He did that already, but if an airlock and a partly closed valve was present
the pressure would not shift the aire in a pipe with a sag in it. It
happened to me some years ago.
Brian

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This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

Blind user, so no pictures please!
"Tricky Dicky" wrote in message
...
Used to get a similar problem at our old house after draining down the DHW
due to partial air locks when refilling The simplest solution was to connect
the washing machine hose cold feed to hot feed and give it a quick blast at
mains pressure until water came out of the main tank overflow, problem
solved.

Richard