15mm or 22mm water feed

NSG666

330d said:

OK. 

So currently I am in the middle of a full house renovation. This included replacing the old regular boiler, cylinder and CWS in the loft with a system boiler and unvented system.

As the house is a building site, its not a lot of effort routing a 22mm pipe to the shower. So this is what i am trying to figure out, is it worth it? 

Put like that, turn the question around. What's the problem in running 22mm to the shower?

It saves all that faffing around reading the instructions, asking the manufacturer for advice, working out flow rates through 15mm pipes. If you do it in 15mm and it doesn't perform as expected you'll kick yourself. If you do it in 22mm and spend an extra £50 and it doesn't perform as expected well, there's nothing more you could have done.

Jeepers_Creepers

330d said:

1) House has a combination of plastic and copper. But the pipe going to the bath tub is 22mm copper. Current shower is 15mm plastic

2) Flow is 20l/min. Cant remember the pressure.

20 lpm is 'good', but nothing special. The pressure is of equal importance, as it's this wot overcomes flow resistance in pipes.

(It's why 15mm pipe is fine for combis, but 22mm required for gravity. The flow from each end with both systems will be similar, but narrow pipes need greater pressures to drive it.)

If, say, your flow was a healthy 25lpm+, and the pressure a solid 3bar+, then I would say you could go '15mm' with no issues at all. Since you flow is 'only' 20lpm, and pressure unknown, as AL says, play safe and go 22mm.

There's another issue too - to get the same flow of water through a 15mm pipe as you would via 22mm means it has to move faster, and that can also create noise - the 'hiss' you can get from narrower pipes. Taking it all into account, I would go '22' from the unvented to the bathroom, and then tee off in 15mm to the shower and other components.

Don't fret about getting the 22mm as close to the mixer as possible - a couple of metres of 15mm pipe at that end will make sodall difference - so run 15mm there if it's easier. And, since you are going '22', then plastic will be absolutely fine.

330d

So to conclude, I will be running 22mm to the shower. 

fezster

The length of pipe is an important factor here. The working (dynamic) pressure will dictate the flow rate achievable, assuming the incoming flow rate is sufficient. The pressure drop difference between a 15mm and 22mm pipe over 1m will be negligible. The same pressure drop difference between the two over 20m will be considerably more. This could mean the difference between an "OK" shower and a "great" shower.

Remember, each fitting (elbow, tee etc) will add to the length of pipe. There's charts available to calculate these for copper pipes.

This becomes even more important if you plan on running multiple outlets/showers, as the pressure drop increases with flow rate.