My heating system is a traditional boiler, loft tank, vented cylinder type
affair. The lower outlet to the coil inside the immersion cylinder tees off
to a vent into the small tank in the loft which therefore vents the boiler
side of things.

The heated water in the cylinder fills from the bottom and the outlet at the
top to the taps also tees off to a vent into the large loft tank. All fairly
normal and so far so good.

A few years ago I was up in the loft for the first time in ages and it was
dripping wet in there. Everything stored had gone musty and water was
running off the roof felt. Turned out the boiler vent was pumping over into
the small tank which was consequently full of hot water and producing clouds
of condensation. I tried the pump on its lowest setting and it still
happened. I examined the pipe runs for the whole system and concluded that
there was no point in a vent being there. The boiler fills up under gravity
feed from the small tank and any excess pressure can just go straight back
up there if it wants. The boiler certainly isn't going to explode and anyway
there's a blow-off valve on the back of it.

So I dug out an old radiator valve, slapped it on the end of the vent in the
loft and turned it off. It's been that way for years and hasn't made a scrap
of difference to anything other than the loft is now dry. So what purpose,
if any, was the vent really serving?

The vent to the main part of the cylinder doesn't overflow of course because
it isn't pumped but again I see no point to it other than maybe to let any
air in the cylinder get out. However wouldn't the air be immediately
displaced out of a hot tap somewhere the first time the system was filled if
there was no vent?
--
Dave Baker
www.pumaracing.co.uk


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