Large extension added to my 3 bed property. Combi has been a pain in the arse
and now won't cut it with 2 main bathrooms.
My question is should I go for a system boiler with an unvented cylinder or get
a conventional boiler and have pressurised ch and again an unvented hot water
Plumbing in the extension hasn't started yet and I plan to put the unvented
cylinder in the gurage.
My plans were for the system boiler but I met a plumber who said to go
A conventional boiler with a cold water tank and a hot water cylinder
allows the use of an immersion heater to provide hot water if the boiler
fails. Simpler boiler with less to go wrong.
Ultimately it is up to you to decide which alternative you prefer. I am
just about to go to a property which has a system combi boiler that is
not working, so I will be cold and have no hot water (kettles apart)
until I fix the problem.
A system boiler implies a conventional system. You have a choice of
unvented, vented or thermal store.
I went for the thermal store as the cylinder isn't pressurised so
doesn't need inspection and testing while still having high pressure hot
A bigger combi might be all you need (and probably the cheapest
option), but a conventional boiler with an unvented
immersion heater back up is the "Rolls Royce" of systems from the
point of view of HW supply with back-up IMO.
replying to Tim+, Darksyphon wrote:
Thanks for the replys. The only thing I'm wondering is if I need the central
heating side pressurised too? I like the idea of not having any tanks. But that
means more gear....another pressure vessel. Just wondering if the extra
reliability of a conventional boiler is worth having to add all the things
seperatly that are now inside system boilers. I'm not bothered about the cost
just want the best system that is reliable. Not looked into Thermal stores. Will
have a read
Ultimately you will have the same major components, its just the
location of them that changes.
Note however you may be able to implement more sophisticated controls
with the system boiler, to give things like split temperature
operation or weather compensation, which are harder to do with
heating only boiler usually.
 Split temp allows you to run lower flow temps through the heating
(for greater condensing efficiency) when its not desperately cold
outside, but still run high temperatures when reheating the cylinder.
 Weather compensation makes the flow temperature automatically adjust
to the outside temperature, and sometimes also the heat loss
characteristics of the building and sometimes also the size of the
difference between the actual internal temperature and the target temp
set on the room stat. So it runs hotter water through the rads on colder
days, or when it needs to heat the place more quickly.
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