CH pipe diameters

[regsmyth]

I'm planning the CH in the new extension, and will run 22mm pipe round to where the rads (3 no. and one plinth heater) will be.
I will branch off this with a 15mm T for the first (upstairs) rad (3-4m from T), and branch off it again for the plinth heater. To get to the second rad (in the hall), the pipes will be laid in a trough in the solid conc hall floor.
The second rad is a tiny one in the hall, but the last, just beyond it, is a big one.
My question is this - after the plinth heater, is it worth continuing in 22mm, (ie bigger channel to cut out of the floor) or should I just do the last two rads in 15mm and not worry about it?

 

[Rico99]

Reg,

Run the 22 as far as you can.

Rico

 

[regsmyth]

Yes, but, I thought that since the second and third rads are so close to each other, (ie on either side of the same wall), and the second one is so small, I could effectively treat them as one rad, and if they were one rad, would there be any point in bringing the feed right to them in 22mm, which would have to be reduced to 15mm to get into the rad? Or, if they were one rad, would you not stop the 22mm once it's branched at the plinth heater?
Sorry if this sounds confusing, or if I sound confused, I guess I am slightly, over flow rates, and pipe reductions and typical arrangements, and the like.
But I don't want to cut a bigger channel in the floor than I have to, so if there's no real benefit to continuing in 22mm beyond the plinth heater, I'd rather not. That's why I'm asking...