if the t/stat youre talking about is the room thermostat then the problem youre having is either the room will not get hotter than 20` or the boiler is not putting enough heat to the radiators to heat the room. Either boiler htg temp control should be turned up - more heat in rads or bare in mind this has been one lousy winter, could the house be more draughty, is the boiler at fault with poor heat transfer/pump? could be many a thing and you would have to state boiler make model and system type before you could get a good guess which it would be cos we aint there.
If you suspect stat is culprit? get someone to wire it out and see if that helps.
x
February 2010
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