do i need to drain unvented water cylinder to fix water inlet pipe of toilet

17 Sep 2020
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United Kingdom

Need some expert advice...

I have a leak on a water inlet pipe of a toilet which looks straight forward to fix. However, I have a unvented water cylinder system (i think that’s what they are called - It has a big tank and a small tank).

It’s a small flat where the hot water cylinder and all taps is on one level, no upstairs or downstairs.

Can someone with knowledge tell me whether I need to drain the hot water cylinder in order to isolate the water to the toilet? I’ve heard about people experiencing problems of their cylinders imploding when they were changing their taps and I am not sure if this could happen I never the event of toilet water inlet change.

My plan is:
Turn off hot water cylinder at the timer.
Turn off water mains
Open cold water taps
Flush the toilet to clear water in tank
Change the water inlet pipe to toilet
Turn mains water on
Test for leaks
Turn hot water cylinder back on at the timer

Would be grateful if some kind person with knowledge to advise. I could be doing without an imploded water tank, leaking toilet and no job. Thanks
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1. If it has a big tan and a small tank, then provided they are both water tanks, less likely to be an unvented system.
2. Please post photographs of hot and cold tanks and pipework around them.
Suggest you fit an isolatation valve on the toilet fill pipe if there isn`t one. If there is one then there then there is no need to drain down anything.
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Sorry I having problems with taking photos. If it were a pressurised system, do I need to drain the tank to change the pipe to the toilet?
The hot water would have nothing directly to do with the cold water to the toilet. Turn off the cold water at the mains and see if the toilet stops filling when flushed.

If it's an unvented hot water (UVHW) cylinder then there may be a balanced cold water supply fed from the UV combination valve serving the cold water outlets in the property but without further information that can't be confirmed. Even then if it is UVHW then you need a qualified engineer in to look at it as it is not something that can be DIY'd.

As @just pumps suggested, does your HW cylinder look something like this?

Small tank above it


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Yes you have an unvented cylinder.

Find your mains stop tap and shut that off and see if the toilet stops filling.
Thanks. Would i need to drain the cylinder to fix the leaking toilet pipe?
The hot water would have nothing directly to do with the cold water to the toilet
Find your mains stop tap and shut that off and see if the toilet stops filling

If turning the mains off stop the toilet filling then it will be safe to repair the cold supply pipe to the toilet. You will have some water left in the pipe so watch out for that.
Thanks Madrab...
I appreciate your patience and helpful advice.
So are you saying that I just need turn off the water mains to fix the toilet inlet pipe and that this interruption will not affect the pressure to the water cylinder?
No need to drain cylinder and have taps open to stop pressure that cause imploding?

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