Question about mains water pressure/connection

Stu666 · 10 Mar 2026

Looking for some advice please:

Around 7 years ago we had a leaking old lead water supply pipe replaced. Due to the layout of the house and to avoid digging up what was then a brand new tarmac driveway, the water supply was relocated to the other side of the house, where there was an existing outside tap fed by 15mm copper pipe. Water pressure here seems quite good, indeed a rudimentary test with a bucket and stopwatch reveals a flow rate of about 18 - 20 litres/minute. However, beyond this point, things start going a bit downhill. The copper pipe from outside tap travels up into loft space above our kitchen, where it branches off into the rest of the house. Flow rate at the kitchen tap is about 8.5 litres/min, yet paradoxically the bath tap upstairs is about 11 L/M. Does this seem right? We have a thermostatic shower running off a new boiler with 30KW hot water output, but flow rate has never been amazing.

Secondly, I was checking for leaks recently and opened up the chocbox for the first time. I was quite surprised to find the water supply is coming up through soil in 15mm speedfit pipe before connecting to 15mm copper pipe. I definitely remember the contractor laying blue MDPE pipe, so I suspect they have teed the MDPE pipe with a three way connector, the third outlet serving the outside tap. I am planning to dig down to check for sure. But something is telling me regular white speedfit pipe buried in soil (and possibly a speedfit type connector) probably isn't right?

Madrab · 10 Mar 2026

Stu666 said:
chocbox

Only ever heartd that for electrics??

No - the water mains should not be coming up out of the ground in 15mm speedfit. If you are getting 18-20L/Min at the outside tap then it shouldn't really be losing half that by the time it gets to the bath unless the dynamic pressure is poor? Do you know what that is?

That being said too - if it's all 15mm then if the dynamic pressure isn't the best then yes, that could impact the flow by the time the water has run all the way around the house.

Wayners · 10 Mar 2026

Stu666 · 10 Mar 2026

Madrab said:
Only ever heartd that for electrics??

Apologies, I think the proper term is an INSUduct box. But I could've sworn overhearing someone calling it a chocbox as it looks like a giant piece of chocolate

Stu666 · 10 Mar 2026

This is what I found after taking the box off. Planning to take a flag up this weekend and see what's going on below.

Madrab · 10 Mar 2026

Ah yep, an Insuduct, not hear it called a chocbox before, though I do see why.

Oh my, nope that isn't right at all if that's your mains supply. I hope that 15mm plastic just runs to the outside tap and the mains is fed in somewhere else. Where is your mains stop tap?

Stu666 · 10 Mar 2026

Madrab said:
Ah yep, an Insuduct, not hear it called a chocbox before, though I do see why.

Oh my, nope that isn't right at all if that's your mains supply. I hope that 15mm plastic just runs to the outside tap and the mains is fed in somewhere else. Where is your mains stop tap?

The mains stop tap is on the other side of that wall in the kitchen loft space (single story extension). It was originally the stop tap to the outside tap. It's difficult to see from the photo, but that is the only entry point in the wall. There is nothing below it, and from memory they didn't drill any new holes at the time, literally connected up to what was already there. They wouldn't have been able to get to the other side anyway, due to fitted kitchen cabinets. There was definitely blue MDPE pipe as I remember helping them dig the trench and saw them lay it. But I didn't pay attention when they were connecting it all up. Definitely suspect they have teed MDPE to speedfit underground, but will need to excavate to check for sure.

Question is, what are the ramifications of this work, and how to best put it right? I'm pretty handy with Speedfit myself having plumbed a few new radiators in with it, but no experience with water mains. I know how to shut the supply off at the street though through bitter experience

denso13 · 10 Mar 2026

Stu666 said:
Question is, what are the ramifications of this work, and how to best put it right?

Open it up first, to see what you are dealing with.

Hugh Jaleak · 11 Mar 2026

I think you'll need to excavate around the base of the Insuduct, and see if you can find the end of the Blue MDPE, if you got that, (then ensure the supply does shut off at the street end!), and remove what you have, and replace with MDPE into the Building at least.

Insuduct offer a 'Hockey Stick', pre made 'L' shape pipe to fit into the duct and get you through the wall, once inside reconnect to the internal Pipework, and I'd run a new feed back out for the Outside Tap.

Stu666 · Sunday at 10:09 AM

Morning all, finally found the time to dig out, and as expected speedfit has been used underground. Thoughts?

Stu666 · Sunday at 10:27 AM

I'm thinking I will probably put a hockey stick in as suggested by Hugh to get it into the house. Beyond this though, would there be any benefit in taking the 25mm MDPE or even 22mm speedfit all the way to near the boiler? All of the existing pipework is 15mm as is the boiler. Or should I just stick with 15mm?

Madrab · Sunday at 10:54 AM

Oh dear, what a waste of time doing that mains replacement work just to jam 15mm speedfit and couplers onto it, that aren't actually designed to be used underground like that. So yep, that all needs to be replaced and as was suggested using the hockey stick pipe plus a proper MDPE coupler onto the mains should sort that all out and then bring that up and into the house onto a new stop tap.

Without knowing the dynamic water pressure and flow it's hard to suggest if increasing the pipework diameter internally will make much difference but certainly up to the stop tap to ensure that is all correct and then see if things improve, would be advised.

Stu666 · Monday at 11:15 AM

Madrab said:
Oh dear, what a waste of time doing that mains replacement work just to jam 15mm speedfit and couplers onto it, that aren't actually designed to be used underground like that. So yep, that all needs to be replaced and as was suggested using the hockey stick pipe plus a proper MDPE coupler onto the mains should sort that all out and then bring that up and into the house onto a new stop tap.

Without knowing the dynamic water pressure and flow it's hard to suggest if increasing the pipework diameter internally will make much difference but certainly up to the stop tap to ensure that is all correct and then see if things improve, would be advised.

Many thanks for the advice, I have a pressure gauge arriving this afternoon and will take some readings to determine the existing mains pressure.

Planning ahead, I think I will probably create a new entry hole for the new MDPE section, alongside the existing 15mm copper pipe. As mentioned previously, this 15mm pipe was originally just serving the outside tap and was repurposed as the mains entry. I am planning to convert this back to how it was previously, and run a new 22mm pipe from the new MDPE stoptap and connect this to the system somewhere in the kitchen. There is an easily accessible 15mm junction in the kitchen loft, which splits the current incoming mains (old outside tap), upstairs taps and boiler/kitchen taps. My thoughts are to tee the new 22mm somewhere near the middle of this junction, but I am wondering if there would be any benefit in continuing it down to the boiler/kitchen taps instead and teeing it into 15mm around here, or would it make no difference?

Madrab · Monday at 12:43 PM

Increasing the system pipework can increase flow, it will have little or no effect on pressure. If the dynamic pressure is low <=1bar then increasing to 22mm will usually have a more noticeable effect on flow, if the dynamic pressure is higher =>2bar then increasing the pipe size will have less of an effect. It's all to do with resistance being higher in a 15mm pipe as against a 22mm pipe

Testing the dynamic flow and pressure would be - connect the gauge to the outside tap and run 2 outlets internally and see how the pressure reacts, that will give a dynamic pressure reading. Whilst doing that, time filling a container over a minute @ full flow, ideally at a tap that doesn't have an aerator or if it does take it out, then measure how much water was delivered, that'll be your dynamic flow figure.

Stu666 · Monday at 4:31 PM

Madrab said:
Increasing the system pipework can increase flow, it will have little or no effect on pressure. If the dynamic pressure is low <=1bar then increasing to 22mm will usually have a more noticeable effect on flow, if the dynamic pressure is higher =>2bar then increasing the pipe size will have less of an effect. It's all to do with resistance being higher in a 15mm pipe as against a 22mm pipe

Testing the dynamic flow and pressure would be - connect the gauge to the outside tap and run 2 outlets internally and see how the pressure reacts, that will give a dynamic pressure reading. Whilst doing that, time filling a container over a minute @ full flow, ideally at a tap that doesn't have an aerator or if it does take it out, then measure how much water was delivered, that'll be your dynamic flow figure.

Pressure gauge has arrived and I have some results:

Static pressure at outside tap = 2 bar

Dynamic pressure (2 outlets fully open) = 3 bar

Dynamic flow = 9.75 litres/minute.

I wasn't expecting the dynamic pressure to increase, so I did it twice in case I was going crackers, results the same both times. I did notice the outside tap appears to be leaking a little bit from the gland when its on, whether this would have any effect on the pressure, I don't know.

diynot

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Question about mains water pressure/connection

Similar threads