Upgrading to unvented cyl on 15mm pipe

[chez_ona]

We are looking to upgrade from a vented gravity fed hot water cylinder to an unvented cylinder.
We are looking to add a shower upstairs as part of a bathroom refurb and the current gravity fed pressure is pathetic.
We've had a few quotes but they don't agree.
We have high flow at the rising main - 42 litres per minute measured at the outside tap. But the internal pipe up to the tank is 15mm.
We've had one person say they would have to install a flow reducer on the existing pipe. Another said we would have to run 22mm pipes up to the tank or the flow would be too low. Has anyone had similar experiences? I'd prefer not to run extra pipe unless we have to as it's the opposite side of the house but I don't want to lose flow rate in the shower every time someone flushes the toilet either.
Thanks for any advice.

 

[muggles]

No reason at all why a 15mm pipe can't be used to supply an unvented cylinder provided mains flow & pressure is high enough.

 

[Madrab]

Depends on what you are planning to feed with the unvented. A 15mm pipe can give plenty of flow, but it needs a higher dynamic pressure to maintain it, that and once the water velocity goes over 2m/s, then it can start to get noisy, especially the more bends and valves there are in the run, it can also promote water hammer and is getting close to the recommended specs. At 2m/s, a 15mm pipe will deliver ~ 16L/Min, so if the demand is much higher then velocity can become a problem.

Can you test the the flow at the outside tap when say the sink and a basin tap are running too?

I would always recommend @22mm feed and supply in and out of an unvented.