Boiler runs continuously even when programmer set to OFF

[garyf]

This has just started happening with no explanation. Potterton E50? boiler, fully pumped system. DHW tank and pump in loft with frost stat. The boiler is running and rads warming up even when programmer is set to OFF. Only way to stop it is turn off boiler. With programmer at OFF on both HW and CH, cycling HW through 'Twice', 'Once', 'ON' and back to 'OFF' momentarily switches boiler off as programmer goes to OFF, but then within seconds its back on again. Cycling CH through the same cycle has no effect.
My guess is the Frost stat, as with outside air temps at present I doubt its that cold in my loft. Would I be ok to disconnect the stat at this time of year to see if its the culprit? Any other suggestions? 3 way valve, would that cause the boiler to come on even if programmer is OFF?

 

[simon_d]

sounds like the frost stat. try temporarly bypassing it and see if the problem goes away

 

[garyf]

Thanks Simon. On further examination, problem only seems to happen late at night / early morning, when at coldest part of day? Took a look at frost stat, after taking cover off, its got graduated scales with numbers 5, 10, 15, 20. Number 5 was at the top, indicating the set point of 5 deg C? I moved it round just a fraction and heard the click of the stat operating. The heating then came on. No way is the temp in my loft 6 deg C! More like 20! Looks to me as if stat is faulty? I have disconnected the live wire and taped it up. Now when I move stat around, click still comes at 6 but no heating. I'll leave it that way for a week or so, see what transpires, if no further trouble, replace stat.

 

[Agile]

Probably no need to replace it!

Its just not showing the correct calibration point.

On many of those you can pull off the knob and push it back on in a different rotational position to correspond with the actual temperature of 11 degrees or whatever you think it is in your loft.

Tony

 

[ollski]

Is it a 3 port valve or 2 x 2 ports?

 

[garyf]

3 port valve. I'll give the recalib a go too

 

[garyf]

Went into th eloft yesterday to recalib the stat. It had manged to recal itself to some extent, with the 'click' coming at around 15 deg C. I took off the dial and moved it around as suggsted. Click then coming at about 25 deg. Thermometer on my mobile phone said 27 deg, so near enough for me. Set the dial to 5 deg, reconnected the power, now watching for repeat occurrences.