Cold water tank overflow

[Wheeler]

Apologies that this topic has been covered before but I haven't found symptoms the same as mine in previous posts.

I have a Potterton Promax boiler, about 10 yrs old I think. This seems to work fine, heating the water and rads via a 3-way valve. The problem I have is that the cold water tank (the large one, not the heating expansion tank) overflows when all the hot water has been used up. This has showed up recently in hot weather when I haven't been heating as much water and it has run out a couple of times. The rads have not been on for some time.

I have replaced the cold feed ballcock but (warm) water still comes up into the tank from the expansion pipe so the overflow constantly runs. When this happened recently I stopped it by running the boiler for 30 mins and heating up some water.

May not be relevant but I have a 3-storey house with the cold water tank in the loft. Hot water tank is just below it in an airing cupboard. Boiler on the ground floor.

Any suggestions ?

 

[picasso]

Is your boiler expansion tank above the cold water tank ? My first thought would be a split in the hot water cylinder coil.

 

[Wheeler]

They are at the same level, both in the loft. If the coil is split, wouldn't it pressurise the tank when it's hot as well as cold?

 

[kbdiy]

I have replaced the cold feed ballcock but (warm) water still comes up into the tank from the expansion pipe so the overflow constantly runs. When this happened recently I stopped it by running the boiler for 30 mins and heating up some water.

This is the bit I'm struggling to understand. I would have expected the flow from the expansion pipe to occur when the boiler is running but you seem to be saying it is exactly the opposite - so when it's cold (no boiler running) water runs from the expansion pipe but not when it's hot?. I'm fascinated to know what is causing that.

 

[Hugh Jaleak]

Heat may cause the metals in the system to expand, thus sealing the leak somewhat whilst the system is running. Once it cools down again, the split will reopen, allowing the water to transfer from primary to secondary side.

Often found with cast iron boilers, a leak in the hex stops when boiler is running, only to manifest once again when boiler cooled down.

 

[kbdiy]

Ok but in this instance both the F&E and Cold Water tanks are sitting at the same level, so there is a greater head of water in the CWS tank than in the F&E tank. If there was a split in the coil then water from the CWS would push back into the F&E and cause that to overflow. However here the hot water cylinder is (apparently) expanding over the CWS tank when the boiler isn't running. So if there was a split in the coil there can't be sufficient static head to cause that to happen.

The OP doesn't mention any problem with the F&E tank so I can't see how it could be a split coil.

 

[Wheeler]

Thanks for the suggestions.

To confirm kbdiy, there is no problem with the F&E (central heating ?) tank. the top of this is significantly lower than the CWS tank.

 

[bellairaphon]

Might be perhaps a mixer tap passing. Back filling the cylinder! You may find condensation on a shower mixer or tap. Just a thought.

Nathan

 

[Wheeler]

Hugh Jaleak, I'm hoping it's not a leak in the coil for obvious reasons! If there is one I think the overflow would run more than it does. Recently it has gone for weeks with no leaks at all then started again when the hot water ran out one day. It seems to be all or nothing.

 

[kbdiy]

For water to pass from the cylinder expansion pipe there must be some kind of pressure being applied to it, otherwise in an idle state the water level in the pipe would be the same as the cold water tank. That's just basic physics.

Just to confirm, you have actually seen water coming from the expansion pipe into the cold water tank and are not just making an assumption?

So, is there a pump anywhere on the hot water circuit (to the taps) and do you have any kind of secondary circulation circuit (to minimise deadlegs) as it is quite a large property? Any possibilty of a mixer tap with mains cold water supply passing through to the hot water side and forcing back through the cylinder (really clutching at straws now!). Something has to be pushing that water over the top.

 

[Wheeler]

I have not actually seen water going into the tank from the overflow but I have replaced the ballcock on the cold water feed (made no difference) and the water in the tank gets warm. Is there any other way this could happen other than from the cylinder expansion ?

There's no pump for hot water (other than on the boiler circuit), do you mean a pump specifically to feed the taps ?

As for a secondary circuit, I'm afraid I don't know the answer to that. I can show you a photo of the airing cupboard plumbing if it helps...

I have two mixer taps which are quite new but also two showers with hot & cold feeds. Is there a way of checking these without replacement ?

 

[kbdiy]

Ok I think you need to prove beyond doubt that the water is coming from the expansion pipe. There can't be many reasons why it does this, especially when the boiler is off and the cylinder is not being heated - therefore no expansion of the heated water would be taking place.

You need to devise a way of catching any water that may be coming from the pipe before making any further assumptions or guesses. A small bowl floated in the cold water tank just below the outlet for the pipe perhaps? It will need to be secured but remain open so it can overflow into the tank if it does fill up. On no account seal the pipe to it as it must discharge into free air over the tank. Then check it on a regular basis and if it does catch any water try and determine what has happened during that period (boiler on/off, showers used etc etc) to narrow down what might be causing it. Then report back.

 

[Wheeler]

Ok, thanks for the suggestions.
I will update you in due course.