In an earlier contribution to this discussion,
kipper wrote:


We have tried driving mains cold water through the system. The pipes
to the taps fill but there is no flow up the water outlet of the
header tank. Instead flow comes through the boiler vent pipe back
into the header tank.

I presume you mean the vent pipe which comes off the top of the hot
cylinder - which provides an easier path for the water than going backwards
through the cylinder and up the feed pipe.

In that case, you'll need to block the vent pipe temporarily while you
back-flush.
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Cheers,
Set Square
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