wrote:

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"

Unlikely I would ahve thought. Most combis will have the blow off valve
set at between 3 and 3.5 bar. Mains water will frequently be more than
that.

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.

That is also the reason there must be a double check valve fitted to the
heating circuit so that water can not escape from it even if the tap
were open and the mains pressure dropped below the system pressure.


--
Cheers,

John.

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