CH hot, DH cold

[doghair]

Gravity primary, pump secondary(CH)
CH on gravity not working. When the CH is on the radiator suck air in when bleeding.
CH off gravity works fine. When CH off radiators bleed as normal.

 

[weareleeds]

Trying to understand your post I believe your pump if fitted on flow pipe & is pumping with a negative system pressure so only bleed rads with pump turned off & turn heating on about an hour after water so you have tank full.Or convert system to fully pumped @ around £700

 

[doitall]

The secondary circuit is a DHW return loop and not the heating.

other than that as weareleeds said

 

[doghair]

What i am trying to say is:-

the gravity feed to the domestic water system doesn't work when the pump is running. when pump is off it works fine. Once the pump is activated the gravity feed loses its flow. I can crack and remove the pipes from the cistern with no spillage!

The pump is pumping to the boiler not from it. The system has been working fine for the past 19 years. Solid fuel boiler for 15 years and oil boiler for the past 4 years

I've checked:- pump, feed and expansion pipes, I've even had mains pressure into the system to force any air out (water flowing out of the expansion pipe)

Could it be a build up of sludge somewhere in the system?

I'm at a bit of a loss.