Rising main goes from 22mm to 15mm before the combi boiler

[diyAlex]

Hello!

Just been playing about in the loft and noticed this: Our rising main comes in at 22mm but goes down to 15mm halfway through the loft. It's a direct system.

There are some junctions on the main in the loft before it steps down the diameter. One is a 22mm feed to a disused (but not removed!) header tank.

The other junction is a 22mm feed for the bathroom's cold water. This pipe itself goes down to 15mm just before it drops through the ceiling.

I'm curious because I'm getting to grips with how the plumbing works so that I can make a few changes. I had thought (assumed?) that all the pipes will be 22mm, and just the "spurs" to things like sinks will be 15mm.

It seems odd to me because the hot water supply from the boiler is 22mm that then steps down to 15. But what is the use of a 22mm hot water pipe if it is coming from a boiler that is being fed from a 15mm rising main??

 

[cross thread]

sounds like in the past some of the cold supplys to your taps have been fed from the cold water tank insted of mains fed ,common in the 70,s

 

[TicklyT]

As above, the chances are the original system was gravity fed, so larger bore pipes were used to achieve reasonable flow rates. Later changes re-used some of the existing pipework, and some new pipework added.

There is a limit to how fast your boiler can heat water, so a 15mm feed to it is probably plenty big enough. 15mm with mains pressure behind it will probably deliver higher flow rates than 22mm or 3/4" pipework with gravity feed anyway.

When planning any changes, check whether you are looking at 22mm or 3/4" pipework. Both imperial and metric sizes were in use in the '70s but the two sizes require different fittings.

For 15mm and 1/2", there is only a small difference, so the majority of fittings can be used for both sizes.