On Wed, 25 Apr 2012 12:51:19 -0700, kent wrote:

Oh dear, I didn't mean to stir the s*** ! Does this mean I am unlikely
to get any rational answers?!

No, it just means you are likely to get some irrational ones ;-)

Multiple boilers in larger properties is quite a sane idea. Generally,
however, they are ganged together so that either boiler can run quite
efficiently at the lower output required for heating in spring and autumn-
ish weather, with both coming on together for winter loads. This also
gives a degree of fault tolerance as a fault in one boiler doesn't leave
you entirely without heating, and may not even be noticed in milder
weather. However the controls required (including a mechanism for
detecting that just one boiler has failed, before the other one goes as
well and you really don't have any heating!) is more complicated and/or
expensive.

Combi boilers aren't generally ganged though: in the larger properties in
which ganged boilers would be called for the hot water demand (e.g.
multiple bathrooms etc) is likely to be greater than even 2 combis
working together can supply.

However a 4 bedroom house isn't large enough to likely require more power
for heating than a single domestic boiler can supply (and if it does
you'd be better advised reducing the heat losses than adding boilers!)
Given that the hot water requirement is the critical factor then some
form of stored water system ('Megaflo'-type unvented, or thermal store/
heat bank) or just a storage combi (e.g. Worcester's Highflows) would be
more appropriate. If you are seriously concerned about backup for when
the boiler goes down a stored water system with immersion heater and a
few cheap convector heaters stored in the attic should do the trick.

--
John Stumbles -- http://yaph.co.uk

The astronomer married a star