Potterton Profile wont stop firing

[stompo]

A strange thing has happened to my boiler... I came home to find the radiators were cold. There was call for heat but the main burner was not lit. The only way to get the boiler to fire was to turn the boiler thermostat down to zero. Turning it up even just slightly would turn off the burner.

I left the boiler on for a while and when it had warmed up there was no way to turn it off using the thermostat control. I switched off the electric and now I notice that the pilot light is still lit.

Any ideas??

[stompo]

OK i've had a further look and the thermostat appears to be working in that as the water temperature increases, so does the switching point of the thermostat. The problem is that its working 'upside down'. When the stat is set below the switch point the boiler runs, when set above, the boiler stops.

The thermostat reads short circuit when the setting is below the boiler temperature and goes open when set higher. This seems to me to be the wrong way round?

-- stompo

[Roger Mills]

Very odd! Could anyone have disturbed the wiring to the stat? Assuming it to have changeover contacts, it sounds as if the output is connected to the wrong contact - NC rather than NO (or vice versa).

[stompo]

Quite! The stat is the boilers own temperature stat and nobody has played around with that. It only has two wires anyway. I wonder if the sense of the stat is in fact correct but something is wrong with the main PCB?

[Roger Mills]

Sounds possible - any ideas, Geoff?

[Ed Sirett]

Indeed it does. It suggests a fault in the thermostat. The new unit is sub

20 quid the only gotcha is the general trickiness of threading the new sensor phial into the right place (which is clipped to the flow pipe and coated in thermal grease).

Presumably if it had gone on for any length of time the OH stat might have worked? or the house got so hot that you turned the external controls off.

[Ed Sirett]

These are fool proof: only two contacts both connectors have white wires.

[Ed Sirett]

I'm 99% sure it is make to fire. The Over heat stat certainly is.

[stompo]

Given that I never have much time (who does!), for the sake of =A320 I've ordered a new thermostat. At least I can quickly try it by just connecting the wires first in case it isn't the problem, usual precautions etc etc.

Would silicon heatsink compound like the white stuff used for power electronics be suitable thermal grease?

I haven't let the boiler reach the OH stat as after half an hour its kettling and gurgling away nd if it turns out thats faulty too....

[Ed Sirett]

Yes. Though you could probably recycle what's already there.

[stompo]

OK so I fitted the new thermostat and it all works fine. I dismantled the old stat and the part of the switch which is usually 'springy' had broken. It would have operated such that if you push down on the springy metal the contact pops up the opposite way but what was happening was when you pushed it down, the contact went down too so 'reversing' the operation of the stat.

Thanks for all your help guys, very much appreciated.

-- Cheers Clive