15mm or 22mm when coming from mains?

[deko]

Last summer I had a plumber in to reroute my water pipes because they were taking a round trip of my house before returning to the bathroom and kitchen (the legacy of an old tank in the loft).

Here's my set-up:

Mains supply comes into my kitchen via 25mm MDPE pipe. Joined to this is a 15mm copper pipe, which after a six inch journey, splits in two. The first to supply the dishwasher, washing machine and boiler, the second to carry on to the bathroom directly above, to feed the cold supply to the toilet, basin and bath. The hot supply comes from the combi boiler below into the bathroom to feed the bath and basin, then continues back down to the sink in the kitchen below.

However, there is a small run in the bathroom where the pipes look to be 22mm, plumber said they were old imperial sizes.

When the rerouting was done I was expecting him to install 22mm pipe from the mains and feed off 15mm pipes from this for all the appliances. However he fitted 15mm along the whole run. He said because my combi boiler was only feeding out 15mm, the cold supply should also be 15mm.
Is this correct?

The pressure on the cold tap in the bath is now much higher than the hot supply, so when I fit a shower it's going to be imposible to balance the temperature. I'm thinking if I change the core supply pipe from a 15mm to a 22mm then it will lower the pressure of the cold slightly.

Since I'm about to redo the bathroom above and change the position of the washing machine in the kitchen, I'll be changing the pipe layout a little again. I'll be doing this myself using plastic push fit, so replacing the whole set up is fairly easy.

So incase I've lost you, is a 22mm common supply with 15mm feeders (22mm for bath) a better choice than my current setup which has a 15mm common supply pipe?

Pressure by the way is fantastic! My toilet currently refills in seconds and sounds like a rocket launching

 

[Agile]

I would say that all you need to do is replace the six inches of 15 mm with 22 mm to the point it splits to the boiler.

The static pressures in your bathroom are the same and you should have little difficulty.

What you should do when replacing any pipework is to make sure you do not have any 22 mm left in the hot feed after the boiler.

Tony

 

[deko]

ok so just to confirm I'm going to try lay this out logically, these are the routes and their associated pipe sizes:

Downstairs
------------
Mains supply (25mm) >
T-junction (J1 + J2) >
J1 (15mm) > Sink > Dishwasher > Washing machine > Boiler.
Boiler supplys hot feed to bathroom above (15mm).

Upstairs
---------
J2 (15mm) then connects to old pipe 22mm.
This pipes feeds toilet (15mm) > basin (15mm) > bath (22mm).

Hot enters from downstairs as 15mm then becomes 22mm.
This pipe feeds bath (22mm) > basin (15mm).
22mm pipe then connects to 15mm pipe before turning down to supply hot tap in kitchen sink below.

So both hot and cold start out as 15mm but become 22mm when they pass through the bathroom. If I remove these 22mm runs surely the end result will be the same?
If you turn on hot and cold taps together, the overall effect is cold.
Is this to do with my boiler not having a high enough flow rate?
Will a thermostatic value be able to compensate?

Really bugging me this one

 

[deko]

...also, if I do the whole run in 15mm, I'll still need to increase to 22mm when I get to the bath right.

 

[BigBurner]

...also, if I do the whole run in 15mm, I'll still need to increase to 22mm when I get to the bath right.

From the main tap take 22mm direct to the combi with no tee offs. Give the combi its own large supply.

From what you say all is fine, except fit a shower mixer with pressure equalisation incorporated. Argos sell one for £149.
http://tinyurl.com/58s8u9

Then if problems of one outlet robbing the combi of flow, insert flow regulators at the offending appliances. One in-ine regulator could do say the washing machine and dishwasher as they do not need to be filled quickly, neither do toilets or basins. http://www.bes.co.uk sell them.

 

[deko]

fit a shower mixer with pressure equalisation incorporated

Thanks for that suggestion BigBurner, found a nice Mira Excel set

 

[OvertonPlumber]

hmm Your confusing pressure and flowrates.
15mm pipe has a finite amount of water it can have flow through it at a set pressure. Yes most combination boilers do only feed out in 15mm however the feed supply to them is 22mm or as damm close as you can.
Id be looking at taking 22mm from the 25mm blue poly all the way to the boiler and then to the main carcas to where the demand splits. Id remove all 22mm pipe on the hot circuit and the large pipes will just result in low flowrate as the combi is never gonna be above say 12ltr/min.
Its worth doing a flow and pressuretest on say an outside tap to see what you have to have with.
I have a customer or has 9 bar and over 29ltr/min on a 15mm main. The maximum you can pump through a 15mm pipe is something silly like 135ltr/min ( w3ould take like 3k worth of pumpset to achieve that though.

 

[conorius]

Slight tangent here
I just put in a 32 mm line from the road to the the house and went down to 28 mm copper in the house with a 28 mm stopcock, I rang 28mm copper up to the 35 kW Combi boiler in the loft , with tees on the way for other cold water users. Just before the boiler the 28 mm reduces to 15 mm.

I had two objectives

a) to be able to flush toilets anywhere in the house with out reducing flow from the boiler to the loft shower.

b) to maximise the water pressure at the shower.


The end result has been excellent. The 28 mm solution was a bit over the top and 22 mm would probably have sufficed.

 

[muggles]

ok so just to confirm I'm going to try lay this out logically, these are the routes and their associated pipe sizes:

Downstairs
------------
Mains supply (25mm) >
T-junction (J1 + J2) >
J1 (15mm) > Sink > Dishwasher > Washing machine > Boiler.
Boiler supplys hot feed to bathroom above (15mm).

I would suspect that this setup is robbing your boiler of it's supply - if you have the DW/WM/both running, and try to draw hot water as well, your supply to the boiler may be struggling to keep up with the demand placed on it. I'd say run 22mm from the stopcock up to the WM, where you should have a 22/15/15 T to split off to the boiler and WM.

I'd run 22 up to the bathroom upstairs and then split to 15 for the basin and bath. There should never be a case where you step up a pipe size, although it is acceptable to have tap connectors that step from 15mm to 3/4" for the bath if required.

Is the combi new or was that an existing installation?