How To Select A Suitable Pump

by Aug 18, 2020Help & Advice

What sort of pump do you need?

There are a lot of rainwater harvesting pumps with different features and specifications available.

How to select a suitable water pump? Let’s narrow it down.

In rainwater harvesting there are three general applications:

1. Garden or external use only
2. Whole of House
3. Complimentary water systems (plumbed to toilets, washing machine, garden tap with automatic mains water switching)


Garden or External Pumps?

Rainwater Tank Pumps Grunfos jpc Claytec
The most affordable pumps in the range, a garden pump is often plumbed to only one tap. They are designed to get water out of the end of a hose at decent pressure, run a small sprinkler system, top up the pool and wash the car.

The Garden pump typically has a maximum flow rate of between 40 and 60 litres per minute and maximum head pressure of between 30 and 50 metres.

Below are the Performance Curves for the Grundfos JPC Series. It shows that the JPC 3-42 has a maximum flow rate of nearly 60 litres per minute and a maximum head of 42 metres. The more powerful 4-54 has a maximum flow rate of nearly 80 litres per minute and maximum head of 54 metres.

What Does ‘Flow Rate’ and ‘Maximum Head Pressure’ Mean?

Flow Rate refers to the total volume of water that is pumped over a certain period of time.  We generally refer to this as  Litres Per Minute.

Maximum Head Pressure refers to the total amount of pressure the pump can build up, expressed in metres. In theory, a pump with a maximum head of 38 should be able to shoot water 38 metres into the air. A maximum head pressure of 38 metres equates to 380 kilopascals (kPa). Likewise, a maximum head of 50 metres equates to 500 kPa.


So why does the graph show a curve that relates ‘flow rate’ to ‘head’? This is the Performance Curve. The actual performance of the pump is somewhere in the middle of the curve. The Grundfos JPC 3-42 will tend to operate at about 30 litres per minute at 250 kPa.  It follows then that if the Grundfos JPC 3-42 was pushing out 60 litres of water it would be doing so with no pressure. Once you start obstructing the flow of water from the pump by adding pipes, valves, filters, tap and hoses, the pressure increases and the flow decreases. If you keep obstructing the flow, by slowly turning the tap off, the pressure will build up to its maxi-mum head of 420 kPa and the water will stop flowing … and the pump should shut down.

Whole of House Pumps

Whole-of-House pumps tend to have a much greater flow rate as they are required to supply water to many outlets and appliances at once with relatively constant pressure. One way of doing this is to have multiple impellers in the pump. The impeller is a bit like a circular fan that’s designed to throw water. The extra impellers operate in stages where one impeller throws to the next and so on. This is called a multistage pump.

A typical multi-stage rainwater harvesting pump will provide maximum water flow of around 100 litres per minute with a maximum head of between 30 and 60 metres.  The Performance Curve below is for the multi-stage range of pumps from Grundfos, the CM Booster range.

From this we can see that ther CMB-SP 3-47 will operate at approximately 35 litres per minute with 350 kPa of pressure (middle of the Performance Curve).

The Performance Curve to the right is for the 5 Stage CM Booster.

Water Tank Pumps - Performance curve

Variable Speed Pumps for Whole-of-House Applications

Variable speed pumps regulate their performance to achieve constant pressure according to water demand. As the flow rate demand
increases, by turning on multiple taps for example, inverter technology increases the speed of the pump to keep the pressure constant. This also provides energy savings as the pump consumes less electricity.

The Scala2 from Grundfos is a robust, composite, variable speed pump with a fully integrated motor, tank, sensor and non return valve. The water cooled motor ensures quiet operation and can be completely covered for further noise insulation.

 Backed by Grundfos 2+2 Year Warranty

Rainwater Tank Pumps Grunfos_Scala2 Med-

“Constant Pressure, Energy Efficient, Quiet, Reliable”

Looking at the Performance Curve for the Scala2 we can see that the pump can operate anywhere in the shaded area.   As water demand increases so does the pump, regulating the flow and pressure at the water appliance. This means perfect water pressure all the time and savings on your power bill.

Complimentary Water Systems

Water tanks installed on new homes with mains water access are generally plumbed to the toilets, washing machine and at least one outside garden tap. Up to 40% of the water used inside a typical home is used for flushing toilets and washing clothes. But the water tanks are quite small and can deplete quickly in dry seasons.

Pumps with mains water switching devices were developed to address this issue. When the tank runs dry the mains water switch opens the mains water line to the toilets and washing machine and prevents the pump from activating.  A good system will then activate at least once per day to test if there is any water in the tank.

In our experience the most reliable mains water switch on the market is the Beltrami Acquasaver. It even comes with an industry leading 5 year warranty.


Submersible Pumps

Submersible pumps are installed inside the water tank. They generally have a separate controller that is installed outside the tank or with a controller that is integrated into the pump.

Submersible pumps are very quiet and don’t take up any space outside of the water tank. For those reasons, submersible pumps have become the choice for Complimentary Water Systems.

Many submersible pumps are multi stage pumps with a maximum water flow of 80 to 100 litres per minute and maximum head of 350 to 500 kPa.