Upgrading water main to 32mm: the diameter of water meter and upstream pipe

5 Sep 2016
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United Kingdom

I'm thinking of having the the 20mm plastic pipe between the water meter and my house replaced with a 32mm MDPE pipe by a WRAS-accredited moling plumber because the shower temperature goes wild when someone fills a kettle or flushes a toilet.

The house has a gas combi boiler and a mains pressure electric shower and no tanks inside the house. We may want to install a second shower, this time from the combi.

The pipe work has 2.2 bar static pressure, reducing to 1.5 bar with 1 tap open and 1.0 bar with 2 taps open. The water meter 7m outside in the pavement has "15mm" printed in its specification.

It seems to me that I could spend £600-800 upgrading the pipe to the water meter but still have a flow rate problem because the meter and/or pipe upstream of the meter are too narrow. My guess is upgrading the meter and upstream pipe will not be cheap.

My questions: is the narrow bore of the meter something to worry about and if so what's the best way to get a decent flow rate for 2 simultaneous showers without spending more than £1000, say? I prefer to live with the problem or ask people not to use both showers at the same time rather than overspend.


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so what's the best way to get a decent flow rate for 2 simultaneous showers without spending more than £1000, say?

A cold water storage tank ( or cistern ) in the loft and a hot water cylinder is one way to achieve that. For a while you can use water from the storage tank faster than the mains can supply water.
I think the bottom line may well be to live with the problem, unless you are prepared to spend a lot of money. Without an accurate reading of available pressures and flow rates at the main, upgrading the entire service may be fruitless if the main is unable to achieve anything better than currently offered.

Pointless IMO replacing part of the supply, if the meter is for a 15mm supply, the restriction will always remain unless the meter and pipework upstream to the connection on the main are also upgraded.
Maybe worth looking at an 'Accumulator', some of the guys more experienced with those maybe able to advise further.
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A 20mm line should supply more than sufficient water. Are you certain the service stop valves are fully open. At what point in your system are you measuring the pressure. Bear in mind that the nearer to an discharging valve you get, the lower will the pressure read, since the only thing creating back pressure is the length of pipe from the point of measurement and the internals of the valve.
No doubt there's some way to calculate the effect of a short constriction in an otherwise larger pipe. The watermain in the road will doubtless be larger than 15mm.

Plastic also has the advantage that it probably has no elbows in it, just sweeping curves.

When I replaced an old lead 1/2" pipe with 20mm MDPE I got a substantial increase in flow, even though there was a short length of 1/2" pipe and a 1/2" stopcock on the old pipe at each end. I happened to have bought the roll already for a planned garden tap, otherwise I would have used 25mm.

I suppose if you are prosperous you could ask the water co for a price to upgrade the meter.
With most of these things an experienced person can make and educated guess at what to expect.

But the exact figures are only measurable on test.

My advice would be to fit the 32 mm tube and see what you get and then consider if its worth spending any more money.

At present all the internal pipework is in 15mm, off one long "tree trunk" shape from the stopcock. I'm thinking (either instead of or in addition to the 32mm pipe) of going to 22 or 28mm downstream of the stopcock and having separate direct pipes in 22 then 15mm to the combi and to the electric shower and putting a pressure reducing valve at 1.0 bar near the stopcock on the reduced 15mm "tree" so that all the remaining valves on the "tree" are disadvantaged.

Will this plan make much of a difference?
Probably not much.

No point in more than 15 mm to the electric shower.

The whole point is to transfer a good pressure to the house distribution.

No point in more than 15 mm after the combi. But some merit in a radial 15 mm from combi outlet to bath, shower and kitchen etc.

You have not mentioned the length of 20 mm supply pipe. But outside pipe is easy to replace.


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