just catching up there - this moved quickly the last few days!
A combi loses mains hot water pressure, are you sure about that?
Yes, absolutely yes, particularly when sized appropriately for the actual heating load of the house. That is the number one selling point of installing a un-vented cylinder is mains pressure hot water.
Can you get a 50kw combi that will deliver 25l/minute? Sure, but it will be grossly oversized for the property which may have a heating requirement as low as 6kw peak and hardly ever get into its condensing mode, constantly cycle and and needlessly chunder loads of gas.
Hmm; trying to ascertain what is meant by vented radiator loop. My combi provides central heating with hot water (on demand), situated in loft which vents to roof.
Vented = not a sealed loop
un-vented = sealed loop
My radiator loop is vented, there is a little header tank in the loft that deals with the expansion and contraction of the water in the system as it heats/cools. It also has a little ball **** which automatically tops up the system as needed.
An unvented loop has a pressure vessel to handle the expansion and you need to fill it manually with a tap somewhere on the system to keep it pressurised.
Do you require radiators to be replaced?
Yes, all but two radiators need replacing, of the 10 or so that need doing 4 are already double panel and will need to grow in size. The others are single panel which need swapping for doubles.
Will the installers perform all additional pipework, which may or may not include pulling apart (and then fixing) floor-board / boxing / trunking work?
It should include putting things back to the way they were but probably not re-decorating.
Will the install also cap the gas mains supply?
I doubt it, you'll have to apply to your supplier to get it done, whoever that is. Octopus are currently doing it for free.