On Thursday, 13 December 2018 19:06:11 UTC, ARW wrote:
If you change to unvented or combi, 22mm pipe has greater volume of water to be run off before the hot arrives at the tap.
Is that not the same when using a header tank he already has?

Yes, but
- the problem is less acute with a tank than with a combi which fires up every time
- with a header tank the larger pipe may be needed to get a useful flow. With a mains pressure system flow will be adequate through a smaller pipe.

Owain