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Gas Back Boiler That Heats Hot Water to Tank and Radiators via a Divert Valve but Pump Wasn't Working?

Postby 24grange » Tue Mar 13, 2018 10:55 pm


I've been trying to solve a central heating problem I've got, which so far has taken me two weeks of fiddling, all to no avail. so I'm asking for advice as a last resort !
Gas back boiler, heating hot water to tank and radiators via a divert valve. Pump wasn't working - it was new about 18 months ago and had been run for about 6 hours in total - no power to it, connections all OK. replaced the pump with a new one, refilled the system, bled all the radiators.
Pump now works and turns on when called for by the programmer and/or the thermostat.
back boiler fires up when hot water or central heating called for
pump runs fine
no heat at all to any radiator
divert valve works
If run hot water only, hot water heated up to tepid no matter how long run
If heat water by immersion heater - water is boiling.

So, What else could possibly be wrong?
Whole system was drained and force cleaned out about 18 months ago.
every element in the circuit seems to work fine. Boiler fires up on demand,
divert valve diverts - even tried it on manual no difference
pump starts and works
have bled and checked for air lock 3 or 4 times now - all OK
At a complete loss - any ideas ?

Thanks in advance

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