Pumped Heating Gravity HW Prob

Hey all

Went to a system today, Pumped heating, and Gravity HW, Prob Was, HW heated fine on its own, but the min the pump comes on for the heating the heats drawn away from the Gravity side.

Only fault I can see is the design of the gravity side, Straight rise from boiler for 2-3 meters then 6-7 meter near hrizontal run in loft (with only 1/2 - 1 ft rise in height)

But the system would have worked at some point, tryed the magnet over the gravity pipework didnt stick to a thing.

any ideas, never came across this before

I've cpme across 2 or 3 that the 28mm flow has been so blocked near the vent T that it is in fact reduced to about 10mm

Only found this out whilst removing HW cyl

Can also be caused by a slight air lock.Had it many times boil up the system also make sure header tank full and cold feed not blocked