Choosing a water tank pump
The pump is the part of a tank system that wears out. Choosing the wrong type is the usual reason it does.
The most common mistake with a tank pump is buying on horsepower alone. A pump that is too strong for the pipework short-cycles, and one that is too weak leaves you with a dribble in the shower. Match the pump to the job, then to the tank.
The three jobs a tank pump can do
Most tank pumps fall into one of three roles. Know which one you want before you look at a single model.
- Garden transfer. Moves water from the tank to sprinklers or a hose. Flow matters more than pressure, and a simple manual or float-switched unit is often enough.
- Pressure pump for the house. Holds steady pressure to toilets, laundry or taps. It has a pressure switch or a controller and usually a small pressure vessel.
- Submersible. Sits inside the tank. It is quiet and never needs priming, but you have to lift it out to service it.
Surface pump or submersible?
A surface pump lives beside the tank. It is easy to reach, but it has to draw water up the inlet pipe, so it can lose prime and it makes noise. A submersible sits below the waterline, so it is always primed and it is almost silent.
| Question | Surface pump | Submersible |
|---|---|---|
| Loses prime? | Yes, if the suction side leaks | No |
| Noise | Audible, needs a sensible location | Very quiet |
| Servicing | Easy access | Has to be lifted out of the tank |
| Runs dry damage | Common when the tank empties | Common when the tank empties |
Neither is better in every case. If your tank is small and close to the house, a submersible avoids the noise. If you want simple servicing, a surface pump wins.
Size the pump on flow and lift, not on the label
Two numbers do the real work. Flow is how much water the fixtures you run at once will want. Head is how high and how far the pump has to push, expressed as pressure. Add the height from the water surface to the highest outlet, then allow for pipe friction over the run.
Oversizing is not a safety margin. A pump much bigger than the plumbing needs will start and stop constantly. Ask any supplier for the flow and head curve of the pump, not just its power rating.
Why tank pumps fail
Three faults cover most complaints.
Short-cycling. The pump starts and stops while a tap is running. Usually the pressure vessel has lost its air charge, or there is a small leak on the outlet side letting pressure bleed away.
Runs but delivers little. Look at prime first, then a blocked strainer or foot valve, then whether the tank level is lower than you think.
Dies after a dry spell. Running dry cooks the seals within minutes. A low-level cut-out or an automatic switch to mains is cheap insurance, and it is the fix most systems are missing.
What you can check yourself, and what you cannot
You can check the tank level, clean an accessible strainer, listen for the cycling pattern and note whether the pump is still wet at the base. That gives a licensed plumber a much better starting point.
Anything that opens the plumbing to the house supply is a different matter. Tank water can never be allowed to flow back into the drinking supply, which is why mains top-up needs an air gap or approved backflow prevention. If the tank water is also meant for drinking, read is tank water safe to drink first, and see sizing a rainwater tank for the supply side.
Frequently asked questions
Is a submersible or a surface pump better for a rainwater tank?
Neither wins in every case. A submersible is quiet and never loses prime but has to be lifted out to service. A surface pump is easy to reach but can lose prime and is louder.
Why does my tank pump keep starting and stopping?
That is short-cycling. It is usually a pressure vessel that has lost its air charge, or a small leak on the outlet side letting pressure bleed away.
Can a bigger pump fix low pressure?
Rarely. Oversizing tends to cause short-cycling. Check for blocked strainers, air leaks on the suction side and pipe restrictions first.
How do I stop a pump running dry?
Fit a low-level cut-out or an automatic changeover to mains. Running dry damages the seals quickly, and it is the most common cause of a burnt-out pump.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- NHMRC. Australian Drinking Water Guidelines
- Australian Building Codes Board. National Construction Code — Volume Three (Plumbing Code of Australia)