I have inherited a house with mainly steel central heating pipes installed at a time in the 70s when there was a world copper shortage. We were going to install a new Vaillant combi but the installer refused to install unless we changed the system to either copper or plastic pipes. This is due to the damage that the steel pipes woulfd cause the boiler - particularly the heat exchanger - through adverse chemical reaction, and they weren't prepared to honour the guarantee.
Another company was not unduly concerned and said that with additives, possibly corrosion inhibitor and maybe magnaclean, this wouldn't be a problem.
Who is correct? Is there any kind of combi out there that would tolerate such a system?
I can't say I have heard this reasoning before and am not convinced it it right. Why not phone the manufacturer and ask what they suggest?
I would though strongly recommend changing the steel pipes anyway as your first installer suggested, but not for the reason he states.
Your pipework is old for one thing, and because it is steel is likely to be corroded and given that only compression joints are used there is a significant risk of it failing because of the extra pressure a combi boiler would exert.
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