How Tank-to-Mains Systems Work | Using Rainwater at Home

More and more homes across New Zealand are plumbing their rainwater tanks for reuse. Whether it’s 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’s a smart, sustainable way to reduce mains-water use and ease stormwater loads on councils — but the part that confuses most people is what happens when the tank runs empty.

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’s where a changeover device (also called a Rain-to-Main system) comes in.

A changeover device automatically switches between tank water and mains water. It has three connections:

Inlet 1: Tank water (from the pump)
Inlet 2: Mains water
Outlet: Supply line to the house

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’s 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.

Importantly, the mains line doesn’t refill the tank. It only supplies water to the house on demand.

Why “Top-Up” Systems Are Outdated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.
It reduces sustainability. You’re effectively mixing harvested rainwater and treated mains water for no real benefit.

Modern changeover devices are far smarter — they use mains water only when absolutely necessary, and only for the fixtures that need it.

What About the Pump Running Dry?

Another common concern is whether the pump will burn out once the tank’s empty.
Promax systems use pumps with in-built run-dry protection.

Here’s how it works:

When the pump detects there’s 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.

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.

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’ll never run out for day-to-day needs.
Whether you’re 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.