In article ,
Peter Smithson writes:
My friends need a boiler to replace the conventional floor mounted one
which works the central heating and the hot water system. The HW is
gravity fed and the CH is pumped round a single pipe system of
radiators. It's a fair sized 3 bedroom detached house.

They'd like a wall mounted one but one quote says they can't have a
gravity fed HW system wall mounted. Any idea why he'd say that?

Gravity fed HW system requires that the boiler directly heats
up a tank of water. Boilers don't work that way any more -- they
heat up a water pipe which has to be continuously pumped whilst
being heated, or meltdown follows within a few seconds ;-)

3) Some people say you can't have a combi with a single pipe system (for
some reason).

There's no problem with a single pipe system. It's not what would
be installed now, but the boiler itself will have no clue about it.
(A condensing boiler with a single pipe system would however be
quite a challenge I suspect.) You will likely need to add some
thermostatic radiator valves to conform to building regs when boiler
is changed.

--
Andrew Gabriel