Cold water pipe size

[cokeysblokey]

Have had an extention built with an ensuite. Im in the middle of planning the pipe work and whilst im saving up for the central heating system i was looking at installing the cold feed myself (i have some basic plumbing skills)

on looking through the house (1965 build) i see the cold feed looks to be 15mm (or imperial equivalent). there are alot of off shoots along the way so im guessing i would be better off replacing the feed to 22mm around all the house?

thanks

Mark.

 

[leakydave]

Depends... what is the current mains pressure and flow rate?

 

[cokeysblokey]

Well i dont know the flow rate but the pressure was tested a while a go at about 3.5bar. pretty high i guess as the plumber was suprised.

 

[bengasman]

Pretty hard to oversize a watermains to a degree that it causes problems.
If you are doing some major work anyway, dig in a 25 mm pipe minimum, or even a size up. The cost in materials will be pretty minimal, and doing it that way you will be sorted for whatever alterations/additions might be wanted or needed for many years to come.
If you size just to current needs, you could find yourself snookered later on. Current pressure and flow are not that important as major upgrades are planned for many areas.

 

[cokeysblokey]

i will have 2 ensuites and a bathroom to feed. so your saying to use the wider dia. pipe and T off with 15mm?

I'm just thinking while im at this stage i might as well do it the best i can instead of T'ing off or extending the existing 15mm.

Im going to have a mains pressure hot water system if that makes a difference. i know it has nothing to do with the cold feed.

am i right in thinking using larger bore pipe will reduce the pressure? or is that irrelevent?

 

[bengasman]

i will have 2 ensuites and a bathroom to feed. so your saying to use the wider dia. pipe and T off with 15mm?

At least 25 mm cold feed, and 22mm tee to boiler/bath/unvented cylinder

I'm just thinking while im at this stage i might as well do it the best i can instead of T'ing off or extending the existing 15mm.

Indeed, espcially if the labour cost is avoided; oversize is the word.

Im going to have a mains pressure hot water system if that makes a difference. i know it has nothing to do with the cold feed.

Oh, but it does.

am i right in thinking using larger bore pipe will reduce the pressure? or is that irrelevent?

As long as the watercompany's pipe is up to spec, it won't reduce the pressure significantly. And even if it would at the moment, the street pipe is bound to be improved at some point, and if the pressure drop is so much that it causes a problem (very unlikely, but possible) don't open the valve/tap to maximum.

 

[TicklyT]

Most pressurised hot water system manufacturers give a minimum mains feed pressure and pipe bore in their blurb. Often it's something like 3 Bar, with a 22mm main, so now would be a good time to prepare for it.


Increasing the bore of a pipe increases the flow. It does not decrease the pressure.

 

[mogget]

am i right in thinking using larger bore pipe will reduce the pressure? or is that irrelevent?

The pressure will remain the same in a large or small pipe, if the water is not flowing (static head)

The smaller the pipe the more friction it has, which will reduce the flow.

A pipe that is oversized will cost more than it needs to.

A pipe that is too small will result in less flow than you need.

However, on hot water pipework, the larger the pipe the longer the hot water will take to reach the tap, as a larger pipe will contain more (cold) water that needs to be run off.

So it's safe to oversize the cold water pipework, but try to get the hot just right.

 

[cokeysblokey]

Interesting stuff guys, thanks

I didnt explain myself well.....i understand the boiler will have a required dia. feed pipe going into it but i was wondering about things like showers running off a mains pressure system. if the cold feed coming in is through different pipe to the hot will this cause an issue with them not being the same flow?

maybe im overcomplicating things here as i would think the cold feed will be servicing more points compared to the hot so i guess will have a slight difference anyway?

 

[bengasman]

If you think you can service 3 bathrooms with hot water from a 15 mm inlet, you will get a cold surprise, unless you never use them at the same time, now or in the future.
15mm to any single tap is fine, as long as it gets bigger at the tee. (looking in reverse flow direction.)
Like Mogget mentioned, the hot side is a bit of a trade off ass a bigger pipe will take longer to fill, reducing the response time and increasing the bill.
For hot, keeping the big pipe as short as possible, and teeing off as soon as is practicable would be best.

 

[cokeysblokey]

good stuff....i will get on with some bigger pipe work then

bit of a bummer as i have to get behind the kitchen units to get to the mains but at least it will give me summat to do and save money for the hot water system