DHW pipe bore size

[BoyElectric]

Am having a condensing boiler installed shortly and am trying to do as much prep work as possible to save on labour charges before getting an RGI in to install the boiler.

The existing gravity feed HW system is plumbed in 22mm pipe - terminating at the bath - with 15mm tee-offs for the other services (kitchen Sink, washing machine, bathroom sink, WC sink). The DHW outlet on the new boiler is 15mm, so . . .

1. Does the entire HW pipe network need to be changed to 15mm?
2. Is it OK to fit a 22>15 reducer to the HW pipes and run the last short stretch (about 2m) to the new boiler in 15mm?
3. If the reducer option is OK, is there a minimum distance the 15mm pipe should run from the boiler before expanding into 22mm?
4. If the reducer option is OK, would it still be preferable to change the entire network to 15mm in terms of better pressure or more even distribution of water when more than one service is drawing HW?

I'm happy to change the HW pipe network myself if necessary as most of the pipework is buried and would be quite labour intensive and expensive to pay someone else to do it. I can run a 15mm in parallel with the 22mm and get the RGI to connect up to the 15mm when the boiler is installed and remove the 22mm afterwards.

[htg engineer]

It is ok to to use a reducer to connect to the 22mm pipework, but it is best to have all 15mm pipework, from boiler to all outlets.

htg