Pipe sizes

[Johnboy54]

Hello to all.
I'm installing new radiators and pipes.
Downstairs are all concrete floors.
Do I need to run 22mm pipes as near to all rads as I can and then reduce to 15mm?
Or, do I run 22mm under the joists and drop down the walls in 15mm?
Thank you.

 

[seco services]

22mm and drop down in 15mm

 

[Johnboy54]

I'm so sorry I've not replied before now.
If you saw the state of my house you'd understand.
Thank you.