In an earlier contribution to this discussion,
Lobster wrote:

My house is fully plumbed on high pressure water; ie no cold water
storage tank and an unvented HW cylinder. It provides a pretty flow
of water at the taps/shower, but the shower is definitely affected if
another tap is run or the dishwasher fires up. I'm shortly going to
install a second shower to cope with the demands of a growing family
(ie, a 13-y-o daughter) and am concerned that if both showers are
run together (which would be the whole idea), then neither will be
satisfactory.

The plumber who fitted our unvented system a few years ago did express
concern about our incoming watermain, and said that we might need to
renew it if the pressure was inadequate. I can't remember what bore
it is where it runs underground 25 yards out to the road (but it was
partly exposed at the time, hence the plumber's remark), but what
comes up through the kitchen floor is just 15mm copper).

Anyway, I'm now considering biting the bullet and installing a
'proper' blue MDPE pipe, which means digging up the drive and
considerable expense/hassle, and so not something to be undertaken
lightly, and unless I'm 100% sure it's going to make a decent
difference to the water flow at the two shower heads. So my question
is, is there a way of finding out? How can I be certain that the
bottle-neck is caused by the length of water pipe under the drive,
or elsewhere (eg the yards of 15mm pipework inside the house, or
whether the water pressure out in the road is not that great in the
first place?)

Thanks for any pointers
David

You could install a pressure gauge close to your rising main, and measure
the static pressure with no appliances running. This would be the same as
the pressure out in the street. Then turn on all your taps, and measure it
again. The difference in pressure would represent the pressure drop along
your underground pipe.
--
Cheers,
Set Square
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