In an earlier contribution to this discussion,
Andrew Beck wrote:

It was British Gas that fitted the Combi for the previous owner.

The boilers manual says minimum flow rate for the boiler is 2.7 l/min
The mains water pressure for maximum flow rate is 0.8 bar

I know the pressure of the mains above 2.5 bar (as it can fill the
closed heating circuit to this pressure)

Andrew


Yes, but I fear that you're confusing static pressure with pressure when
there's a significant flow. From what you said earlier, the flow to the
combi more or less stops when you turn cold taps on. This indicates that,
when water is flowing elsewhere, the pressure drops to a level which is
insufficient for the combi - presumably to below the required 0.8 bar. This
is almost certainly because of restrictions in the supply line - which will
cause a pressure drop which is dependent on flow rate.

To illustrate with a guesstimate example: Attach pressure gauge to pipework
just above the stop-cock. Static pressure, with all taps off: 2.6 bar. Turn
kitcken tap full on - giving 10 litres per minute, or whatever. Pressure now
0.6 bar - because a flow of 10 litres per minute causes a pressure drop of
2.0 bar down your 15mm supply pipe. [These aren't the actual figures, of
course - but I have a shrewd suspicion that they ain't that far out].
--
Cheers,
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