Strange radiator issue

[Timsy]

Heating system is Oil Rayburn, 8 rads, gravity fed in Bungalow. The other day I drained the system to flush it then add some boiler quietner, initially flow from drain valve near front door was good but soon slowed to trickle, (stop cock on) hmmmmm, turned stop cock off, shut drain valve and added solution to radiator, turned stop cock on and system wouldn’t fill. Climb into loft and find heating header tank not filling, operate ball cock a couple of times and water fills tank and system. Bleed radiators and turn heating on, get six of the 8 rads working. Rad in bed room bleeds ok but won’t get hot, filled many cups with slightly grubby water, pipes down wall are cold. Connect hose to drain valve below rad, open valve and nothing comes out. If I open the bleed screw and blow into hose air comes out of the bleed hole as well as water. Not a drop comes from the drain valve on the return whatever I do. Help!

[Razor]

You have a blocked cold feed between the feed and expansion tank and the system. If you post pictures where it joins we can probably pinpoint the exact blockage