In article , F
writes
Relative lives in an inner, three story town house and needs a new
boiler. The current ~20 year old boiler is located on the ground floor
with the flue running up through the other two (concrete) floors.
There's a garage to the front and a kitchen to the rear and the boiler
is in a walk-in cupboard between the two.

She's been told that the boiler must go on an outside wall with the flue
through the wall, or on the top floor with the flue through the flat
roof. Question is, why can't it go where the existing boiler is? Is
there a limit on the length of flues for modern boilers?

It t' old days this was not a problem as a conventionally flued boiler
could draw air from the room in which it was sited and the exhaust gas
was so hot that it could easily find its way up a long large bore flue
to the top of house.

Nowadays it is not considered safe for new installations to draw air
from the room in the appliance is sited and the regulated requirement
for condensing boilers having low exhaust temperatures means that the
flue gasses no longer have the oomf to make it up a long flue on their
own.

The solution is fan assisted balanced flues which are frequently
concentric (exhaust inside and inlet outside) and although extension
pieces are available they are expensive and there is a limit on the
length that the fan can suck and push the gasses.

Boilers do exist that are capable of driving long flues but this is a
specialist requirement so there aren't that many around and some of them
are expensive. Almost universally they split the flue into 2 pipes to
drive long distances. I know of:

Keston - Reasonably priced boilers, and flues are formed from low cost
(but inflexible) 50mm muPVC drainpipe. I have a Keston Celsius 25 boiler
but I'm not sure I would give it an unqualified recommendation

Mann Micromat - A top quality German boiler but pricey, extension flue
is flexible corrugated plastic tube which is cheaper than metal
extension but still expensive for long runs. Formerly distributed by Eco
Hometec but I've lost track of who are doing them now.

Ariston - Saw these in a block of flats a while back and noticed they
were using long flues via adaptors, no detailed info.

To cut a long story short, if you chose to go the long flue route then
you will be making life more difficult for yourself, you will be paying
more and tying yourself to a number of specialist suppliers whose
support network may not be as extensive as the mainstream market and as
a result you may have difficulty with spares in the future.

More mainstream boilers are most easily fitted to outside walls but many
can drive concentric flues of 3 or 4m which may mean that you can fit
them in a cupboard and route the flue to the outside.

See also the group boiler choice faq from Ed Sirett:
http://www.makewrite.demon.co.uk/BoilerChoice.html
--
fred
Plusnet - I hope you like vanilla