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Preventing water backflow


What is backflow?

Backflow is an unwanted flow of potentially contaminated water in the reverse direction.

Backflow of water presents a risk of contamination to the water supply and the distribution lines in an establishment. The use of prevention devices at key locations can help you prevent backflow and the contamination of the water supply.


The Canadian Food Inspection Agency (CFIA) created this document as guidance to help food businesses comply with the requirements set out in the Safe Food for Canadians Regulations.

It's your choice

You may use other guidance developed by provincial governments, industry associations, international partners or academic bodies as long as they can achieve the outcomes identified in the regulations. Always ensure that the guidance you choose is relevant for your particular business, product or products, and market requirements.

What is included

The document outlines:

  • Why backflow occurs and how you can prevent it

Refer to the Tell me more! section for additional sources of information that may help you with backflow prevention.

What is not included

While the document provides examples of measures to prevent backflow, it is not exhaustive ­the preventive measures needed will depend on the size and complexity of the food business and be unique for each business. The document does not cover:

  • The protection of municipal water systems that supply establishments
  • Fire service piping systems

Roles and responsibilities

Food businesses are responsible for complying with the law. They demonstrate compliance by ensuring that the commodities and processes for which they are responsible meet regulatory requirements. If a written preventive control plan (PCP) is required, the food business develops a PCP with supporting documents, monitors and maintains evidence of its implementation, and verifies that all control measures are effective.

The CFIA verifies the compliance of a food business by conducting activities that include inspection, and surveillance. When non-compliance is identified, the CFIA takes appropriate compliance and enforcement actions.

Preventing water backflow

Methods used to prevent backflow may be simple or complex depending on the nature of the food processing environment. You may choose to have a qualified party assess your facility's backflow prevention needs and maintenance.

Why does backflow occur?

There are two causes of backflow:

1. Back-siphonage – where contaminated water or undesirable liquids are sucked back into the potable water system.

Back-siphonage can occur when the following conditions take place at the same time:

  • pressure in the potable water system drops below atmospheric pressure
  • a supply valve is open
  • the outlet of a supply valve is immersed in a fluid other than potable water, and
  • there is no protection from back-siphonage, an existing device or method is malfunctioning

2. Backpressure- where contaminated water or undesirable liquids are pushed back into the potable water system.

Backpressure occurs in one of two ways:

  • Mechanical means, causing backpressure when the water supply system is connected to another system that is operating at a higher pressure
    • For example, a booster pump or elevated piping can cause the higher pressure
  • Thermal expansion, causing backpressure when water is heated. Excessive thermal expansion of water results in pressure on the system and, if left unchecked, may cause backflow
    • For example, thermal expansion can be caused by a malfunctioning water heater or boiler

Let's look at a common example of backpressure causing backflow:

If your facility has a secondary or alternative water system, such as a sprinkler system, there is a risk of forming back pressure if the two systems are interconnected. Backflow is most likely to occur when the following conditions exist:

  • backpressure rises above the supply pressure, or vice-versa
  • a supply valve is open
  • the potable water system is connected to a device or system containing a fluid other than potable water
  • there is no backflow prevention device installed (or the device fails)

How can I prevent backflow?

Federal, provincial, or municipal plumbing codes require backflow prevention for potable water systems. A well designed and properly maintained air gap at the supply fixture is the best means available for protection against backflow. However, an air gap is not always practical and is vulnerable to bypass arrangements that can nullify its effectiveness. If you do not use an air gap at the supply fixture to protect against backflow, a mechanical backflow preventer will serve the same purpose.

You can use several types of devices to prevent backflow as long as the device effectively prevents back-siphonage and can withstand backpressure if needed. A trained professional can help you choose the device that best fits your facility and system design.

Selecting prevention devices

There are six basic types of devices that can be used to prevent backflow:

  • Non-mechanical
    • air gaps
    • barometric loops
  • Mechanical
    • atmospheric and pressure vacuum breakers
    • double check valves with intermediate atmospheric vents
    • double check valves
    • reduced pressure devices

Inspection and testing

Inspection and testing activities differ for non-mechanical and mechanical backflow preventers. The following describes the type of inspection and testing that should be done for each of them.

Non-mechanical backflow preventers:

  • visual inspection to ensure that they are in place and have not been altered

Mechanical backflow preventers:

  • visual inspection of internal seats, springs, and moving parts that are subject to fouling, wear or fatigue
  • visual inspection to ensure they are still in place and have not been by-passed intentionally or unintentionally
  • testing to ensure they are functioning properly

Note: The CFIA recommends that a qualified person test backflow preventers. Provincial water and wastewater associations or municipalities may provide lists of certified or licensed testers.

Keep in mind

This document does not replace provincial or municipal plumbing codes. Regulated parties are responsible for ensuring that existing and new installations made in their facility meet applicable plumbing codes and regulations whether federal, provincial or municipal.

Tell me more! – Further reading

The following references contain information that helps explain food safety controls, demonstrates how to develop them, and provides examples. The CFIA is not responsible for the content of documents that are created by other government agencies or international sources.

CFIA references

Other references