Replacing water main with MDPE pipe - will it help my pressure?

[Lobster]

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

[Set Square]

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.

[nick smith]

I can definitely confirm that replacing your old main which terminates in 15 mm copper with 25mm MDPE wil very substantially increase your flow rate whatever the pressure is. I recently replaced my old steel incoming pipe which was about the same ID as

15mm copper, and old and internally corroded, with 25mm MDPE and the two ran "side by side" for a while, with valves to allow the house to take via the old or the new.

The max flow rate into the bath with both bath taps running, filling a bucket, was 13 litres a minute on the old incoming pipe and a massive 45 litres a minute on the new ! ! I fact it was more, as a fair amount was spraying around the bucket the force / flow rate was so good! - I had also replumbed everywhere with 22mm copper as far as possible so pressure drop was minimised in the house - anyway, shower now gave a flow rate of some 20 litres a minute which was too much ( on a meter now) so fitted the smaller bore shower hose which dropped it a bit to some 15 litres a minute, which is still good! If you heat water on demand (combi) I would say you also need to consider whether the boiler will cope with two showers running ? Mine was a 105,000 BTU , and I like cooler, high flow showers - flow reduces in winter and / or with hotter showers.

Go for a 25 or 32 mm re-plumb and keep the bore big in the house as far as possible. The water company may charge more for larger pipe sizes !

Nick

[IMM]

expense/hassle,

You need 22mm from the stop c*ck to the cylinder and also a full bore stop c*ck. If either of these is not done, get it done, and then see if the performance improves. the plumber is probably right in the mains being too, small, but you still those two points seen to.

[Broadback]

Unlike Nick I replaced lead pipe with 25mm MPDE, it made no difference that I could tell. I must say I am somewhat confused by flow rates, with MPDE an insert is fitted at the joints to reinforce, these have a relatively small hole compared to the pipe, so surely that is the same as fitting a small pipe?

[John]

SNIP

expense/hassle,

"If" the main under the road is of adequate size then a drop in flow must be related to your supply branch pipework. However if you and your neighbours are being supplied by a somewhat smaller street main then a pressure loss due in part to the resistance of the street main will affect you "and" your close neighbours. Try asking your next door neighbour if turning on your taps can be noticed in the flow rate available in their house too. A practical test may be needed. The most likely scenario is the loss occurs in your suply branch but if you are at the end of a street or live in a terrace then the shared supply may be the problem.