Roger Mills wrote:
In an earlier contribution to this discussion,
Bart C wrote:


Disconnected this and there is a leak from the inlet service valve of
about 1 drop every 10-15 seconds.

If that's the cause then it's amazing such an innocuous looking drip
should pressurise my CH system.

Yes, but think about it. Four or five drops per minute - how many litres is
that is several weeks? Introducing that amount of extra water will
*certainly* pressurise it!
--

It's not really that many drops entering the system over a period of
time more that the pressure in the two systems will try to equate over
a period of time. If the mains is the same pressure as the "sealed"
system then no water will enter. The reason the filling loop *must* be
disconnected in the opposite problem - if the mains pressure drops
water from your boiler system could be introduced into the mains water.