Plumbing supplementary - how much 22mm to use?

[David WE Roberts]

Our current plumbing goes straight from a nice blue plastic main down to

15mm copper. Pressure and flow rate seem good tested by plumbers cup (sounds slightly dodgy and/or masonic).

We are sizing to potentially run two showers from a single combi so want to have as good a flow as possible for as much of the pipe run as possible.

So are there benefits in doing some of the pipework in 22mm?

The maximum flow will be governed at the combi (probably a Vaillant 37kW or Worcester Bosch equivalent) but apart from that the hot water flow will depend to an extent on the pipework.

Is it likely that using 22mm for the cold supply to the combi and the hot supply to the showers will allow more flow of hot water, or is it pointless because 15mm will carry as much cold water as the boiler can heat?

If there are different answers for plastic and copper (perhaps because of different internal diameters and restrictions from plastic pipe inserts) then it would be good to know :-)

As usual I don't have the full details but plumber seemed happy that there was enough flow for a big combi.

TIA

Dave R

[Andrew Gabriel]

It's going to depend on the pipe run.

One thing I would say - if you do just part of it, do the cold water part. 22mm on the hot water means you have to run off twice as much water before it starts coming through hot (and for a small draw-off (e.g. hand washing), you then waste twice as much hot water afterwards as the pipework cools again.

Plastic has same external diameter and thicker walls, so less crosssectional area for water flow.

[John Rumm]

The main flow limitation is the speed the combi can heat the water.

Some but not much unless its hugely powerful.

In our previous place the incoming main was 15mm, and there was about 3

- 4 bar of static pressure. However flow rate was limited to about 17 lpm. The combi was a 35kW and could manage two showers at once. However if there were any other users of water in the place at the time two showers were going on, then you noticed it!

The effective distance will come into play as well. 22mm up to the boiler would be worthwhile.

Not much point using it for the hot from the boiler just on ground of capacity - although depending on layout it may make sense to take 22mm for some of the way, and then split into two 15mm legs.

Note that more 22mm will mean longer dead legs of water to clear each time.

Both have pros and cons. The copper might have a slightly larger bore, but also has a higher heat capacity so will take slightly longer to heat up.

even 37kW can only do 15 lpm at a 35 degree rise. It will drive a pair of moderate showers, but not anything with body jets or soaker heads.