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I've got a pretty good track record at diagnosing problems i've had with my CH system over the years but this one has me stumped. Its a standard vented gas CH system. The boiler is firing up fine as usual and we've got hot water, but no heat in the rads. I immediately suspected the 3-way motorised valve, but that appears to be OK - all three connected pipes are hot (the supply pipe, the HW cylinder heating loop input and the rad input) but obviously the hot water is not getting through to the rads. The pump is very silent so now i'm suspecting that - but if the pump is knackered then how is it i'm getting hot water? Its been like this for a week or so now since I noticed it, so could have been like this even longer.
Cheers.
Possibly enough gravity circulation on hw circuit.
Pump diminished in power / dirty / sheared impeller
Blockage in ch circuit
Faulty motorised valve just letting enough water through ch port to warm the pipe a few feet.
Swollen ball in valve can cause this although not so common now they used to react with Fernox
Low system water.
Think that's enough to be checking!
Doesn't the pump have a large screw on the front? - undo it (towel/bucket underneath) and you should be able to see if the pump is working.