John Rumm writes:

Tim Lamb wrote:

If you stick a phosphate dosing unit (like a "Combimate"
for example) in front of the combi then it should not
suffer scale problems. That should then bring the
decision back to the basic one of which better suits your
requirements.

I plan to put a conventional ion exchange water softener in front of mine.

Even better ;-)

Curiously, the installation instructions for my combi say that
softening the water isn't recommended. Any idea why?

(at a guess, the extra sodium ions might react with the
aluminium?)
--
Jón Fairbairn
http://www.chaos.org.uk/~jf/Stuff-I-dont-want.html (updated 2009-01-31)