Looking at the other posts it would appear that you have been introduced to a range of replies.
Provided:
1) The existing system, when it worked, worked well enough to heat all the radiators enough to heat the house properly. &
2) There is absolutely no hint of sludge or scale in the existing circuit
then keeping the existing 8mm and 10mm pipes will be a good choice.
Obviously 8mm pipework is even more prone blockage than larger sizes but isn't automatically a problem.
The existing system worked on the basis that the difference between the flow and return temperatures would be somewhat more than a system using larger pipes. This is all to the good when using a condensing boiler.
Inasmuch any domestic system is 'designed' rather than estimated (usually quite well) then microbore systems have a larger difference between flow and return temperatures than systems using 22& 15mm.
I'm a bit confused by references to different sized pipes here . Do you mean the two pipes to and from each rad are different sizes( I'm presuming you don't mean that) or,as in my case different rads have different sized pipes( 8mm or 10mm) connecting them to the 22 mm F+R ( either from tees or from a manifold) ?
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