On Friday, 13 April 2018 21:44:06 UTC+1, Jonno wrote:
I want to install a direct un-vented hot water cylinder in a flat. The install
instructions call for 22mm diameter water supply line, the main riser supply
to the building is 22m and then reduces to 15mm before the stop valve. Do i
need to isolate the main riser into the building in order to replace the 15mm
stop valve with 22mm or can I step up the pipe diameter back to 22m after
the 15mm stop valve without affecting the performance of the cylinder.

15mm is likely to give unsatisfactory flow.

If the unvented cylinder is over 15 litres you must have an Unvented Hot Water certificate to do this work.

Owain