If you have a combi then unless you reduce the overall pressure in the system this is bound to happen until you have.
If you have a more conventional system with an F&E tank in the roof the feed will keep feeding until it is drained and if you haven't isolated it it will keep feeding until you feel ready to draw pension.
Yes the pressure in flow and return has to be the same and if you open both then it is bound to come out of both.
No you are not draining the "Whole" system but the water has to find a level and if that is still above the valves for the radiator then ..........
Do you think a plumber might be in order for this job?
"Do you think a plumber might be in order for this job?" Lol, yes I do, but the bank balance says no.
I sorted it,
The header tank was knocked off via a bit of pipe under the float so no water was coming through there. what was confusing me was that when draining the hot water storage tank with the downstairs taps it emptied out, so i presumed that it was empty. Obviously the pipes feeding this radiator were much futher down the tank than the pipes feeding the downstairs taps. I think that is what was happening.
Thanks for the replies
BTW Im an ex bricklayer so it's no wonder i don't know what I'm doing :)
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