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