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Backflow Prevention and Monitoring Program


Backflow prevention ensures that the supply of drinking water, in your facility and the City water mains, is protected against the entry of any contaminants, pollutants, infectious agents, or other materials or substances.  In most cases, this is achieved by the installation of an appropriate backflow prevention device at the water connection where this potential exists.  In some circumstances, the main water supply must also be isolated with the installation of a backflow prevention device at or near the water meter.

A building permit for plumbing will be required for the installation of any new testable backflow prevention device.  Replacement of existing testable devices would require a permit if the type, size, or location of the device is being changed.  Removal of a testable Source Isolation backflow preventer also requires a permit.

In accordance with the Ontario Building Code and City of London Water By-law #W-8, any existing or new reduced pressure backflow preventers, double check valves and pressure vacuum breakers are required to be inspected and tested upon installation, when cleaned, repaired or overhauled, when relocated - and annually thereafter - by a tester registered with the City of London. No person shall without the permission of the City remove any cross connection control or backflow prevention devices.

A list of certified Cross Connection and Backflow Prevention Device Testers who have registered with the City of London as of the date noted at the top of the list is available. Please contact these Registered Testers (see pdf in right-hand menu) directly as they are not employed by the City of London. While every effort is made to ensure that the most up-to-date list appears on this page, changes can occur frequently. Current lists are available from the Development & Compliance Division at 519 661-4555.

Testing and Inspection reports for the Backflow Prevention & Monitoring Program, along with instructions for submitting them, are now available on-line for use by registered testers only.

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  • What is a cross connection?

    A cross connection is any actual or potential connection between a potable water system and any source of pollution or contamination. The most common examples in a home environment are a garden hose attached to an outside tap or hose bib with the end of the hose connected to a pesticide sprayer or left submerged in a pool and underground lawn watering (irrigation) systems. Other examples are handheld shower heads immersed in water, alternative heating systems, auxiliary water sources, boilers, fire protection systems and in-home water treatment (softeners) systems, high pressure washers, etc.

    To help keep our water clean, the city asks that you eliminate any cross-connections in your home and protect those that cannot be eliminated. Anything that happens to the water will affect you, the resident, first. It is in your best interest to make sure the water does not come in direct contact with harmful substances.

  • What is the difference between Premise Isolation and Source Isolation?

    Premise isolation is the installation of backflow prevention at or near the water meter, prior to any other connection to the water service for the facility. This protects the City water distribution system from possible contamination. Source isolation is the installation of a backflow prevention device within a facility to isolate a cross connection from the internal water distribution system. This protects people within the facility.

  • What are the five basic products used for protection from cross connections?
    1. Air Gap
    2. Atmospheric Vacuum Breakers – which also includes hose connection vacuum breakers
    3. Pressure Vacuum Breakers
    4. Double Check Valve Assemblies
    5. Reduced Pressure Backflow Preventers

    The product used is determined by the process and the hazard involved.  An Air Gap is the best form of backflow protection but may not be practical in all situations.

  • Why do cross connection control devices have to be tested periodically?

    If you are required to install a testable cross connection control device, it is important to realize that these backflow preventers have internal seals, springs and moving parts that are subject to fouling, wear or fatigue. Therefore, all testable backflow preventers have to be tested periodically to ensure that they are functioning properly and protecting the public water system.

  • How does backflow occur?

    Backflow can occur in two ways, back-siphonage and backpressure.

    Back-siphonage can occur when there is a significant drop in pressure in the main distribution pipes which can pull water back into the system from any nearby connections. The drop in pressure could be caused by a large volume of water being removed from the system, for example due to firefighting or a watermain break. An example of this is a garden hose left in a pool, if a back-siphonage event were to occur, the pool water could be pulled back into the watermain and delivered to the tap of a nearby business or residence.

    The second way backflow can occur is by backpressure. This occurs when the pressure on the customer's side of the connection is greater than that in the City's distribution system and pushes water back into the system.

  • How do you register as a backflow device tester?

    There are standard requirements to register as a backflow device tester in the City of London Backflow Prevention & Monitoring Program.  To obtain an Application Form and list of requirements, please contact our office at 519-661-4555.

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