I'm planning the refurb to be futureproof and wanted to run 28mm pipes to the UFH manifold, so any future ASHP would have easier life. For the time being the combi will continue the service though, and it uses 22mm connections. Any issues with changing midway the diameter from 22 to 28mm (on feed, so 28 to 22 on return)?

I would face a similar situation when my gas boiler packs in, currently piped in 22 mm to manifolds and 15mm to ground floor rads and 10mm microbore on first floor to rads.

found 2 suitable locations for ASHP, preferred location is in drive approx 3m lower than house so may need secondary pump

interested to watch this thread!

My current rads are 15mm copper from 22mm boiler, and previously I had microbore with combi, which makes me think that it does not matter - the flow speed must change accordingly to cross section (with all the consequences, like resistance/pressure drop) and that's about it. The plumber we parted ways today mentioned whistle effect if I forced him to go 28mm (I checked - it happens due to high flow velocities, quite the opposite large diameter does), hence the question before I make my hands wet.