In article , Tony Bryer
writes
In article , Mike Barnard
wrote:
I'm looking at ordering a new combi on monday, but the maker offers
two flue pack options, a four inch and a five inch. Can anyone tell
me why there is a difference and which would be the better option to
choose? And maybe why?

Haven't checked the tech info myself, but at a guess the 5" allows a
longer total flue length.

Much longer. The Vaillant Ecomax 824E manual (for example) shows the max
length of a 4" flue to be 9 metres (vertical) but a 5" flue is good for
26 metres (vertical). Woster Bolshi will be similar. Don't forget to
allow for 90 and 45 degree bends which reduce the overall limit.
PS If you are going condensing then 26 metres of 5" flue is going to
cost a *lot*, one reason why people go for Keston or Man-ecotec which
use much cheaper 2" plastic kitchen waste. The latter is available as
a combi, and is expensive but if you are staying put for 20 years
cheaper in the long term.
--
Andrew