Overflowing immersion heater water-tank with ball valve (problem)

5 Jul 2015
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United Kingdom

I have a strange problem with my immersion heater water tank overflowing. It overflows right over the top of the tank. It does this when the bath mixer tap is running BOTH hot and cold water, but not either one on their own. Strange. Why would this be? I will explain the story below with photos to paint a clearer picture.

This is my bathroom mixer tap.


If I just run the cold water (from the mains) from this tap there is no rise in the level of the water in the immersion heater tank. If I run just the hot from this tap, as you would expect the level goes down in the tank and the ball valve then successfully kicks in and the water level returns to normal without issue. No problem there.

However if I turn both the hot and cold on the bathroom mixer tap, the level of the water in the immersion heater rises and eventually pours over the top of the tank. This is my immersion heater tank (the top of it).


When the bathroom mixer tap is running both cold and hot water, no water comes out of either the ball valve or the grey hole or the hole labelled there with the red arrow. Water must be filling the tank from elsewhere, not from the top there. Eventually water comes pouring out of the hole in the tank labelled with the big red arrow. The water will continue to rise and pour over the top of the tank. The way to stop this from happening is to turn the bathroom mixer tap off, so no hot or cold.

I also have a mixer tap in my kitchen. I tried the same thing with the kitchen mixer tap, both hot and cold and no problem. The water does not rise. There is no issue with the kitchen tap.

This is the kitchen mixer tap.


So, does anybody know why this would happen with the bathroom tap? Thanks.
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Yes, your bath taps have been plumbed so that the cold water comes off the high-pressure watermain and the hot comes from your low pressure tank. This is a poor design which makes it impossible to get a satisfactory mix.

You have an unsuitable mixer tap which allows the high-pressure cold supply to push up the hot pipes and into the cylinder.

You could fit non-return valves on the hot pipes, or you could buy separate bathtaps instead of the mixer, or you could feed the cold bath pipe from your cold water storage tank.

UK-designed kitchen mixers are designed to prevent this happening, so that the drinking water is not contaminated.
The mains cold is back feeding up the hot pipe as they are not equal pressures. Ideally you should have separate hot and cold taps on a bath if the pressures are not equal. A non-return valve fitted on the hot pipe to the bath tap is a cheap fix but they can (and do) fail.
Thanks guys. I think it is going to have to be two new taps I think.
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Bristan are very good quality, and they make some in traditional styles.
You might want to check your overflow pipe isn't blocked. Not the cause of the problem, I accept.

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