Cold water feed and heating vent

A simple question, is the 150mm gap between the cold water feed and the heating vent pipe a maximum or a minimum?
Second question, Why?
Richard
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Tricky Dicky wrote:

Max
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so as to minimize the difference in pressure between the two points, to reduce the risk of pumping over or sucking air into the system.
I'm sure this is all in the FAQ (or if it's not it should be).
cheers
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