Novice - Two Water Tanks - Small One Overflowing?

20 Aug 2009
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United Kingdom
Hello, thanks for clicking on the post, hopefully you can help?!

First of all some background, we have two cold water tanks in our loft; A small one and a large one. The small one was filling up to the overflow pipe and was dripping onto our patio.

My wife called out a Plumber who looked at the problem and replaced both ballcock's. It seemed to have done the job but after that week it started dripping again.

Now, on investigating, when you press the ballcock down on the big watertank, water comes flowing into the tank, like you'd expect. However, the same doesn't happen on the small tank, no water comes in at all when you press it down. Weird right?

Now the small water tank has three pipes attached to it. Two which run down the hot water tank in the airing cupboard. One pipe attached at the bottom part and one hanging over the top of the tank. On the other side of the tank is a pipe which feeds the ballcock inlet. It goes the other side of the wall and I can't see where the pipe leads; Seems to the bathroom though.

Now what I can't work out, is if the ballcock inlet isn't letting in any water, how on earth is it getting to the overflow.

Can someone point me in the right direction and maybe explain...
- The purpose of two water tanks?
- Why 3 pipes to the small water tank..What does each one do?
- What do you reckon the fault is?

Thanks in advance!
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BIG TANK supplys your hot and cold water to bathroom, shower, kitchen etc.
SMALL TANK is a totally separate supply to your heating system.

Three pipes are a mains supply to fill the tank, a cold feed/supply to heating system and the over the top's your safety device or expansion pipe.

Is the small tank lower or at same level of the big tank?

The mains pipe that lets the water into the big tank should be connected to the pipe that fills the small tank. Is there a valve between the ballcock on the small tank and the point where it joins the supply to the other tank?
The small cistern is the feed and expansion tank for your heating system.
The large cistern is the cold water storage feeding your hot water system.

The F&E for the heating system should normally just have enough water in it to cover the bottom outlet by a couple of inches. You will probably have to push the float down almost to the bottom of the tank to open the valve.

The two are completely seperate systems. They only come into close contact where the heating system water runs through a coil of pipe inside the hot cylinder to heat it. If that pipe leaks inside the cylinder, it can allow water from the cold storage via the hot cylinder into the heating system, causing the F&E to overflow as the water in the two tanks tries to find it's own level.

Turn your heating off and shut off the mains water feed to the F&E. Bail out most of the water. If the level in the F&E tank rises, there's a pretty good chance the cylinder coil has failed.

To confirm, shut off the cold water feed to the hot cylinder and open a hot tap. the water flow should stop quite quickly. Monitor the F&E level. If it stops rising, or starts falling, then it's probably time for a new hot cylinder.
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if the ballcock dosent supply the tank when pushed down it could be the ballcock washer has perished and will need replacing. when you run either hw/ch go into the loft and see if any water comes from your expansion pipe the one which will come over the top. indicates overheating could be blockage faulty pump. if your coil has gone then you will notice that your water maybe dirty if no hot water has be drawn off. a way to check is attach an hosepipe to a drain point and drain your central heating by isolating the mains stopcock if the big cistern becomes empty then the coil has had it.

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