Col wrote:
Forgive any mis-use of terminology - I'm no expert on boilers etc.
I currently have two gas boilers, both in my kitchen. One gives
instantaneous hot water, the other does the central heating only.
With a re-fit of the kitchen coming up I am considering replacing
these two with a single combi boiler (to save some space), but am
unsure of what to get or what is actually available. The few combis
that I have seen have a pressurised part (the heating circuit, I
presume) but I'm not keen on risking my ageing pipework with one of
those - so is it possible to get combis which are un-pressurised?
No.
Your pipework should be up to it, combis only operate on 1-2Bar.
I recently fitted a combi on such an ancient system.
There was some small leakage from 1 or 2 the rad valves which was easily
fixed with the Fernox stuff in a cartridge.
The new setup is greatly appreciated by the resident. Less than 1/2 the
space is used by the new more efficient boiler and she lost all the grottty
old header tanks, separate pump etc.

(and what's the advantage of pressurised anyway?)
BTW - The only other room which has hot/cold water is the bathroom,
which is next to the kitchen, and both are on the ground floor.
Therefore I'd rather not go to the hassle of hot-water cylinders etc
upstairs.