New boiler - heating pipes step down from 22 to 15 mm?

[volks]

Having new boiler fitted - the central heating flow and return is 22mm pipe. These pipes seem to step down to 15mm before reaching the radiators and obviously step back up on the return.

Is this done for a reason? I noticed one radiator has the 22mm step down right at the thermostatic valve, is this normal?

Obviously you can't reliably run radiators on 22mm pipe or so I think, so why do boilers come with larger diameter that has to step down shortly after leaving the boiler?

 

[ianmcd]

that is a good way of doing it, too many run 10mm plastic to rads and it is crap

 

[oldbuffer]

The radiators are run in parallel across the flow and return pipes. Large diameter pipes ensure that adequate hot water can flow at a speed which doesn't cause excessive noise. Spurs are taken off these large pipes, and do not need to be as great a diameter because the radiators are in parallel. Hence the common 22 mm spine and 15 mm feeds to radiators. There are variations.
Also, most radiator valves have 15 mm (or smaller) inlets, to there would be no point using pipes of greater diameter.
I would guess your radiator with the step down at the valve was a matter of convenience for the installer.

 

[volks]

Thanks everyone for the insight, I learned a lot!