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Radiators upstairs hot, as normal.
Radiators downstairs cold.
Up and down on the same circuit controlled by a 3 way valve (for the hot water as well).
Any clues.
TRV’s turned down or stuck.
No TRV's.
OK this morning.
Not OK this afternoon.
Airlock?
Warm water rises, so how long has the system been running? What's the system pressure if it's an indirect closed loop? Have you bled the rads yet?
Pump stopped working.
Could be your pump that has failed. Should be able to feel it vibrate slightly if its working.
Edit. Beaten to it!
if it is a stuck pump then they can often be restarted - there is usually a big silver screw in the face of the pump - remove it ( a bit of water will come out) and then you can access the pump spindle to give it a flick - sometimes gets it running again for a short time
Air in the pump will stop it - as above, undo the screw in the middle of the pump.
Also turn off all upper rads and see if heat circulates downstairs - air lock.
The main suspect is the pump.
I increased the main thermostat to 'call the boiler on'. Cannot feel any vibration in the pump, nor hear it running.
Co-incidently, the central heating has been a bit 'noisier' than usual just recently. Perhaps that was a signal of the pump giving up.
Will call a pal tomorrow morning (he is a heating engineer).
Thanks chaps.