Advice on choosing combi boiler?

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? (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.
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Col wrote:

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.

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Col wrote:

For you a combi boiler (see the main FAQ) is probably the way to go.
The pipework is probably going to be OK unless it's buried in concrete. The rads if they are already seriously corroded may fail when they first have to take pressure. Some boiler instructions tell you to go up to 2.7 bar as a commissioning test, if they survive that and yu add corrosion inhibitor you have nothing to fear.
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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