JoeJoe wrote:
I have a brand new combi running 18 radiators + 1 shower in my house. No
complaints there.

Problem is that the radiators seem to be getting very cold very quickly as
soon as the boiler is switched off. My suspicion is that the cause could be
because some of the pipes (copper) hat feed the downstairs radiators are not
insulated. I have a 1m high space under the house, where the pipes are
located, and it is freezing cold and drafty there.

Just a thought - check that all your radiators are properly bled of air
- a part full rad looses heat faster when the boiler is switched off

Charlie