central heating won't shut down

9 Feb 2006
Reaction score
United Kingdom
We've got a Halstead boiler that feeds both hot water & central heating system, fully pumped with a 3 way valve. A couple of months ago had problems with lack of hot water & have since replaced programmer with a Horstmann Centaurplus C27 & cylinder stat with a Grasslin clamp-on stat. Everything was fine til this evening when I noticed it was getting really hot in the house. Appears that the boiler is not shutting down when the room gets to set temp (using a room thermostat). When I switched both hot water and CH controls to "off" position, boiler and pump continued to run. Only way to stop them was to turn off main switch. Is this likely to be a room stat, valve or boiler problem?!!
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You have an electrical fault of some sort which should be easy enough to sort out.

The first thing I would check would be the switched lives from CH and HW from the programmer. If both turn off, I would suspect the the boiler, if it has a permanent live.
More likely to be a problem with the mid-position valve. Basic electrical checks will identify this but you need someone who understands (or can work out from circuit diagrams) the way the controls work. I think there's more detail on this in the sticky FAQ topic.
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Thanks for the suggestion guys! May need get our mate in who knows about electrics then!!

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