"Aidan" wrote in message
ups.com...

Dave Baker wrote:

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.

With all due respect, you're daft.
Take it off.

So what purpose,
if any, was the vent really serving?

In normal operation it releases any air that gets into the system.
Typically dissoled oxygen in the boiler return wil be released from
solution when the water is haeted in the boiler and is discharged from
the open vent.

In the event of a control failure, it is a safety device; it releases
steam, prevents the system becoming over-pressurized and bursting. In
your system, such a failure would probably now cause the contents of
the heating system to be discharged into the loft and the boiler would
dry-fire to destruction.

It was pumping over probably because it was piped incorrectly. Get
someone competent to fix that problem. Conversion to a sealed system
would be an improvement, if the boiler is suitable; if you'd like that
done, get someone competent to do that.

The boiler certainly isn't going to explode and anyway
there's a blow-off valve on the back of it.

I find your faith in a cheap safety valve to be touching.

They don't have to do anything in normal operation. If called upon to
operate in anger, a disturbing number fail to work. Many have been
dripping inconspicuously for years, the water evaporates from the hot
safety valve and the accumulation of limescale deposits concretes the
valve immovably shut. Have you tested the valve?

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?

Kin 'ell! Is this a wind up? Leave it alone.

Immersion heater contacts frequently weld themselves together, so the
heater fails on. The open vent would then discharge steam. If you block
it, the steam will blow the contents of the cylinder into the storage
tank. If that discharge route is blocked, this happens;

http://www.waterheaterblast.com

That is a small water heater (12 US gallons). Drivel loves posting that
link to promote his thermal stores, but someone proposes plugging the
vent on a domestic hot water storage system and he makes no comment; he
is a dangerous idiot.

Unvented water heaters are good,

I disagree. Heat banks give better performance as they can take higher
pressures becaus eof the plate heat exchangers, open vented and a minute to
zero chance of an explosion. Unvented cylinders require an annual service
charge. That alone is enough to discount them totally...especially when a
safe, service free, better performing unit is available....the heat bank.

but they have at least 3 sequential
safety devices for each heating system. They are costly. A vented
cylinder cannot be converted.