Building Construction, Renovation & Maintenance

Float Valves and Ball Cocks

Controls For Filling Water Tanks

Hidden away in our water tanks there is usually a small device that controls the level of the water in the tank. It turns the flow of water on to fill the tank and turns the water off when the tank is full. There are two types in very common use around the world, these are the ball cock and the float valve. They work in different ways so let us look at what they are and how they work.


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I recently came across a frightening case of a man who came home to find his water tank overflowing. The tank was set on the top of a high water tower and the electrical panels for the house were installed in the bottom of the tower. Water running down from the tank was drenching the main electrical distribution box. Obviously the electrical installation should never have been underneath the water tank. Water tanks will quite often have a fault and overflow.

Why water tanks overflow

When your water tank overflows you can bet your life it will be the float valve.

A float valve is a small device that controls the flow of water into your water tank.

There are, in fact, two different types of valves in water tanks.

Ball Valves

The first is the humble ball valve. These have been around since Victorian times and are simple and reliable. A plastic (formerly copper) hollow ball on the end of a brass rod floats on the surface of the water. If the water level drops the float falls a little and opens a valve allowing more water to flow into the tank. The water pipe to the tank is pressurised directly by a water pump and as soon as the valve opens the pump senses the drop in pressure and switches on so filling the tank until the ball rises again to shut off the valve. These valves are simple and usually very reliable but have one distinct disadvantage - the valve is on or off dependent on a single water level. The tank is always full. This arrangement means that as soon as you use any water the pump will start and as long as water is being used the pump will be running which effectively means you are not able to make use of the tank's storage capacity (unless you have a power cut).

Float Valves

A far better arrangement is to have a high and low water level. The tank fills to the high level, then water can be used progressively until the water falls to the lower level at which point the tank starts filling again back up to the high level. This means the pump that fills the tank is only operating now and again.

To achieve this most tanks these days have an electric float valve which switches the water pump on and off. The valve has a high and a low water level. Water can be used until the valve senses that the low water level has been reached at which point it switches the pump on. The pump runs to fill the tank until the high water level is reached at which point the valve switches the pump off.

These valves operate day in day out for years without any attention but, every now and again they have a habit of failing at which time you either have no water or an overflowing tank.

How does a float valve work?

An electrical switch is mounted in the top of the tank which is operated by a vertical cord hanging down inside the tank. Pull the cord down the switch switches on, let it go and the switch switches off - simple.

An electrical cable runs from the switch down to the pump to switch the pump on and off.

The cord has two floats attached to it, one set at the high water level and the other set at the low water level. These floats have to be adjusted so that the weight of one float is not enough to activate the switch but the weight of the two floats together is.

The floats are plastic cylinders about 8cms long by 3cms in diameter. They are made in two halves that screw together in the middle. You can increase their weight by unscrewing them and allowing water inside.

Adjusting a float valve

To adjust them it is necessary to fiddle about adding or taking water away from the two floats until they operate effectively and with some leaway. When properly adjusted the weight of the upper float alone will not operate the switch but the weight of both very reliably will. Obviously both floats must still be light enough to float.

The heights should be adjusted to give a good range of water between the high and low water levels so you are making good use of the volume of the tank. This means that the pump will not switch on and off so often and will run for a reasonable length of time, this will minimise wear on the pump.

Why do float valves fail?

When they fail float switches of this type usually fail in the ON position for one simple reason, water leaks into one of the floats over time thereby increasing its weight, it no longer floats and so doesn't deactivate the switch. The pump keeps running and the tank overflows.

If you have a faulty float valve it is not easy to solve the problem. Float valves are usually not easy to get at and fumbling around at the top of a tank tower can be pretty dangerous.

If your tank starts overflowing the best option is to switch off or unplug the water pump and call someone to come and fix it - in daylight. If you run out of water in the meantime you can always switch on the pump again for a half an hour or so.

If your tank is in your roof space you may have a lot of expensive damage if the float valve fails and the tank overflows onto your ceilings particularly if your ceilings are Gypsum (plasterboard).

As a final comment it is dangerous but not unusual in some parts of the world to have electrical circuits under tank towers. This is not a good idea so perhaps you might want to check and, if need be, arrange to get them moved.


Copyright © Phil Wilson January 2010
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