22mm up to combi, 15mm everywhere else?

[silly_savage]

For reasons that are too longwinded to explain here, I'm going to have to get my supply pipe replaced. Currently 25mm Alkathene enters by the front door and runs approx. 10m in 15mm to the back of the house where it splits off to the taps, appliances and combi boiler.

My question is: do you think it's going to be worth paying the premium for 22mm in the replacement? The problem I am trying to solve is an apparent bottleneck happening when I'm using the shower and someone runs a tap (hot or cold) or flushes the loo, the flow on the shower dips considerably, enough to make it useless if two of these happen at the same time.

The 22 mm will run directly below the combi boiler, but the existing 15mm up to the combi will probably be harder to replace. Will I still see any benefit if the 15mm to the boiler tees off the 22mm supply first, which then goes on to taps/appliances? My logic is that one 22mm will feed two 15mm better than one 15mm will feed two 15mm.... if that makes sense. Or to I need to run 22mm all the way to the boiler to see any benefit?

All of the rest of the hot water plumbing is, and will remain, 15mm.

 

[oldbutnotdead]

First thing you need to find out is what dynamic pressure and flow rate your water supplier is delivering to your house- if its the statutory minimum (1 bar 12l/min) you're unlikely to see any improvement. Get in touch with your supplier, they can test this for you & should be free.
If you have good pressure/flow (2 bar plus, 20+ l/min) then yes you'll see improvement. Prob not worth replacing the short leg of 15mm into the combi, flow rate thru that will only be 12 odd l/min at shower temperature.

 

[silly_savage]

Thanks. I've logged a request with my water board to test.

 

[Madrab]

As mentioned, the key is pressure, a 15mm can supply plenty of flow if it has the dynamic pressure to push it, all it gets is noisy when the water velocity increases to over 2m/s.

As that is the case as suggested, you need your mains tested and that then needs to be compared with the pressure and flow in other areas inside the house - an outside tap - appliance valve - basin tap - feed to the shower, filters in shower etc. I'd be ruling out other things first before replacing the mains, especially as it's 25mm alkathene, you may find it's not the mains after all, rather something else.

 

[silly_savage]

Thanks, it's not the main I'm replacing but the supply pipe from the front door to the back of the house, that's currently 15mm and pondering using 22mm as the replacement.

 

[Madrab]

Ah ok, apols, didn't read that properly

The info still holds true though, it's all about the dynamic pressure. 15mm can deliver plenty of flow, albeit more noisily, it's all down to whether the mains pressure can sustain it. If you have the pressure then changing to 22mm will always be better from a flow/noise perspective