Thanks again, (sorry about the diagram-it still might not be clear but
hopefully lines up and clearer than trying to put into words)


BleedValve
I
HWCyl-----TRV-----------------I-------------I
I I
I I
______I MV
I I I
I I I---ToRads
gV1 I
I I
I I----------
HWCyl--- V1-------------I I
I I
I Pump VentPipe
_____I I I
I I I-----------------I
I I I
I I I FeedfromExpTank
I I I I
I I I------------------------I
I I I
I I
Boiler From
Return Boiler



The system is about 20 years old (just bought the house).
The rads do get hot when CH (& CH+HW) set to on.
The Gatevalve (gV1) is fully open. I did actually close it fully in
trepidation (but reasoned the boiler shouldnt explode fingers crossed.
Still the same- luke warm at HWCyl - V1 , the boiler forced up hot
water through the vent pipe and stopped, so at least that bits
working!)
I am wondering could there be air in this boiler feed / boiler
return / cylinder coil circuit that would cause this problem so that
the pump is unable to pump effectively to & through coil.
How probable is it that valve (V1) is partially blocked, or God forbid
the cylinder coil itself.
Or could the pump itself not be pumping at a high enough pressure.
What does it mean if there is still no hot water coming out of coil
even when the Gatevalve (gV1) is fully closed.

The CH overhaul sounds like a good idea, I just wish it was an easy (&
quick) job.
Thanks
Steve