Backflow prevention is a critical requirement in factories, laboratories, and medical facilities across New Zealand and Australia. These environments commonly use chemicals, additives, or process fluids that pose a serious contamination risk if they enter the potable (drinking) water supply.

Why is backflow prevention required?

Any connection between a potable water supply and a system containing chemicals or contaminants presents a high hazard. A common example is a potable water connection to an HVAC system, where corrosion inhibitors, biocides, or glycol may be present.

If backflow were to occur and these chemicals entered the potable water system, the consequences could be severe, including serious illness or death.

A real-world example often referenced in the industry involved a medical centre in the Midwest United States, where two patients died after consuming water that was believed to be potable. The potable supply feeding an HVAC holding tank had no RPZ (Reduced Pressure Zone) device installed. When backflow occurred, chemicals migrated through the system and discharged from basins and sinks throughout the building.

While this incident occurred overseas, the risk is the same in NZ and Australia, which is why water authorities classify these connections as high hazard and require RPZ backflow prevention.

Fixtures used for personal hygiene – a common design mistake

A frequent design error is connecting fixtures used for personal hygiene downstream of an RPZ simply because they are located in the same room.

This results in a system that is:

• Non-compliant

• Not fit for purpose

• A potential health risk

Example: Cleaner’s room

In a typical cleaner’s room you may have:

• A sink used for mixing chemicals

• A basin used for handwashing

The sink must be protected by an RPZ to prevent chemical contamination of the potable supply.

However, if the handwash basin is also supplied downstream of the RPZ, the design allows water that is not potable to discharge from a fixture where a user reasonably expects drinking-quality water. This creates a serious risk of accidental consumption.

Best practice:
Only fixtures that require protection from backflow should be supplied downstream of the RPZ. Personal hygiene fixtures must remain on a potable supply unless specifically approved and clearly labelled.

How does backflow occur?

Backflow is the reversal of the normal direction of water flow within a plumbing system.

It most commonly occurs due to:

• A sudden drop in pressure in the upstream water main

• Water main breaks or maintenance

• Firefighting demand

• Pump failures

When upstream pressure drops, contaminated water can be drawn back into the potable supply unless a backflow prevention device is installed.

Without correctly installed backflow prevention, contaminants from one building can enter the water supply and affect multiple downstream buildings, not just the site where the issue originated.

Sizing RPZ valves correctly

RPZ valves must be sized based on flow rate and allowable pressure loss, not simply matched to pipe size.

Manufacturers provide performance graphs that allow designers to:

• Select the appropriate RPZ size

• Understand the expected pressure drop across the valve

It is common—and acceptable under NZ and Australian standards—for the RPZ size to differ from the inlet pipe size.

Example

For a flow rate of 0.6 L/s
(approximately a 25 mm pipe at 1.5 m/s maximum velocity):

• A 20 mm RPZ or a 25 mm RPZ may both be suitable

• Both options may result in a similar pressure loss

Selecting the smaller valve can often reduce cost without compromising performance, provided manufacturer limits are met.

Drainage requirements for RPZ valves

RPZ valves are designed to discharge water under certain fault or pressure conditions. This is normal operation and must be allowed for in the design.

Manufacturers provide discharge graphs that show:

• Expected discharge flow rates

• Discharge volume at various inlet pressures

Designers must ensure there is a suitably sized drain beneath the RPZ to safely accommodate this discharge.

Example

A 20 mm RPZ with an incoming pressure of 483 kPa can discharge approximately 2.4 L/s.
This is often far more water than designers expect, and undersized drains are a common issue during commissioning.

Other important design considerations

When designing RPZ installations in NZ and Australia, also consider the following:

• Strainers
  A strainer should always be installed upstream of the RPZ (and is typically required by regulations) to protect the valve from debris and damage.

• Internal installations
  RPZ valves are relatively large and not particularly aesthetic. When installed indoors, coordinate locations early with the architect to ensure adequate space, access, and drainage.

• Alternative backflow devices
  In situations where the risk of cross-contamination is lower, alternative backflow prevention devices may be permitted and can be more cost-effective than an RPZ. Always confirm hazard classification with the local water authority.

• Signage
  Fixtures supplied by RPZ-protected water must be clearly labelled with
  “Warning – Not for Drinking” signage, as required by NZ and Australian standards.

Are there any other backflow design considerations you’d like to see covered in this article?
Let us know in the comments.

If you want, I can also:

• Tighten this for a technical engineering audience

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Tighten this for a technical engineering audience

Got it — here’s a tighter, more technical version aimed squarely at engineers, hydraulic designers, and specifiers, with less storytelling and more compliance-driven clarity, aligned to NZ & Australian practice.

Backflow Prevention Design for Factories, Laboratories, and Medical Facilities

Technical considerations under NZ and Australian standards

Backflow prevention is a mandatory requirement for factories, laboratories, and medical facilities throughout New Zealand and Australia, where potable water supplies are connected to systems containing chemicals, additives, or other contaminants.

Under AS/NZS 3500 and local water authority requirements, these connections are generally classified as High Hazard, requiring Reduced Pressure Zone (RPZ) backflow prevention.

Why RPZ backflow prevention is required

Any potable water connection to a system containing chemicals (e.g. HVAC dosing systems, laboratory equipment, process plant, or cleaning systems) presents an unacceptable contamination risk.

If backflow occurs, contaminants may be drawn into the potable water supply, creating a serious public health hazard. For this reason, potable connections to such systems must be protected by an RPZ device in accordance with:

• AS/NZS 3500

• Local water authority requirements (e.g. Watercare, Sydney Water, SA Water)

RPZ devices provide protection against both back-siphonage and back-pressure, making them suitable for high hazard applications.

Separation of personal hygiene fixtures

A common design non-compliance occurs when fixtures used for personal hygiene are connected downstream of an RPZ because they are located in the same room.

This is not acceptable.

Example: Cleaner’s room

Where a cleaner’s room contains:

• A sink for chemical mixing (high hazard)

• A handwash basin for personal hygiene

The sink requires RPZ protection. However, the handwash basin must not be supplied downstream of the RPZ.

Supplying personal hygiene fixtures from an RPZ allows non-potable water to discharge from fixtures where potable water is expected, creating a significant health risk and a non-compliant design.

Design principle:
Only fixtures that require backflow protection due to contamination risk should be supplied downstream of an RPZ.

Mechanisms of backflow

Backflow is the reversal of normal flow within a water supply system and typically occurs due to:

• Sudden drops in upstream supply pressure

• Water main failures or maintenance

• Firefighting demand

• Pump or system failures

Without appropriate backflow prevention, contaminants can migrate into the potable water supply and affect not only the source building, but also other connected properties.

RPZ valve sizing

RPZ valves must be sized based on design flow rate and allowable pressure loss, not simply matched to pipe diameter.

Manufacturers provide performance curves that allow designers to:

• Select the appropriate RPZ size

• Quantify pressure loss at design flow conditions

It is acceptable for the RPZ valve size to differ from the connected pipe size, provided velocity and pressure loss criteria are met.

Example

For a design flow of 0.6 L/s
(approximately a 25 mm pipe at 1.5 m/s maximum velocity):

• A 20 mm RPZ or 25 mm RPZ may both be suitable

• Pressure losses may be comparable for either selection

Valve selection should be confirmed against manufacturer data and system pressure requirements.

RPZ discharge and drainage design

RPZ devices are designed to discharge water during fault or abnormal pressure conditions. This discharge is a functional requirement and must be accommodated in the hydraulic design.

Manufacturers provide discharge data indicating expected flow rates at various inlet pressures.

Designers must ensure:

• A drain is provided directly below the RPZ relief port

• The drain is sized to handle the maximum anticipated discharge

Example

A 20 mm RPZ at an inlet pressure of 483 kPa may discharge approximately 2.4 L/s, which often exceeds typical assumptions if manufacturer data is not reviewed.

Undersized drains are a common cause of commissioning issues and non-compliance.

Additional design considerations

The following items should be addressed during design and coordination:

• Upstream strainers
  Strainers should be installed upstream of RPZ valves to protect internal components and are typically required by regulations and manufacturers.

• Access and space requirements
  RPZ valves require adequate clearance for testing, maintenance, and relief discharge. Internal installations should be coordinated early with the architect due to valve size, drainage requirements, and visual impact.

• Alternative backflow prevention
  Where hazard classification permits, alternative devices may be acceptable and more cost-effective. Hazard level should always be confirmed with the relevant water authority.

• Signage
  Fixtures supplied by RPZ-protected water must be clearly labelled
  “Warning – Not for Drinking” in accordance with NZ and Australian requirements.

This article focuses on the most common RPZ design issues encountered in industrial and laboratory environments.
If there are additional technical topics you’d like covered—such as testable devices, commissioning requirements, or authority approval processes—please let us know.