Search the forum,

Discuss How to isolate hot water from unvented cylinder to change tap in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

Messages
54
Hello all,

To start off I'm not g3 qualified.

I have been reading around about unvented cylinders imploding in some situations when cold water to them is isolated and hot water tap is opened, often in an attempt to reduce the pressure in the system to carry out some repair on a hot water outlet.

In order to change a tap, a washer, a shower etc is it necessary to drain down the whole cylinder (according to MI's)?

Does a cylinder need a complete drain down in order to temp isolate hot water.

Many thanks

This is a great forum.
 
No. Just shut off feed to cylinder and open the tap you want to work on- it’ll stop after a short time.
Open one on the floor below if there is one , leave that one open and do your job.
 
No. Just shut off feed to cylinder and open the tap you want to work on- it’ll stop after a short time.
Open one on the floor below if there is one , leave that one open and do your job.

Thank you for your reply. Can I ask how the problem of imploding occurs? I appreciate the pressure differences but how does it occur practically? Not opening the correct tap in the correct place in the house?

What does opening one on the floor below do? and if the tap is on the ground floor should one above it be opened?
 
Imploding occurs when as you say there is a pressure differential. Cylinders are designed, capable of quite high positive pressures but it does not take much under vacuum conditions for any cylinder to collapse. The position of the cylinder in the house also has an underlying affect on vacuum pressure generated. If you have a cylinder in the loft and the cold fill somehow became isolated and someone unknowingly open a hot tap on the ground floor the vacuum pressure created is enough for the cylinder to implode and as far as I know there are no domestic cylinders with internal baffles to strengthen its rigidity. In this case and in any potential vacuum situation an anti vacuum valve should be installed to prevent the above. These are installed on hot water out pipe from cylinder and under usual working conditions are held closed by the water pressure. In the event of a vacuum situation arising the valve opens allowing air into the unit where its internal pressure equalises with atmospheric pressure, thus preventing an implosion.
I must also add that these are used for the same reason if the hot water outlets can discharge water quicker than the cold fill can replenish.
 
Imploding occurs when as you say there is a pressure differential. Cylinders are designed, capable of quite high positive pressures but it does not take much under vacuum conditions for any cylinder to collapse. The position of the cylinder in the house also has an underlying affect on vacuum pressure generated. If you have a cylinder in the loft and the cold fill somehow became isolated and someone unknowingly open a hot tap on the ground floor the vacuum pressure created is enough for the cylinder to implode and as far as I know there are no domestic cylinders with internal baffles to strengthen its rigidity. In this case and in any potential vacuum situation an anti vacuum valve should be installed to prevent the above. These are installed on hot water out pipe from cylinder and under usual working conditions are held closed by the water pressure. In the event of a vacuum situation arising the valve opens allowing air into the unit where its internal pressure equalises with atmospheric pressure, thus preventing an implosion.
I must also add that these are used for the same reason if the hot water outlets can discharge water quicker than the cold fill can replenish.


Many thanks for this. So how would you go about changing a tap on the ground floor so that a vacuum doesn't occur when you are doing so?
 
Isolate cold supply, use the combined temperature and pressure relief valve to discharge the small head in cylinder and keep open as you open the downstairs tap. This should prevent any vacuum pressure being created.
Having said that are there not isolation valves on hot and cold to taps?
 
Isolate cold supply, use the combined temperature and pressure relief valve to discharge the small head in cylinder and keep open as you open the downstairs tap. This should prevent any vacuum pressure being created.
Having said that are there not isolation valves on hot and cold to taps?


Sadly there aren't no. Thank you for your reply. Very informative.

Will leaving the PRV open mean that if a vacuum is created by opening the tap that the "suck" against the walls of the cylinder will be prevented as the open PRV will allow air in, in effect acting as air admitance valve?

Are you saying that in reality the approach taken to isolation of the hot water will be depend on the specific install location of the unvented cylinder? And that there is no simple procedure for all situations.
 
If an unvented cylinder was installed in the loft and there was a problem and a ground floor tap was open this can generate enough vacuum to implode the cylinder, this of course shouldn't happen if the cylinder was on the ground floor and the tap in the loft. Implosion occurs when the pressure inside the cylinder is less than atmospheric but by keeping the combined temperature and pressure relief valve open, or installing an anti vacuum valve pressure can be kept equal to the atmosphere and imploding shouldn't occur. This of course is the same for any cylinder, open vented or unvented. As long as you can understand this then you should be fine.
 
If an unvented cylinder was installed in the loft and there was a problem and a ground floor tap was open this can generate enough vacuum to implode the cylinder, this of course shouldn't happen if the cylinder was on the ground floor and the tap in the loft. Implosion occurs when the pressure inside the cylinder is less than atmospheric but by keeping the combined temperature and pressure relief valve open, or installing an anti vacuum valve pressure can be kept equal to the atmosphere and imploding shouldn't occur. This of course is the same for any cylinder, open vented or unvented. As long as you can understand this then you should be fine.

Brilliant thank you

so basically cylinder height than tap then open valve

cylinder in sane floor or below no need just turn off cold feed and open hot to be worked on?
 
Yes. If the tap is above the cylinder you will only drop a little trapped water in the pipe and then safe to work on, after of course you've isolated cold to cylinder
 
Yes. If the tap is above the cylinder you will only drop a little trapped water in the pipe and then safe to work on, after of course you've isolated cold to cylinder

Thank you very much. I love finding out answers and the belt and braces way of going about things. Thank you for you time
 

Reply to How to isolate hot water from unvented cylinder to change tap in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

Creating content since 2001. Untold Media.

OFFICIAL SPONSORS

OFFICIAL SPONSORS

Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock