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im a gas engineer not a plumber, i hate plumbing, too much risk of ruining customers carpets!
sorted it eventually after countless attempts. held a plastic bag over the end of the mixer tap to create a better seal and could immediately hear the water rushing up the pipes, worked first time.
im not kidding you, i connected a hose from the bathroom sink cold tap to the hot and i pumped gallon after gallon of water into the pipework, it wasn't appearing in the header tank and when i turned the hot tap on there wasn't even a trickle, god knows where it was going !
i did fear that there was a blockage in the cold feed or the hot pipework which i wasn't looking forward to sorting out.
the old man who is vastly more experienced at plumbing than me reckons the hot water cylinder empties quicker than the header tank can re-fill it so a solution would be a larger header tank. i have also noticed that whoever installed the system has left a slight incline on the cold feed pipework to the cylinder whch can't be good, the tank coud do with raising up slightly to get a better flow.