More and more homes across New Zealand are plumbing their rainwater tanks for reuse. Whether it is a simple roof-collection system, a stormwater retention tank, or a SMAF-compliant dual-use setup in Auckland, the principle is the same: harvest rainwater, store it in a tank, and reuse it for non-potable purposes like toilet flushing, laundry and outdoor taps. It is a smart, sustainable way to reduce mains-water use and ease stormwater loads on councils. The part that confuses most people is what happens when the tank runs empty. This article, the first in our Using Rainwater at Home series, answers that.
The Common Question: What Happens When the Tank's Empty?
When your tank runs dry, you still need water to flush toilets or run the washing machine. That is where a changeover device, also called a Rain-to-Main system, comes in. A changeover device automatically switches between tank water and mains water. When the tank water line has pressure, the changeover device allows tank water to flow to your toilets, laundry and outdoor taps. When the tank is empty and the pump stops producing pressure, it automatically switches over to mains supply, ensuring continuous water flow without you having to do a thing. You do not get a notification and you do not need one, because nothing about your day changes.
How a Changeover Device Is Plumbed
The device itself is straightforward. It has three connections:
- Inlet 1: tank water, from the pump
- Inlet 2: mains water
- Outlet: supply line to the house
The important detail is what the mains connection does, and what it does not. The mains line does not refill the tank. It only supplies water to the house on demand. Tank water and mains water both reach the same outlet, but they never mix in storage, and the tank stays a rainwater tank. That single design decision is what separates a changeover system from the older approach, and it is the reason the rest of this article makes sense.
Why "Top-Up" Systems Are Outdated
Older systems used to keep tanks topped up with mains water once they dropped below a certain level. That approach is inefficient for two reasons. It wastes water: if it rains the next day, all the mains water that topped up your tank gets displaced and overflows straight out. And it reduces sustainability, because you are effectively mixing harvested rainwater with treated mains water for no real benefit. You have paid to treat that water to drinking standard and then flushed it down a toilet, or watched it run out the overflow. Modern changeover devices are far smarter. They use mains water only when it is absolutely necessary, and only for the fixtures that need it.
What About the Pump Running Dry?
The other common concern is whether the pump will burn out once the tank is empty. Promax systems use pumps with in-built run-dry protection, so it will not. Here is how it works. When the pump detects there is no water in the tank, it automatically shuts itself off. It waits for around 24 hours before retrying. When rain refills the tank, the pump detects water again and resumes normal operation. That means no damage, no manual resets, and no wasted mains water. The pump looks after itself, which is exactly what you want from a piece of equipment sitting outside that nobody thinks about until it stops.
Why This Setup Makes Sense
Plumbing a tank for reuse with a changeover device offers several benefits:
- Saves water. You use every drop of rainfall before touching mains supply.
- Automatic operation. No manual valves or intervention needed.
- Compliant design. Works with rainwater, stormwater and SMAF dual-use systems.
- Pump protection. Smart run-dry safety built in.
- Council-friendly. Meets NZ plumbing and backflow requirements for non-potable reuse.
Taken together, these are the reasons the changeover approach has become the default rather than an upgrade. Nothing on that list asks anything of the homeowner.
The Bottom Line
A properly installed rain-to-main system is simple, efficient and sustainable. It makes the most of free water from the sky while ensuring you will never run out for day-to-day needs. The tank does the work when there is water in it, the mains covers the gap when there is not, and the switch between them is not your problem. Whether you are building new, retrofitting, or designing a SMAF-compliant site in Auckland, a Promax tank paired with a changeover device is the smart way to go.
The Rest of the Series
This article covers how the system works. The other three parts of Using Rainwater at Home cover what you put in it and what you do with it afterwards:
- Selecting the Right Pump and Changeover Device, on choosing the hardware described above.
- Plumbing Layout and Council Compliance, on how the pipework is arranged and what your council expects of it.
- Maintenance, Troubleshooting and Handover, on keeping the system running once it is in.