MPDE pipe and pressure

[muh3]

Good afternoon all,

I building a posh garnen room with bathroom in the back of my garden, 40m from the house. The house is on a shared water main galvanized pipe and from there it goes through a softener all basically 15mm copper pipes. I was planning to connect the garden room after the softener. I was planning on using 25mm MPDE pipe.

I just had the builder around who is doing the trench and ground work and he said I should use a MPDE pipe as due to the bigger pipe I will have not enough pressure.

Question is I'm i correct in using 25mm pipe or should I be using a smaller pipe, my understanding would be the smaller pipe has more resistance and that would mean less flow, even less pressure.

Thank you.

 

[Madrab]

Pressure isn't really dictate by size of pipe, flow is though. 25/22mm pipe will always flow more than 15mm pipe at a given dynamic pressure. The trouble you have is someone is their mis-directed wisdom has dropped your internal pipework to 15mm, therefore that is the throttle point, that and the softener.

15mm can flow as much as 22mm/25mm pipe as long as the dynamic pressure can drive it but the velocity has to increase with a corresponding increase in noise and friction, which is undesirable.

What you need to know first is the max dynamic flow and pressure at a full flow tap close to the mains, say an outside tap and then at the point you would be branching off to the supply to the outside room to give you an idea of what you have.

 

[muh3]

Hi Rob, if it helps i did connect a 40m run of the 25mm pipe direct to the outside tap (before softener) to check if the flow is good, I managed to fill a 12l water bucket in 50 seconds, so I do have about 15l/minute. I also think that the shared main will not last for ever, the house is from the 50s, so I will probably convert to 25 mains coming in and at that point I probably need a new softener which will be 22mm.

We also tried to connect the same 25mm pipe to the utility room (would be easier to get to), which is far away from the mains and all in 15mm copper that left me with about 6l/minute.

I do understand that if somebody has a shower in the garden room that the main house will probably not have much pressure left for somebody else in the shower.

Question is should I stay with the 25mm pipe or get something smaller.

 

[Madrab]

I do understand that if somebody has a shower in the garden room that the main house will probably not have much pressure left for somebody else in the shower.

If the house mains is first then you'll probably find that if someone opens a tap in the house, the person in the shower is getting cold.

With that drop off in flow I'd suggest that the dynamic pressure isn't the best on such a long run over that pipework. You want to get a pressure gauge, attach it to the outside tap and then open a few taps in the house, that'll give you an idea of the dynamic mains, or get the transporter out to test the mains.

As far as 25mm pipe then no reason not to but you will be unbalancing the system against the house. If you will have that potential draw then the mains would ideally got to 32mm. Knowing the mains dynamic pressure is key to the (edit) decision though.

 

[muh3]

Pressure gauge is ordered. I'll report back tomorrow. Thank you.

 

[muh3]

Hi, 40psi static pressure. 25psi dynamic. I got somebody coming round tomorrow to look at putting a new main in at the same time the shed gets connected. But I don't think they will connect a 32mm. Thanks

 

[Madrab]

You're kinda on the limit really @ 1.5bar dynamic. Ideally you want a min 2.5 bar dynamic especially if you want to the feed a garden room 40M away and not have much of a drop off in either location due to increasing loading units, especially when 2 people are using the shower and then when the washing machine kicks in and then someone else runs another tap then things may be impacted quite significantly. it will run on diversity and as long as it can be managed so everyone knows when things are being used then it can be

May be an idea to put a call into your transporter and get them to check out the district mains to see what may be available to you, that will then dictate if your supply can eventually go to 32mm, if so then it could be worthwhile laying 25mm out to the garden.

 

[muh3]

Thanks for that Rob and if they can only lay a 25mm would that mean best to go with 20mm MPDE?

Cheers.

 

[Madrab]

Physically if they can lay 25mm then they can lay 32mm but they may have a limit on any domestic supply size. That is usually borne out of the available district supply pressures and mains demand/domestic property density.

If 25mm is the largest domestic supply pipe they will connect then no point in going to 25mm to the garden room, 20mm would be more than enough to supply 'one bathroom' if you see what I mean. You do need to be careful that you don't compromise the house by shortcutting the mains to the garden room with a larger pipe and that then becomes the primary draw, if you see what I mean.

When it comes to adding extra outlets to a system though, some principles should be followed. The way domestic pipe sizes are worked out is through a calculation using loading units, available supply pressure and flow whilst maintaining a standard of max water velocity to minimise noise and friction (wear). It usually doesn't really apply to domestic situations due to diversity and the lower limit on outlets but when things start to get added and bad design principles are adopted (e.g.15mm distribution pipework) that's when things are made a lot more complicated to achieve a well designed and working system.

I know it sounds somewhat overcomplicated, after all it's 'just plumbing', which is something I hear all the time but a well working system is one that's has the correct design principles applied. That's the difference between a qualified plumber and someone that 'calls' themselves a plumber. If the standards aren't followed then significant drop offs and pressure fluctuations are introduced which is undesirable.

 

[muh3]

Thanks for that Rob, if you would be near me I would give you the job

 

[muh3]

Just an update, I had a new water main fitted in 25mm. This gets split into 2 one for the house and one to the Garden Room. I now have 3 Bar static pressure and 2.8 bar dynamic pressure on the outside tap. Without changing the house plumbing (all 15mm) I can now flush the toilet whilst somebody else has a shower, with little impact on the shower.