One of the reasons why most rainwater systems use too much power is because because the pump is too big. For other reasons see Rainwater – Pump Issues
Most pump suppliers and design charts will specify a larger pump than you really need (They sell a more expensive pump, It’s less likely that the pump will be returned because it isn’t big enough, and they aren’t paying the power bill)
If you want to pick the most economic pump here is how to go about it.
What is the most flow you need at any one time. Here are some figures for typical house fittings.
10 to 15
Tap with Flow Restrictor
4 to 6
Low Flow Shower
7 to 9
4 to 10
4 to 6
3 to 5
10 to 15
*To get to the lower figure you will need to close the supply valve this will add a few minutes to the wash but will help with issues like Water Hammer.
**It’s really better to irrigate the garden with a separate pump.
Add together the highest flow rate fittings that you think you will want to run together, which will give you a Total Flow Rate ‘Q’
You need to aim for a pressure at the fitting of around 150kPa (15m of Head)
To get this pressure you need to:
Measure the height of the furthest fitting above the lowest level in the tank ‘Hs)
Calculate the Pressure Loss ‘Hf‘ due to Friction in the pipes See table below
Max Flow Rate Litres/min
Hf m head per 100m of pipe
The Required Pump Pressure is then calculated from:
Required Pump Head ‘P‘ m = 15 +Hs + Hf
Or Required Pump Head ‘P‘ KPa = 150 +(Hs +Hf)) x 10
To order a pump you just need to quote the Total Flow and Required Pump Head
A pressure tank, which I think is the most important upgrade to a standard rainwater supply system, looks like the photo on the right.
Small tanks are mounted on the pump, larger tanks like this one are on the ground.
What you get is a cylindrical storage tank storage tank with an internal membrane.
The top part of the tank, above the membrane, is filled with compressed air.
The bottom part of the tank will be filled with water by the pump from a connection at the tank base.
In the top half of the tank there is an air valve where you can check the air pressure and add more air if necessary.
How It Works
The top of the tank is pre-charged with air to a pressure slightly lower than the ON setting of the pressure switch.
When the pump starts water is pumped into the tank compressing the air until the maximum pressure is reached.
If you turn on a tap the compressed air will push water out of the tank until the tank is almost empty (when the pressure will have dropped to the minimum level)
If you use less water than that stored in the tank the pressure doesn’t drop enough so the pump doesn’t start.
Once the pressure switch turns the pump back on, the pump fills the tank while at the same time supplying water to your system. Even if you turn the tap of the pump will continue to run until the tank reaches maximum pressure.
Although very small tanks are available about the smallest tank I would recommend would be an 18litre tank, which would give a flow volume of around 5litres between pump starts.
A 35litre tank with a flow volume of around 11litres between pump starts would ensure a toilet flush would not exhaust the tank.
Why Should You Get A Pressure Tank?
It can cost more than some pumps to buy the pressure tanks so why buy it is an important question.
To find out the problems with a standard set up see the post “Rainwater – Pump Issues”
The pressure tank should:
Double, or even treble the life of the pump, by reducing the number of pump starts and pump run time.
Significantly reduce the power usage by ensuring the pump mainly runs at the design rate.
Reduces the noise nuisance by less frequent pump runs.
This photo shows a fairly typical pump installation.
An electric pump with pressure sensor control. It’s even got a fairly large diameter flexible suction hose between the tank and the pump.
So…..What are the issues?
Well water is an incompressible substance so every time some water leaves the system the pressure drops enough to trigger the sensor and the pump starts…………..That happens whether you are having a shower, putting a splash of water on your hands, or even a couple of drops dripping from a slightly leaky toilet valve.
This leads to Three Issues.
Electric pumps are designed to pump efficiently at their designed rate. If you are using water at a slower rate than the design rate the pump wastes energy trying to pump at its designed flow.
A lot more power is used to start the pump then when the pump is running for some time.
A Recent Study for the CSIRO showed that a typical domestic rainwater system used more energy than traditional centralised water treatment and distribution systems.
The main thing that wears pumps out is continually starting rather than running.
Because the pump runs frequently you are more likely to notice the noise. If you have got a leaking tap or toilet valve it will be starting up several times through the night.
If you have got a large block with a slope of several metres it may be possible to install a header tank. This solution however is not normally possible for most of us.
The best solution for the average house is to upgrade by installing a Pressure Tank. Follow the link to find out more.