Thanks to Dennis from Aquatrek (Supadiverta) for this explaination as I was tossing between the variable speed pump vs a standard power with auto pressure switch and a 100litre pressure tank set up.
PUMPS: This comes down to the use.
A variable speed pump will deliver constant pressure because its small pressure tank (set at a high pressure) delivers the initial water demand at the correct pressure while the pump is spooling up. This is important if the pump is going to deliver water to the shower as there will be no pressure fluctuations like you will have if you have a standard pump with a pressure tank. A variable speed drive pump will start every time but because it only delivers the required flow of water, it is very energy efficient and there will be energy cost savings.
A pressure tank connected to a standard pump will save on a lot of start – stops but a 100 litre pressure wave tank is about $400. You can get a Davey 60 litre tank for about $200 though. The tanks have about 30-35% draw down, depending on the pressure setting.
The pressure tank’s factory pressure setting can be changed and most pump pressure switches are either 20/40 psi, 30/50 psi but some (rarely) are 40/60 psi. The higher the cut out pressure – the less draw down but less the pressure fluctuation. For a shower, you shouldn’t consider a 20/40 psi due to the fluctuation.
The pump must have pressure greater than the cut out setting, in other words, if the pump had a maximum output of 55 psi, you wouldn’t have a cut out of 60 psi.
The pump’s cut in is set 2-3 psi above the tank’s pressure setting so that the tank doesn’t run out of water prior to the pump cutting in. The pump will operate at its most efficient at its peak power because it will be supplying both the fixture in use and the tank. Energy savings will be from the reduced number of starts and the fact that it will not be strangled when otherwise supplying short duration runs when supplying low flow fixtures like cisterns.
If the pump has an electronic timer, it must be factory set to the required pressure settings. An electronic timer also has run dry (cut out) protection but the switches are not adjustable. If you fit a manual pressure switch (they are adjustable), it will not have run dry protection and if the tank runs dry, the motor will keep running.
Some manufacturers make electronic pressure switches that are set for pressure tanks and they can often be used on other pumps but there would be an additional expense to have one connected.
You can link a larger pressure tank to a smaller one already fitted to a pump but the pressures must be the same and as per the smaller tank’s factory setting.
There are some cheaper pumps available now that have pressure tanks (and factory set electronic pressure switches) fitted and the two in mind are the 800 W Commercial Electric (Bunnings brand) and the Einhell 550 W. Both are available with 20 litre pressure tanks but they are set at 20 psi. The Einhell is $259 complete and both pumps are similar in that they have a pre pump filter, full parts back up and a 2 year warranty. They have given these pumps every chance to have a long life and an additional 60 litre Davey tank gives 80 litres at around $460 plus some connecting hose and parts, say $500 all up. The additional pressure tank doesn’t have to be near the pump.