Water cistern overflow leaking after turning stop cock, a mistery?

[Ambergreen45]

Hi,

I had to go away for a few days, so turned off the stock cock and opened the bath's cold water tap (to make sure water was off).

On returning my bathroom carpet was soaked with water. I discovered the water was coming out of the toilet water cistern overflow and leaking onto the floor rather than going outside. The tap inside the water cistern does not leak and the ball valve always shuts the tap, so how could water still rise and overflow? I does not make any sense. Just wondering if anyone could help me to understand this mystery?

Many thanks.

 

[Newboy]

In some installations the WC cistern overflow links to the bath waste/overflow so it's possible for the bath waste water to leak through the WC overflow

 

[Ambergreen45]

In some installations the WC cistern overflow links to the bath waste/overflow

The WC cistern is completely independent. As the bath tap was turned on but the mains shut, there would not have been water pressure against the WC cistern tap. If somehow water was siphoned due to air in the pipes, it should not have gone passed the WC cistern tap. So, I'm still puzzled...

 

[Newboy]

where does the WC overflow run too?

 

[Ambergreen45]

where does the WC overflow run too?

The overflow runs to the outside wall but I've discovered it's not joined properly just before it passes through the external wall, therefore leaking onto the floor.

 

[Newboy]

Well that limits it to the cistern overfilling.

The supply wasn't shut off completely and the cistern oveflowed due to a very slow let-by from the float-valve

 

[TonyW2]

If cistern is fed from a cold water storage tank, then turning off mains at stopcock still allows it to fill from the store.

 

[Ambergreen45]

The supply wasn't shut off completely and the cistern oveflowed due to a very slow let-by from the float-valve

That seems the most logical answer, it does make sense now. Thanks again!

 

[Ambergreen45]

Well that limits it to the cistern overfilling.

The supply wasn't shut off completely and the cistern oveflowed due to a very slow let-by from the float-valve

Sorry to resurrect this thread again (last week's), but I'm trying to find out if I need to replace the toilet cistern float-valve or not, so I did a couple of tests on it.

Test 1: I did not use the water cistern for a couple of days and left the mains water supply on. The water level in the water cistern did not rise at all, therefore it did not overflow. The same thing happens when I get up in the morning, the water level is still the same, which makes me think the float valve does not leak with normal water pressure.

Test 2: I turned off the mains stop cock and left the bath's cold water tap turned on to make sure any excess water if any, would go out that way, being the lowest point. The bath tap level is about 4 inches lower than the water cistern float-valve. However, after several hours, the water cistern float-valve lets water through very very slowly which would build up for an overflow. Is this normal? If so, what causes it?