched Live, Brown).

Hope that lets you sort it.

Transformer's about £50, Thermostat depends which model, may be £80 ish.

These boilers tend to die of leaking heat exchangers, by the way.

 

[galwag]

If my memory serves me and with ChrisR mentioning the orange wires you have a Mclaren/ITT gas valve. This may be the obvious but then a lot of faults often are - after you relit the pilot did you turn the contol knob on the gas valve to 'on'
Also there is an in line fuse on one of the orange wires

 

[ChrisR]

That would account for it. Blue or white, round knob. Expensive to replace though I saw them cheaper recently.

Fixed a combi the other day; HW worked but ch wouldn't come on. The switch SHOULD have been the first thing I checked.

 

[ChrisR]

.....just noticed it's a Honeywell gas valve, so it isn't that!

 

[galwag]

ChrisR
you are right they did use a honeywell gas valve but that was on the later models determined by serial number.
On the earlier models it wasnt the Mcclaren Itt gas valve as I first thought but it was in fact a Robert Shaw gas valve which also has a turn control knob.
I have a breakdown of both versions on my pc but am unsure how to show them on the forum

 

[masona]

I have a breakdown of both versions on my pc but am unsure how to show them on the forum

What is it are you unsure of,can we help?

 

[galwag]

I thought maybe ChrisR or anybody else interested might like to view breakdown I have of boiler.
I know how to scan image but am unsure how to insert it into here

 

[ChrisR]

Just checked the EPICS part of HRPC site - there are a coupla dozen fuelsavers diags there, ta.

 

[breezer]

I know how to scan image but am unsure how to insert it into here

see this

 

[galwag]

Thanks breezer I'll try that next time.