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Water Pumps Switching On and Off

Cycling Water Pumps

When a water system has a problem the water pump may start switching itself on and off. Depending on what the cause is this can become an ongoing problem. It may start slowly and progressively get worse. A pump that is continuously switching on and off is known as "Cycling".

The sound can be very annoying but worse that that, if the problem is not found and repaired the pump can be damaged. Here we look at water pump cycling, what causes it and how do we stop it.

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Pumps are operated by internal pressure switches

water pump pressure switch

Pumps that are constantly switching themselves on and off are pumps that are operated by internal pressure switches. If we can understand how these switches work we can start to understand why our pump is constantly switching itself on and off.

A water pump works using an electric motor that turns an impellor (it is rather like a fan in an enclosed housing I suppose) which draws water in from your well or tank through an inlet pipe and pushes it out through an outlet pipe delivering the water to taps, toilets and showers around your house. Inside the pump are two switches which are operated by the pressure of the water in the outlet pipe.

The first, the low pressure switch, needs pressure in the water to keep it switched off. If the pressure in the water drops the switch switches the water pump on.

The second, the high pressure switch, is switched off when the water pressure gets too high.

So, we get in the shower and turn the water on. When we open the tap the pressure in the water pipe is released, the low pressure switch in the pump senses this and switches the water pump on.

When we have finished our shower we turn the tap off. The pump carries on running for a micro second until the water pressure in the pipe builds up and is high enough to operate the high pressure switch which switches the pump off.

Pressure switches must be set correctly

Of course both of these switches can be adjusted so we can control how much pressure we have in our shower. Under normal circumstances the low pressure switch is set at around 20 psi or 1.4 Kg/sqcm (Kilograms per square centimetre) and the high pressure around 40 psi although some people prefer higher water pressures than this.

If these pressures are set too close together then only small changes in the water pressure will switch the pump on and off.

Is your water pump too large?

If you have a large water pump then one small shower will not be able to keep up with the copious supply of water coming from your pump.

In this situation you turn the shower on and the water starts to flow releasing the pressure in the pipes and switching the pump on. But the water can’t get out of your shower head fast enough so the pressure in the water pipe builds up and quickly switches the pump off.

The shower head can now catch up and the water pressure drops once again which switches the pump on again. This cycle of the pump switching on and off is repeated ad nauseum. You can usually tell if this is happening because the water coming from your shower will pulse as the pump switches itself on and off. The lesson from this is don’t use large pumps to supply water to bathrooms. If you have lots of bathrooms use several smaller pumps rather than one large one. This is particularly the case if you are supplying bathrooms on more than one floor level because you will need higher pressure to pump water up to the second floor.

Adding a pressure tank to your water system

There is a way around the problem of a large pump supplying a small tap or shower head. We add a pressure tank sometimes known as an accumulator. You will have seen water pumps with a painted steel canister attached to them usually about 20 cms diameter and 30 cms long.

Compressing water is not easy, it doesn’t compress very much at all. Because water doesn’t compress you only need to release a couple of drops of water from a water pipe for the pressure to fall.

Air is somewhat different, it is very compressible. A large volume of air will cause only a small difference in pressure.

So we attach a tank full of air (the air is inside a rubber bladder) into the water pipe after the water pump. This we know of as a pressure tank. The water pump will now switch on and run for a while filling the tank and compressing the air until it is up to high pressure and the pump switches off. Now when we turn on the shower the air in the tank will slowly expand pushing the water out of the shower until eventually the pressure falls to the level at which the pump switches on and starts to pump the pressure back into the tank again. Note for those people who are actually following this technical diatribe that it is important that the low pressure setting in the pump must set higher than the pressure of the air in the pressure tank in its relaxed state or the pump will not switch itself on.

Pressure tanks need maintenance, sometimes the air escapes in which case the tank will need to be pumped up with air again. Sometimes the rubber bladder bursts and needs to be replaced. In either case once you lose the air the pressure tank will not work and the pump will once again start to switch itself on and off continuously.

Let us consider what will happen if we have a small leak in your house’s pressurised water system. A small leak of water will quickly reduce the water pressure in the pipes so the pump will switch on. It is only a small leak so quickly the pump will pump the water back up to pressure and the pump will switch itself off again. Then another small amount of water leaks out, the pressure drops and once again the pump switches on.

This is a very common cause of water pumps cycling - switching themselves on and off repetitively and sometimes quite quickly. The slower the clicking the smaller the leak.

Water tank ball valves and level sensors

Water pumps can also be switched on and off by valves or sensors in our water tanks.

For many decades water tanks were filled using a ball valve. A ball valve is a brass valve with an arm attached to it and a round ball float on the end of the arm. When the tank is filling up the float floats on the surface of the water eventually lifting the arm and switching the valve off. You turn on a tap and the water level in the tank falls, the float drops and this allows the valve to open. When the valve opens the pump will sense the lower pressure in the water and will switch itself on. As the tank fills up the ball valve will close, the water pressure will increase and the pump will switch off.

Ball valves are simple and reliable and have been used for more than 100 years particularly for filling toilet cisterns. There is a downside to these valves in that the tank is always full. As soon as the water level drops even a small amount the valve will open and the pump will switch on. This is unlikely to be the cause of your pump cycling on and off.

These days float sensors are widely used in water tanks which switch the pump on when the tank is empty and off when the tank is full. These do not cause pumps to cycle on and off.

How to find out why your pump is switching on and off

So if your pump is constantly switching on and off it is most likely a small leak in your water system. Try this, go around the house and make sure everything that uses water, hot or cold, is turned off. Is the pump still switching on and off? If it is check again, make sure there are no toilets running or taps dripping, ones that are often missed are a dripping jet washer in the toilet or the pressure relief valve on your water heater particularly a solar water heater on the roof. The pump still cycling? Pressure is leaking out of your system somewhere. Cycling slowly means a small leak, cycling quickly a larger leak, continuously on a very large leak or a badly set or non working pressure switch.

Go around the garden and see if you can find evidence of water leaking such as a patch of exuberant foliage (always a giveaway) or wet patches on the driveway when you don’t have a dog. Leaking pipes in walls (often caused by plumbers forgetting to glue the pipe joints) is a common problem. Try turning stop cocks in the piping system off to see if you can find what area of the pipework may be leaking.

If you cannot find the cause of the problem call a plumber. Get him to check the water pump pressure switch settings which may be set too close together. If the pump has a pressure tank then get him to check the air pressure in the bladder. If the bladder is torn it will need replacing, this is not expensive.

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Copyright © Phil Wilson May 2013
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