New Water Supply 25mm or 32mm

rossyl · 26 Aug 2021

Hi All

I see this has been covered before, and I've searched, but grateful for a bit of advice.

Thames Water have been renewing the mains and pipes, up to the pavement stop cock. Everything is new.

I went a spoke to a couple of the blokes working for Thames Water doing the work in the road...they told me...
that the pipe is 25mm coming to the pavement stop cock.
That the connection will take a 25mm pipe.
That a 32mm pipe would require an upgrade
That they don't see the point in having a 32mm pipe into the house as the pipe from the main to the pavement stop cock is 25mm.
They also advised that pressure is very good, and well over 2.5 bar.

The house is 3 storeys.
Has 3 bathrooms + additional WC
Has 5 bedrooms and 3 reception rooms.
It's currently a renovation project, but will have an Unvented system put in.

Should I stick with the 25mm?
Or, pay the extra for the 32mm?

Thanks

Londoner2 · 26 Aug 2021

25mm will do, i had the same concerns but 25mm is fine, (if you want 32mm then all internal pipework will need to be upgraded from 15mm, not worth the expense).

Madrab · 26 Aug 2021

A 25mm pipe could deliver a similar volume of water as a 32mm pipe as long as the dynamic pressure is adequate, it just gets a bit noisier through the 25mm due to the velocity being higher, to deliver a similar volume.

Mike2007 · 26 Aug 2021

Bigger the better, but harder to terminate and wrestle with.

Bouy · 26 Aug 2021

Assuming a flow rate of 18l/min down the supply pipe (which is very good) the head loss down a 25mm pipe would be about 1.3m and down a 32mm pipe about 0.4. Since your supply pressure is 25m the difference in the supply pressure presented at your house is minimal i.e. 23.7 or 24.6 so you would see very little actual flow difference from your house taps. Calc is based on a 50m length of supply pipe, the shorter the length the less the difference, similarly the restriction of your internal pipework (15mm) would decrease the the supply flow below 18l/s so again the pressure loss difference within the supply pipe would more than likely be less than that calculated.

Mike2007 · 28 Aug 2021

Lets not forget for a minute that the more water you get the bigger your bills (with a meter). Long term, water conservation will be a plumbing concern.

rossyl · 30 Aug 2021

Thanks all.

A question on the below post please...

Bouy said:
Assuming a flow rate of 18l/min down the supply pipe (which is very good) the head loss down a 25mm pipe would be about 1.3m and down a 32mm pipe about 0.4. Since your supply pressure is 25m the difference in the supply pressure presented at your house is minimal i.e. 23.7 or 24.6 so you would see very little actual flow difference from your house taps. Calc is based on a 50m length of supply pipe, the shorter the length the less the difference, similarly the restriction of your internal pipework (15mm) would decrease the the supply flow below 18l/s so again the pressure loss difference within the supply pipe would more than likely be less than that calculated.

I'm sure the above answers my question, but I don't think I've enough between my ears to fully understand your post!

Am I right in that what you are saying is...
- if all pipework from the Main to the Pavement Point is 25mm
- then, increasing the pipe from the pavement point into the house to 32mm
1. would make minor improvement to pressure
2. would make virtually no difference in flow

Is that right?

Thank you

Bouy · 30 Aug 2021

Basically thats about it. I have worked on the total distance of pipe involved being 50m and assumed the starting pressure and flow rate from the connection at the main supply pipe is 2.5 bar and delivery is18l/min. If the water company could supply a flow fig and pressure at the boundary stopcock then a more definitive calc could be done. The fact is that the 25mm supply pipe to the pavement point already controls the flow so, unless the 32mm is 100m plus long, having 32mm pipework through your garden instead of 25mm will not have a dramatic effect on the flow/pressure at your house stop valve

rossyl · 31 Aug 2021

Thanks very much. Makes sense to me. Also eases much concern, so thanks.

I was wondering how to benefit from the increased 25mm.

Would this make sense.
22mm or 25mm
- To Unvented Cylinder.
- From Unvented Cylinder to 3 showers/bath
- Stepping it down to 15mm as close as possible to the bath/shower

15mm to all others outlets (toilets, sinks, washing machines etc)

Aim is to reduce impact if e.g. a toilet is flushed when 2 showers are being used.

Or would the benefit be negligible by doing this?

Many thanks

Bouy · 31 Aug 2021

Will have to pass on a theoretical analysis of that but any increase from 15mm to 22mm (especially on bends, tees etc.) could reduce frictional resistance by quite a bit but to how much benefit??

Madrab · 31 Aug 2021

Yes, that's the preferred process but I would leave that up to your qualified unvented installer.

You mentioned that you had 2.5bar at the mains, have you (or your installer) tested the dynamic pressure and flow rates? It is these figures that will determine how much of a drop in pressure and flow you will see with multiple outlets running at the same time.

No one would be able to guesstimate how much of a drop you would see without these figures.

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New Water Supply 25mm or 32mm

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