I have to change a friends kitchen sink and taps which looks manageable
enough but I note that the water/heating is an unvented cylinder with
expansion and pressure safety devices aplenty. I know the system delivers
hot water at cold mains pressure but how do I shut off the hot supply so I
can disconnect the tap(s). Does turning off the Cold main isolate the hot
supply as in combi boiler?

Thanks

"wounded horse" wrote in message
...
I have to change a friends kitchen sink and taps which looks manageable
enough but I note that the water/heating is an unvented cylinder with
expansion and pressure safety devices aplenty. I know the system delivers
hot water at cold mains pressure but how do I shut off the hot supply so I
can disconnect the tap(s). Does turning off the Cold main isolate the hot
supply as in combi boiler?

yes but you may find the stored pressure takes some time to run down

In an earlier contribution to this discussion,
John wrote:

"wounded horse" wrote in message
...
I have to change a friends kitchen sink and taps which looks
manageable enough but I note that the water/heating is an unvented
cylinder with expansion and pressure safety devices aplenty. I know
the system delivers hot water at cold mains pressure but how do I
shut off the hot supply so I can disconnect the tap(s). Does
turning off the Cold main isolate the hot supply as in combi boiler?

yes but you may find the stored pressure takes some time to run down

And you may have to re-establish the air bubble when you've finished - for
which there should be some instructions on the hot cylinder.

If you're *very* lucky, there may be isolator valves on the pipework under
the sink. If you can turn these off, and just disconnect the pipework on the
tap side of the valves, you won't have to worry about the pressurised
system.
--
Cheers,
Set Square
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Vot is re-establish air bubble pliz. There are no isolators under the sink.
so I need to know this before I decide to pass on this job. Thanks

In an earlier contribution to this discussion,
wounded horse wrote:

Vot is re-establish air bubble pliz. There are no isolators under
the sink. so I need to know this before I decide to pass on this job.
Thanks

I am by no means an expert on these things. But, AIUI, an unvented hot
cylinder has to have some trapped air above the water, to allow for
expansion when the water is heated - otherwise it would burst. These things
are sufficiently dangerous, anyway, that you need appropriate qualifications
to install them - it's not a DIY job. But I believe that there *is* a user
procedure for re-establishing the air pocket when necessary - typically when
the pressure release safety valve has operated. A friend of mine has got
one, and there are some instructions attached to the cylinder, detailing how
to do this.
--
Cheers,
Set Square
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I am by no means an expert on these things. But, AIUI, an unvented hot
cylinder has to have some trapped air above the water, to allow for
expansion when the water is heated - otherwise it would burst.

Note that only some unvented cylinders require this. I believe Heatrae Sadia
Megaflo is one. Other designs use conventional pressure vessels.

Christian.

"Christian McArdle" wrote in message
...
I am by no means an expert on these things. But, AIUI, an unvented hot
cylinder has to have some trapped air above the water, to allow for
expansion when the water is heated - otherwise it would burst.

Have you blocked up your safety pressure relief valve then?


Note that only some unvented cylinders require this. I believe Heatrae
Sadia
Megaflo is one. Other designs use conventional pressure vessels.

Christian.

In an earlier contribution to this discussion,
John wrote:

I am by no means an expert on these things. But, AIUI, an unvented
hot cylinder has to have some trapped air above the water, to allow
for expansion when the water is heated - otherwise it would burst.

Have you blocked up your safety pressure relief valve then?


I assume that's not a serious question! I was giving a simplified
explanation of how these things work to someone who had never come across
one. I suppose I could have said "otherwise the pressure would build up to a
dangerous level". Obviously, the pressure release valve would open before
the cylinder burst in the case of loss of air bubble - but that only happens
in exceptional circumstances, and I was describing the *normal* operation.
--
Cheers,
Set Square
______
Please reply to newsgroup. Reply address is invalid.

"Set Square" wrote in message
...
In an earlier contribution to this discussion,
John wrote:

I am by no means an expert on these things. But, AIUI, an unvented
hot cylinder has to have some trapped air above the water, to allow
for expansion when the water is heated - otherwise it would burst.

Have you blocked up your safety pressure relief valve then?


I assume that's not a serious question! I was giving a simplified
explanation of how these things work to someone who had never come across
one. I suppose I could have said "otherwise the pressure would build up to
a
dangerous level". Obviously, the pressure release valve would open before
the cylinder burst in the case of loss of air bubble - but that only
happens
in exceptional circumstances, and I was describing the *normal* operation.

It was "serious" insofar as the pressure would not rise until the cylinder
burst because the (3 bar) valve would protect it. You and I know it would,
but the uninitiated might have considered your response as a true statement
of fact and run screaming down the road whenever they saw an unvented
cylinderg

Unvented cylinders are multilevel protected - Back up limit thermostats,
Over pressure safety pressure relief valves, Over temperature relief valves
etc. The biggest problem is keeping them running not stopping them