The float valve on a rainwater tank
A small valve decides whether your tank tops up quietly or runs the mains all night.
A float valve is a mechanical tap that shuts itself off when the water reaches a set level. On a rainwater tank it controls a mains top-up, so the pump never runs dry in a dry spell. When it works you never notice it. When it fails you either run out of water or waste a lot of it.
How a tank float valve works
A float rides on the water surface and is linked to a valve. As the level drops, the float drops and opens the valve to let mains water in. When the level rises again, the float lifts and closes it. There is no electronics in a simple one.
Some tanks use a different set-up, with an automatic switchover at the pump that draws from the mains when the tank is low. Check which you have before you diagnose anything.
Why a float valve sticks
Most failures are small. Grit and scale stop the valve from seating, a worn washer lets a trickle past, and a bent arm leaves the float jammed. Insects and debris get into a tank that is not well screened.
- Valve will not close: the tank overflows on dry days, or the water meter keeps moving.
- Valve will not open: the tank runs low and the pump struggles.
- Hissing at the inlet: water is still passing through a worn seat.
A stuck valve is not the same as an overflow problem
If the tank spills only in heavy rain, the valve is probably fine and the overflow is the issue. Our guide to why a water tank overflows separates the two.
| Symptom | Likely cause | What to do |
|---|---|---|
| Overflows on dry days | Valve not closing | Licensed plumber |
| Pump runs dry, tank empty | Valve not opening, or mains shut off | Check the supply tap, then a licensed plumber |
| Water meter moving with no one home | Valve leaking through | Licensed plumber |
Is it worth keeping the mains top-up?
For a garden-only tank, probably not. Letting it run low costs nothing. For a tank feeding toilets or the laundry, a top-up is what keeps the house supplied in a dry spell. Decide based on what the tank feeds, not on habit.
How to read a float valve problem without touching it
You can learn a lot from the ground. Work through these checks before you ring anyone.
- Find out which set-up you have. Some tanks use a float valve. Others use an automatic switchover at the pump. The checks below differ.
- Look at the water level. Note whether it sits high, low or at its usual mark, and whether it changes on dry days.
- Listen at the inlet. A hiss means water is still passing through a worn seat.
- Check the water meter. If it keeps moving with no one home, the valve may be leaking through.
- Check the supply tap. If the tank is empty and the pump struggles, make sure the mains supply is on before blaming the valve.
- Separate it from an overflow. Spilling only in heavy rain points at the overflow, not the valve.
Write down what you saw, then call a licensed plumber. A mains top-up joins the tank to the drinking supply, so adjusting or replacing it is licensed work. Our tank overflow guide helps you separate the two faults.
Does your tank need a mains top-up?
The answer depends on what the tank feeds. Decide on that, not on habit.
| The tank feeds | Top-up needed? | Reason |
|---|---|---|
| Garden only | Probably not | Letting the tank run low costs nothing |
| Toilets | Usually yes | A top-up keeps the house supplied in a dry spell |
| Laundry | Usually yes | Same reason as the toilets |
| Hot water system or kitchen tap | Ask a licensed plumber | Water quality and plumbing standards both matter |
| Several uses at once | Yes, if any use cannot stop | Decide on the use you cannot do without |
If you keep a top-up, it must be fitted so mains water can never be drawn back from the tank. That is a backflow rule, and it is licensed plumbing work in every state and territory. See how a tank is plumbed for where the valve sits in the line.
Frequently asked questions
What does a float valve do on a rainwater tank?
It opens when the tank level drops and shuts when it is full, so a mains top-up keeps the pump supplied without overfilling.
Why does my tank float valve keep running?
Usually grit, scale or a worn washer is stopping the valve from seating. A bent float arm can do the same.
Can I replace a tank float valve myself?
If it is on a mains top-up, no. It connects to the drinking supply and is licensed plumbing work.
Do I need a mains top-up on a rainwater tank?
Only if the tank feeds things you cannot do without, such as toilets or the laundry. A garden-only tank does not need one.
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)