Advice on Overflow on Cold Water Storage Cistern

[FozzieBear]

I am looking for advice on a problem I have with my 50 Gallon cold water storage tank which regularly overflows.
Previously the ball valve was a Torbeck Diaphragm Valve but it kept getting grit in it and overflowing. I have therefore changed this to a good quality brass float valve with a plastic float.

However after a period of days the water level rises and the overflow drips continuously until you run off water.
I think the issue might be the overflow. Inside the tank is a plastic elbow type fitting (almost like a Bylaw 30 kit but not), that extends approximately 100mm down into the tank.
I think what might be happing is that when the water rises it create a siphon and the water then siphons out of the tank via the overflow. As the water level drops the valve opens and more water comes in so the process continues until a toilet is flushed or hot water run off which breaks the vacuum.
I have tried bending the brass lever to lower the water level slightly but will I be safe in drilling a small hole in the plastic elbow so that it lets in air and breaks the vacuum so preventing the siphon? If the water does rise due to failure of the valve it will still run out the overflow.
Any thoughts would be appreciated.

 

[Burnerman]

Can you just turn the elbow so that it becomes horizontal? It is syphoning, as you say.
A type 2 brass valve with an adjustable float may make your water level task easier, but buy a quality one!
John

 

[oldbuffer]

Interesting problem. Some thoughts:

1. For a syphon to start, the water level would have to rise sufficiently to cover the top of the overflow pipe (not just the dip pipe). If it doesn't, the air from the overflow pipe would break any syphon.
2. If its rising that far, then the overflow is doing its job and the cistern is overfilling.
3. Could be a fault elsewhere, e.g. a pin hole in the hot water cylinder primary coil.

You could try:
A. Your own idea of a small hole in the elbow. You could re-seal it with a stainless steel self tapping screw and some PTFE / silicone later.
B. If feasible, make sure the cistern is full, and isolate the cold feed to the cistern for a day or two and see if the problem recurs.
C. If there is an expansion pipe from the hot water cylinder over the cistern, is this running intermittently?

 

[terryplumb]

Forget the idea of siphoning ,its not happening. Nor do you have a pin hole leak in your hot water cylinders coil.You need to establish why the level is getting so high as to be anywhere near the overspill level. Do you have any mixer taps / shower valves that are supplied with mains cold water and gravity fed hot ? If you do ,one of them is allowing mains cold into the hot distribution pipework ,and onward into the CWSC.

 

[FozzieBear]

Can you just turn the elbow so that it becomes horizontal? It is syphoning, as you say.
A type 2 brass valve with an adjustable float may make your water level task easier, but buy a quality one!
John

I can try turning the elbow. I have bought and fitted a good quality brass float valve and float from a reputable plumbers merchants not a cheap one from Wickes or Screwfix.

 

[FozzieBear]

Forget the idea of siphoning ,its not happening. Nor do you have a pin hole leak in your hot water cylinders coil.You need to establish why the level is getting so high as to be anywhere near the overspill level. Do you have any mixer taps / shower valves that are supplied with mains cold water and gravity fed hot ? If you do ,one of them is allowing mains cold into the hot distribution pipework ,and onward into the CWSC.

No all my shower mixers and sink mixers are supplied from the storage tank and HW cylinder at the same pressure including kitchen sink which has a separate CW drinking tap. The HW cylinder was replaced a couple of years ago for a larger cylinder and was clean as a whistle with no scale as we have a water softener installed.
I am convinced the water is siphoning as the dip tube goes down into the water at normal fill levels. Can I safely drill a hole in the elbow to break the vacuum?

 

[FozzieBear]

Interesting problem. Some thoughts:

1. For a syphon to start, the water level would have to rise sufficiently to cover the top of the overflow pipe (not just the dip pipe). If it doesn't, the air from the overflow pipe would break any syphon.
2. If its rising that far, then the overflow is doing its job and the cistern is overfilling.
3. Could be a fault elsewhere, e.g. a pin hole in the hot water cylinder primary coil.

You could try:
A. Your own idea of a small hole in the elbow. You could re-seal it with a stainless steel self tapping screw and some PTFE / silicone later.
B. If feasible, make sure the cistern is full, and isolate the cold feed to the cistern for a day or two and see if the problem recurs.
C. If there is an expansion pipe from the hot water cylinder over the cistern, is this running intermittently?

Thanks for your suggestions. See responses to your questions:
A. Will probably try that approach
B. Not feasible as would drain the tank with showers and WC flushes
C. No the expansion is fine. No overflow from HWC

 

[terryplumb]

You need to establish what is causing the water level to rise in the tank ,after the ball cock has fully closed. It is definitely not the overflow ,that is absurd.
Bend the float arm so the water level does not reach the overflows down pipe when the valve is fully closed. That will prove to you that the rising water level is nowt to do with a siphonic overflow !

 

[dilalio]

I am convinced the water is siphoning as the dip tube goes down into the water at normal fill levels

Wow. That's means I can drain HW cylinders and cisterns without having to suck on a hose, from now on

 

[DP]

Do you have mixer taps or shower that is on mains cold and gravity hot water?
If yes, look at that fitting as this can sometimes raise CWC water lever to result in overflow situation

 

[dilalio]

Do you have mixer taps or shower that is on mains cold and gravity hot water?
If yes, look at that fitting as this can sometimes raise CWC water lever to result in overflow situation

Been covered above mate.
OP says all off tanks

 

[terryplumb]

Been covered above mate.
OP says all off tanks

And could possibly be wrong !

 

[dilalio]

And could possibly be wrong !

That's why I added the at the end of my post

 

[terryplumb]

That's why I added the at the end of my post

I got that ,but it was real subtle