Cold water cistern overflow mystery...

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29 Sep 2004
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Good Evening,

My cold water cistern has been overflowing. The float valve seemed to be working OK but I replaced it anyway. Somehow the cistern is still filling up and the only other culprit would seem to be the cylinder vent pipe.

The problem's intermittant and the cistern is a bit difficult to reach so there's no way I can confirm this short of camping out in my attic for a day or so.

So my question is, what would cause water to travel up the cylinder vent pipe into my cold water cistern and what can I do to fix it, or what else could be causing the problem?

Cheers in Advance. ;)
 
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When you say you replace the float valve! did that include the valve seat where the rubber disc seals the water off. I had a situation where the plastic valve seat had a hair line crack in it and as the float came up to its highest point and put pressure on the valve seat it open up the crack.
Regarding the vent pipe, As the water gets hot it does increase in volume with the excess going into the water cistern. Now suppose the water level in the cistern is quite high and the overflow pipe going outside is low it may not take much expansion to have a overflow situation.
If you have a lower water level in the cistern you also have a lower level in the expansion pipe, which means theres more room in the cistern for the excess without having a overflow situation. I suggest you lower the level by an inch or so.
:rolleyes: :rolleyes: :rolleyes:
 
Cheers for the reply. I dried the whole area round the valve off and watched for a few minutes so I'm pretty sure it's nothing to do with the valve.

I have a new, exotic theory as to what might be causing the problem...

The basin mixer tap in my bathroom has started hissing. Put your head next to it and you can hear water flowing through the tap when it's switched off! I'm wondering if cold water is flowing down the hot water pipe flooding the cylinder and cycling back into the cistern causing the overflow. It's one of these modern sealed mixer taps and I have no idea what goes on inside these things so I don't even know if this is plausible.

I'm going to cut off the supply to the tap and see what happens. Anyone come across anything like this before?
 
Hi Chops,

Have you got water coming into your feed and expansion tank - the small one?
 
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L_Plate said:
Hi Chops,

Have you got water coming into your feed and expansion tank - the small one?

Hiya,

Not sure what you mean; basically the expansion tank seems to be operating OK and isn't overflowing.

Cheers.
 
I was thinking it is possible that you may have a perforated heat exchanger in your hw cylinder. This will mean that your primary circuit (CH) and your secondary circuit (HW) are effectively one and the same and you will need to have your cylinder replaced. Bare in mind that you will see no evidence from the F&E overflow but you should notice water coming in via the (F&E) cold feed; it could be small depending on the size of the perforation.

What this means is that you will not see any water coming in through your vent pipe filling up your cw cistern. Instead the excess water will be coming from the F & E cistern into the cylinder. The only way the excess water can go is up through the hw distribution pipe from the cw cistern. This will eventually cause the overflow.

You describe the problem as intermitent in which case this possible solution would appear less likely.
 
Ah OK, that's a possibility.

I'm going to try some other things before I check the F&E cistern for the simple reason that my loft hatch is 2' square hole over a 30' deep stairwell and I am not great with ladders. :eek:

I'm going to start another thread about my hissing tap, because even if this has got nothing to do with my cistern, I figure a tap shouldn't hiss.

Cheers.
:D
 

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