In earlier parts of this series we covered how a tank-to-mains changeover device works and how to lay out a compliant system. This part focuses on choosing the pump and the changeover device themselves. They are central to the performance and reliability of a rainwater reuse system, and they are where most of the decisions that determine how the system feels to live with actually get made.
Why the Pump Matters
The pump maintains flow and pressure across the fixtures being supplied by rainwater. When the pump is matched properly to the size of the home and the usage pattern, the system runs smoothly and reliably. When it is not, the symptoms show up at the tap rather than at the tank, which is why pump selection deserves more attention than it often gets.
When selecting a pump, consider:
- Flow rate: must meet fixture demand.
- Head pressure: must be enough to reach all outlets, particularly in multi-storey homes.
- Automatic operation: the pump should operate seamlessly with the changeover device.
- Run-dry protection: essential to prevent pump damage when the tank is empty.
- Noise and location: mount the pump on a stable, dampened base and protect it from weather exposure.
Powered vs Passive Changeover Devices
The changeover device determines how the system switches between tank water and mains supply when the tank runs low. Both types provide continuous supply, so the choice is not about whether you keep water at the tap. It is about what you want the system to do when conditions are less than ideal.
Powered Changeover Devices
- Use a solenoid valve to control supply.
- Require mains power to operate.
- Deliver reliable switching and stable pressure.
- Do not supply water during a power outage.
Passive (Pressure-Based) Changeover Devices
- Require no power, and operate using water pressure.
- Continue to supply mains water during power cuts.
- May provide slightly lower outlet pressure.
- Less suitable for complex or high-demand systems.
Choose passive where resilience during outages is important, or powered where consistent pressure is the priority.
Pre-Plumbed Tank and Pump Combos
For domestic homes and small commercial installations, a pre-plumbed pump and changeover system can greatly simplify the installation process. Rather than specifying, sourcing and matching the components separately, you take delivery of a tank with that work already done, which removes the step where mismatches usually creep in.
- The pump and changeover device are factory fitted to the tank.
- Components are tested for correct operation before delivery.
- On-site installation time is reduced.
- Council sign-off is simplified due to consistent configuration and documented specifications.
Pre-plumbed tank and pump configurations are available across the Promax slimline tank range.
Choosing the Right Tank Size
Tank size is set by one of two things, and it is worth knowing which applies to you before you start comparing volumes.
If the tank forms part of a compliance requirement, such as SMAF or a dual-purpose detention and retention system, the minimum volume will be set under the resource or building consent. In that case the number is not really yours to choose, and the task is confirming the tank meets it.
If the system is being installed voluntarily, tank size can be chosen based on available space, household usage, and sustainability goals. For guidance on household and seasonal performance, use the Promax tank size calculator.
Installation and Best Practice
A handful of details separate a system that goes in cleanly from one that has to be revisited:
- Confirm the changeover device includes a built-in dual-check valve for backflow compliance.
- Use lilac non-potable pipework and clear signage.
- Mount the pump in a sheltered position, securely fixed.
The backflow point is the one to confirm first, because it comes from compliance rather than preference. The lilac pipework and signage do a related job in a different way, making it obvious to anyone working on the property later which supply is which.
Testing and Handover
The system is not finished when the last fitting is tight. Test system operation by allowing the tank to empty, which confirms the automatic switching actually happens rather than being assumed. This is the check that proves the pump and the changeover device are working together as intended, and it is the one most likely to be skipped on a busy site.
Then provide the homeowner with the installation schematic and maintenance instructions. Someone will need those in a few years, and it will not necessarily be the person who installed the system. This series also covers maintenance, troubleshooting and handover in more detail.
Why a Pre-Configured Promax System Makes Sense
Everything above is a decision someone has to make. A pre-configured Promax system makes most of them in advance:
- Fully tested and matched components
- Automatic mains backup included
- Faster on-site installation
- Council documentation and compliance support
- Reduced installation risk and variability
The value is less about any single item on that list than about the variability it removes. A system assembled from separately chosen parts can work perfectly well, but it depends on every one of those choices being right and on someone having the time to verify them. A matched system starts from a configuration that has already been tested.