I have just bought a 2 bed lower flat into which I am intending to
install a condensing combi system. Based on the on-line radiator
calculators I need about 8.5kW of output in total for the six
radiators and hot water, so my questions to the group are a few:

1) I had though I could use 15mm (plastic) pipe for the feed and
return to the radiators, but it has been suggested to me that this
should be upped to 22mm - is this good advice or would 15mm suffice
for this sort of installation?

2) Being a condensing boiler I understand it will run more efficiently
with a lower circulation temperature - does this mean I have to derate
the radiators below their quoted capacity?

3) Since all the boilers I have seen are substantially more powerful
than I require for this building, I will have to put in an over
powerful model. I am guessing that I should also over spec the
radiators, to an extent, in order to provide a decent load for the
boiler and use a room stat as an additional step to prevent short
cycling (the other rads will all have TRVs on them). Is this
sensible?

4) I have collated a chunk of info / FAQs on this, but if anyone can
point me in the direction of substantial / useful resources, again
this would be greatfully received.

Many thanks in advance for any help,

Mark