pump is running, radiators are cold!

I have an interesting situation with my CH system (please forgive lack of proper terminology - CH novice!!)

my system has one pump immediately before what looks to me like a 3 port valve.

1) When the hotwater cylinder thermostat "demands" hot water, the pump comes on, a minute or so later the pipe warms up and heats the water in the cylinder - as expected

2) When both the hotwater cyl thermostat and the room thermostat in the hall downstairs demand hot water, the pump comes on, all the surrounding pipes warm up, and all the radiators and hw cylinder heats up - again, I'd think this is pretty normal

3) But... when ONLY the room thermostat demands hot water, the pump comes on, but it does not switch on the boiler so only cold water circulates in the radiators. If I turn both thermostats up and let it run for a while, then turn the cylinder thermo down, the pipes stay warm around the pump (which stays running) but gradually the pipes and radiators cool down.

any ideas? both thermo's are rather old satchwells, I forget the make of the valve and pump but can go look and post if it'll help troubleshooting.

The wire from the valve which turns the boiler on (orange) in CH position is connected straight to the pump for older boilers. For newer ones the pump is controlled by the boiler. Ergo your system is wired wrongly.

VERY common!

We had something similar with our boiler, but in this case the pump didn't come on either. It was due to a valve that was up the duff. We had to run the bath to stop the water reaching temp making the heating go off ! Took 3 different pumbers to eventually suss it.

Thanks for the prompt feedback!

ChrisR said:

Ergo your system is wired wrongly.

VERY common!

Click to expand...

Could this be fixed easily with a bit of DIY rewiring - or best left to a professional?