Unvented loft cylinder installation – partial 22mm pipe upgrade query

[Monsieur27]

Hi all,
Looking for some real-world advice on a pipework upgrade for a upcoming heat pump and unvented cylinder install. The new cylinder is going in the loft.
Current setup:

• Main supply comes into a crawl space via a lead pipe to the stopcock.
• From the stopcock, it transitions to 15mm copper running through the house.
• Mains pressure: 3.8 bar static / 3.2 bar dynamic (measured at garden tap).

The issue:
The installer wants the main feed upgraded to 22mm. The problem is that a large section of the 15mm pipe runs under the floorboards of a recently finished bathroom and bedroom before dropping into the void. It’s incredibly difficult to access.

My questions:

1. If I can only easily upgrade the final 3 metres of the run (from the first floor up to the loft) to 22mm, will this improve the flow rate at all? Or does the earlier 15mm section completely cancel out the benefits?
2. In your experience, will the installer refuse to sign off the benchmark certificate if they see 15mm pipework between the stopcock and the first floor?

Any thoughts or alternative suggestions would be massively appreciated. Cheers!

 

[Madrab]

The benchmark is there to ensure that the Unvented (edit) has been installed as per the MI and that everything is as it should be. The mains supply to the unvented does not really come into it as far as the size of the pipe is concerned.

The preferred solution to satisfy an unvented cylinder is to maximise pressure and flow hence why a 22mm supply is ideal. Target starting rates for an unvented would be => 2bar dynamic @ 20L/Min. Whist a 15mm pipe can flow large volumes of water, to do that the velocity has to increase significantly, that usually means turbulence, ergo noise and wear, if the pipework has lots of changes of direction and can also mean a significant drop in dynamic pressure. 22mm pipe carries a lot more flow at a lower velocity with less of a drop in dynamic pressure absorbed by the run, hence why 22mm is always recommended

Have you measured the dynamic flow? Can I suggest you check dynamic figures at peak times during the day with at least 2 other outlets running at the same time. Can I also suggest that if the cylinder is being placed in the attic that you try and avoid a horizontal unit, they are known to be troublesome.

 

[Monsieur27]

Have you measured the dynamic flow? Can I suggest you check dynamic figures at peak times during the day with at least 2 other outlets running at the same time. Can I also suggest that if the cylinder is being placed in the attic that you try and avoid a horizontal unit, they are known to be troublesome.

thank you for your input, i've tried now 11pm the flow rate is about 16L/m from a single tap(bathroom upstairs).
for the cylinder I opted for a vertical one.

 

[Madrab]

OK - Try it again, say around 6pm - that should be when the district main's under a typical high load, again with the gauge on the outside tap with a couple of outlets running. That'll give you the worst case scenario.

Good to hear about the vertical cylinder as most clients I hear about that have horizontal cylinders, never seem to be too happy with them.