Replacing water main with MDPE pipe - will it help my pressure? (reprise!)

[Lobster]

Hi all

I posted a query here a few months ago asking whether it was likely to be worth my while biting the bullet and replacing the old incoming water main with a modern wide-bore MDPE affair, specifically to prevent my shower pressure from being so badly affected by other water outlets in the house (it's all fully mains pressure, with an unvented HW system). I was advised that the best course of action was to take some water pressure measurements close to the stopcock, which should help diagnose the location of the pressure drop (see original thread below:)

I've got around to buying a gauge, and have put it to use. There's a threaded outside tap very close to the stopcock, so I was able to attach the gauge there.

Here's some data:

a) Static pressure, no water flowing: 3.9 bar (Flow rate measured at kitchen sink, no other water running: 20 L/min)

b) Shower running (nothing else): 3.8 bar

c) Shower running, plus bathroom sink tap and toilet flushing - causes severe reduction in shower flow rate: 2.7 bar

d) Every tap/shower in the house running - causes shower to stop completely: 1.8 bar

All pressures measure at the outside tap.

Now, I'm still not clear whether this helps. Presumably, if the pressure had stayed at 3.9 bar under all 4 test conditions, that would have told me that my incoming pressure was OK, and that the restriction was inside the house, right? But these results would seem to be intermediate, so I'm not sure how much benefit there would be if I went ahead and had the new MDPE pipe fitted?

Any thoughts, or any other diagnostic measurements I should make? :-)

Thanks David

[Ian Stirling]

Right. So, (approximately) if you were to cut the pipe immediately after the stopcock, it'd read 0 bar. With every tap/... in the house running, about half the pressure loss is in the inlet, half in the house. Increasing the bore of the pipes in the house, and the taps will significantly increase your flow. Is the house one or two stories?

[Lobster]

Thanks. So are you saying that you reckon increasing the bore of the pipes inside the house would have the same effect as renewing the incoming pipe, and that presumably these effects would be additive? Neither would be very easily accomplished, all the same, mainly due to the geometry of the house. It's the shower that's the issue, so which pipes should I be looking at inside: between HW cylinder and shower? Stopcock and HW cylinder? Stopcock and shower? All the above?(!)

Two.

Thanks David

[Ian Stirling]

Ok. Part of the problem is that it's two stories, the shower will see at best

0.4 bar less than a tap in the lower story.

Does the heating come from a combi boiler, or do you have a header tank?

Another test - possibly not needing the boiler. Does a tap downstairs cause the same problem as an equal volume of water/ minute drawn from an upstairs tap?

If not, this might indicate that there is some constriction in the pipe upstairs.

[Doctor Evil]

He said he had an unvented cylinder.

Before digging the ground up, I would get the pipes inside right first.

1. Install a full bore stop c*ck.
2. Have a dedicated 22mm pipe directly to the unvented cylinder.
3. Tee off at the stop c*ck and run to all cold supplies in the house on a dedicated cold pipe.
4. Take the cold of the shower from the cylinder cold feed line, just before it enters the cylinder.

If that sorts it then all is fine. If there is still problems replace the mains pipe. Either way the internal pipes need sorting, so get them done first.

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[James Salisbury]

Check the isolation valves bewteen the pressure reduction valve and the cylinder, sometimes these are not full bore, or are part turned off. Try checking that that toilets and the cold side of the kitchen sink are not fed from the pressure reduction valve. Check the strainer in the pressure control valve is it furred up.

[Lobster]

dedicated cold pipe.

enters the cylinder.

I've been looking at this again, and TBH the big problem is that I'd just about have to rip the house apart to make all the changes you suggest... eg must be about 17 metres from the stop c*ck to the HW tank; that pipe is buried behind tiled boxing, under carpeted floors, goes up and down levels, etc etc, all of which I'm sure contributes to the pressure problem. I can't begin to imagine how much work it would take to put it all right.

What it boils down to is, "is the problem severe enough to warrant all that upheaval", and the answer has to be "no"!

One reason this issue has come now is that we're probably going to get our drive done this summer, so now is the time to be digging it all up to fit a new MDPE pipe out to the boundary. It certainly won't happen after the new drive is laid.

So perversely, maybe it's the external pipework I'm looking at maybe changing. Given the pressure readings in my last post, which seem to me to indicate a 50:50 split between external and internal, would it still be worthwhile changing only the outside pipework?

Lost me here... where would the pressure reduction valve be, and why would I have one/need one, especially if I have a low pressure problem?

Thanks David

[Doctor Evil]

on a dedicated cold pipe.

before it enters the cylinder.

David, you said you had an unvented cylinder, so one will be there.

In your case as you are having the drive ripped up, get a new mains pipe run in and see how that goes. If a long run then use 32mm. They may reduce it and connect to 25mm at the street stop c*ck, but the resistance to pressure/flow will be far less. And have a "full bore" stop c*ck in the house.

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[Lobster]

I noticed you mentioned fitting a 'full bore' stop c*ck again elsewhere today... by that do you just mean a stopcock with 22mm output as opposed to 15mm, (like

or is the 'full bore' descriptor refering to something else?

Thanks David