Rainwater Reuse Systems Explained

Rainwater reuse systems collect rainwater from your roof and store it in a tank for use in toilets, laundry, and outdoor taps. This reduces mains water use, lowers stormwater load on council networks, and helps homes become more resilient and efficient.

This guide explains how the system works, how to lay out the plumbing correctly, how to select a pump and changeover device, and what maintenance is required long term.

1. How Tank-to-Mains Rainwater Systems Work

The core of a rainwater reuse system is the changeover device (also called a rain-to-mains valve). It ensures that when the tank has water, the house uses rainwater, and when the tank is empty, the system automatically switches to mains water.

It connects three points:

• Tank water inlet (via the pump)
• Mains water inlet (backup supply)
• Outlet to fixtures (toilets, laundry, outdoor taps)

When rainwater is available at pressure, that is used. When the tank runs low and pressure drops, the device switches to mains. The mains supply does not refill the tank.

Modern systems avoid the wasteful old approach of topping up tanks with mains water.

2. Plumbing Layout and Council Requirements

Most compliant systems follow a clear, repeatable layout.

Standard Layout

• Roof to tank: gutters → leaf guard → optional first flush diverter → tank inlet
• Tank to house: tank outlet → pump → changeover device → supply lines
• Overflow: tank overflow → approved stormwater drain

Backflow Prevention

A dual-check valve is typically suitable for low-risk domestic reuse (toilets and laundry). Many Promax changeover units include this built in. :contentReference[oaicite:1]{index=1}

SMAF Dual-Use Tanks (Auckland Only)

In Auckland SMAF zones, a single tank provides both stormwater detention and rainwater reuse.

• Upper volume = controlled detention
• Lower volume = reuse water supply
• Flow is restricted via a calibrated orifice to reduce peak stormwater runoff

Tools to assist system design and consent:

• Promax SMAF Resources
• Slimline Specification Tool
• Underground Tank Installation Tool

All reuse pipework must be lilac-coloured and labelled Non-Potable Water – Do Not Drink.

3. Selecting the Pump and Changeover Device

The pump maintains water pressure and the changeover device controls supply. :contentReference[oaicite:2]{index=2}

Choosing a Pump

• Match flow and pressure to household size
• Use a pump with run-dry protection
• Mount securely and protect from weather

Changeover Devices

Powered devices: stable pressure, require electricity.

Passive devices: work during power cuts, slightly lower pressure.

Pre-Plumbed Tank Systems

For faster installation and easier sign-off, Promax offers systems supplied with pump and changeover device already fitted and tested.

• Slimline Tank & Pump Combo
• View the full Slimline range
• Tank Size Calculator

4. Maintenance, Troubleshooting and Handover

Routine checks help ensure long-term performance. :contentReference[oaicite:3]{index=3}

Maintenance

• Clear leaf guards and inlet screens
• Clean pump filters periodically
• Check overflow mesh
• Confirm signage on reuse outlets

Download the O&M Guide: Slimline Tank Operation & Maintenance Guide (PDF)

Troubleshooting

• Pump not starting → check power and tank level
• Low pressure → clean filters and confirm pipe sizing
• No water during power cut → consider passive changeover device