An Introduction To Choosing The Right Pond Pump

Every pond pump driven by electricity is fitted with a motor. The motor is normally rated in watts or kilowatts. This is a measure of the amount of energy the motor can generate at absolute maximum and at 100% efficiency

Have you ever tried carrying a bucket of water up a flight of stairs? It can be a bit tiring. Now try carrying 2 buckets of water up the same stairs and you will feel extra tired ... in this second instance you have used twice as much energy to lift twice as much water to the SAME height. Let's now think of a pump in a pond. Normally you want to pump water up to a waterfall and when you do this you may have noticed the water flow is less than what was coming out at ground or at a lower level.

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This is because the pump has a motor operating at its maximum operating point. In practical terms this means if you want to lift the water higher than the pond surface then with a fixed amount of energy the pump compensates by reducing the amount of water it can lift.

You can thus see that any pump will share its energy ... some to flow and some to lift. The more the lift the less the flow and vice versa.

Every pump box states MAXIMUM HEAD and MAXIMUM FLOW. What do these bits of information means and why are they pretty useless bits of information?

MAXIMUM HEAD

This means the height at which flow STOPS ... eg if this figure is 6 feet then at a height anywhere between 0 and 5.9 feet there will be some, but different amounts of, water flow. However at 6 ft there will be NONE. The number is only useful for comparing models (and then not very useful)

MAXIMUM FLOW

This happen at the level of the pond surface and is the flow with no pipe attached to the pump. Probably the best practical situation that closely resembles rerality is when all you need the pump for is simply circulating the pond contents. In other words there is no lift required or there is no significant length of pipe attached to the pump

The energy from a pump's motor is shared between flow (actually mass) of water and the height (head) to which it must be pumped. In a real pond situation it becomes a bit more complicated because when water flows through a pipe, or via a "T" piece, "Y" piece bend, fountain nozzle, or through a valve etc some energy is required to overcome friction losses in these different parts of the system.

Because the pump's motor (energy output) is fixed it means that the water flow is reduced for any specific height pumped. We refer to these "things" as flow restrictions. In practice you need to remove or lower the impact of any restrictions to flow. You therefore will get better pump performance if you do the following ...

  • Use widest diameter pipe you can

  • Use shortest possible pipe length

  • Do not kink the pipe (a big problem with thin walledplastic pipe)

  • Do not use valves, T pieces, Y pieces, sharp bends etc in your pipework if you can avoid it.

  • Prevent build-up of algae inside the pipe by using NON-transparent piping.

  • When you use stepped connectors cut off the smaller diameter steps.

To totally and accurately specify a pump you need to know 3 things

  1. Volume of water flow required

  2. Height to which this flow must be pumped

  3. Head (or friction loss) loss due to restrictions in pipe and fittings.

This last component is complex to calculate and this is where you should use my calculator.

In a normal situation if you add 20% to the height you want to pump this will normally be safe. You would use this new number .... Actual Height + 20% .... to specify the pump.

How To Convert Different Measurement Units

  • To convert gallons to litres multiply by 4.54

  • To convert gallons to US gallons multiply by 1.2

  • To convert US gallons to litres multiply by 3.78

  • To convert cu.feet to gallons multiply by 6.23

  • To convert cu.feet to US gallons multiply by 7.48

  • To convert inches to cm multiply by 2.54

  • To convert feet to metres multiply by 0.305

  • To convert pounds to kilograms multiply by 2.2

  • To convert sq. feet to sq. Metres multiply by 0.093

  • To convert cu metres to litres multiply by 1000

Of course to convert in the other direction you divide by the factor. For example to convert litres to gallons you would divide by 4.54