In article ,
Christian McArdle wrote:
Also, having two boilers means that your minimum modulation
gets worse. In a marginal heating situation, bumping along the
bottom at 4-8kW, you want the entire load on one boiler so it
doesn't have to cycle unnecessarily. With 2 combis, you could
have 2 boilers cycling instead of one pootling along at
minimum, at much greater efficiency.

Yes, but it's a simplicity v. efficiency v. redundancy equation.
At home I've got one combi, 2 heating zones controlled by
2xCM67's controlling 2MV's: maximum efficiency but no redundancy
and the extra failure point of the MV's. Two combis (or one combi
and a system boiler each feeding its own heating zone might lead
to increased cycling in mild weather but would reduce the risk of
being without heating and would, as IMM says, require much less
wiring. But I'm less than convinced that it really makes sense in
a normal home, though you might argue that the £50 for the
additional boiler service is a much cheaper form of insurance
than BG or a boiler maker's emergency service cover.

Our church heating, oft cited here, has two boilers and one
heating zone with a basic control system to try and ensure that
only one boiler runs when only one is needed. The upside is that
we always have some heating even if a boiler fails; the downside
is that four pumps are running instead of two. If there's some
problem with church heating it's a bit late to discover it at
1000 on a Sunday morning

Ultimately IMO there is no 'best': each option has its own
advantages and disadvantages.

--
Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk
Free SEDBUK boiler database browser
http://www.sda.co.uk/qsedbuk.htm