Backflow is water flowing the wrong way in a plumbing system, from a building or piece of equipment back into the drinking water supply. Backflow prevention devices stop it, and most municipalities in Canada require higher-risk buildings to install approved devices and have them tested regularly by a certified tester. As the owner, you are responsible for both.
How backflow happens
Water normally flows from the municipal main into the building. Two conditions can reverse it:
- Back-siphonage: pressure in the supply drops, for example during a main break or heavy firefighting draw, and water is siphoned backward out of the building.
- Back-pressure: pressure inside the building's system rises above the supply pressure, for example from pumps, boilers or elevated tanks, and pushes water back.
Either way, water that has been in contact with chemicals, process equipment, irrigation systems or fire sprinkler piping can enter the drinking water supply. A connection where this can happen is called a cross-connection.
Common cross-connections in buildings
- Boilers and closed heating loops with treatment chemicals.
- Fire sprinkler and standpipe systems.
- Irrigation systems.
- Chemical dispensers, dishwashers and commercial kitchen equipment.
- Laboratory, medical and dental equipment.
- Cooling towers and process equipment.
- Hoses left submerged in tanks or sinks.
Types of protection
The required protection depends on the degree of hazard. Device types are covered by CSA B64 series standards, and the plumbing code and local bylaws set out which device is acceptable for which hazard.
| Protection | How it works | Typical use |
|---|---|---|
| Air gap | A physical gap between the water outlet and the receiving vessel | Where practical, for any hazard level |
| Reduced pressure principle assembly (RP) | Two check valves with a relief valve that discharges if either fails | High-hazard connections, such as chemical or process systems |
| Double check valve assembly (DCVA) | Two independent check valves with test cocks | Lower-hazard connections, often fire sprinkler systems |
| Vacuum breakers | Admit air to break a siphon | Hose bibbs, some irrigation and fixture connections |
The reduced pressure principle assembly is worth understanding in a little more detail, because it is the device most often found on high-hazard connections and the one most often misunderstood when it discharges. It has two independent check valves with a zone between them. A relief valve on that zone opens to atmosphere whenever the pressure in the zone gets too close to the supply pressure, which is what happens when either check valve leaks or the flow tries to reverse.
Many municipalities also require premise isolation: a device at the building's water service that protects the public supply from the whole building, in addition to devices at individual hazards inside.
Testing requirements
Testable assemblies, such as RP and DCVA devices, must be tested when installed and then at intervals set by the municipality, commonly once a year. Tests must be done by a certified backflow tester, and results are usually submitted to the municipality on its forms. Requirements differ between cities in Alberta, British Columbia, Ontario and elsewhere, so check your local water utility's cross-connection control bylaw.
Devices are also tested after repair or relocation. A device that fails must be repaired or replaced and retested.
Keep the relief valve clear. RP assemblies can discharge water from their relief port, sometimes in large volumes. They need proper drainage, and they must not be installed where they can be submerged. A discharging RP is a sign of a problem that should be investigated by a qualified plumber or tester, not plugged.
What a test involves
Owners do not do the test, but knowing what happens helps you prepare and read the report. A certified tester typically:
- Arranges a short shutdown. Testing requires closing the device's shutoff valves, which interrupts water to whatever it serves. Plan the timing with occupants, and for fire sprinkler connections, notify the monitoring company and follow the fire protection procedures for impairments.
- Connects a test kit to the test cocks and checks that each check valve holds and, on an RP, that the relief valve opens at the right point.
- Records the readings and the result on the municipality's form, along with the device's location, make, size and serial number.
- Restores service and confirms the shutoff valves are fully open again.
- Reports any failure and the repair needed. After repair, the device is retested.
Keep the report with your building records and check that it has been submitted to the municipality if the tester does not do that for you.
What owners should do
- Find out what your municipality requires for your type of building.
- Make an inventory of every backflow device, with location, type, size and serial number.
- Make sure each device is accessible for testing and protected from freezing.
- Book annual tests with a certified tester and keep copies of the reports.
- Submit test results to the municipality as required.
- Repair or replace failed devices promptly, using licensed plumbers.
- Review cross-connections whenever equipment is added, such as new chemical feeders or irrigation.
Common problems found
- Devices missing on equipment that has been added since the building was built.
- Devices installed in pits or locations where they can flood or freeze.
- Test records missing, or tests overdue by years.
- Bypass lines installed around devices.
- Devices of the wrong type for the hazard.
A typical case
A building's new owner receives a notice from the municipality saying that two backflow devices are overdue for testing. The previous owner's records show only one device. A walk through the building finds three: the premise isolation device at the water service, a double check valve assembly on the sprinkler connection and an RP assembly on a boiler make-up line added during a past renovation that was never registered.
The fix in a case like this is to build a proper inventory, have all three tested by a certified tester, register the third device with the municipality, and set a single annual test date for all of them so none drifts out of schedule. While the tester is on site, it is worth asking whether any newer equipment, such as a chemical feeder or irrigation, has created cross-connections that are not yet protected.
How ehZee can help
Our air and water filtration system inspections include checking that backflow devices are present where they are needed, accessible and tested on schedule. We do not perform certified backflow testing, but we can coordinate a certified tester and licensed plumbers, and track results. See also water filtration systems for buildings and plumbing red flags in an inspection.
Common questions
Does a house need a backflow preventer?
Most houses have basic protection such as vacuum breakers on hose connections. Testable devices are usually required only where there is a higher hazard, such as an irrigation system or a fire sprinkler connection, depending on local bylaws.
Who can test a backflow device?
Only a tester certified under the program recognized by your municipality. Check the municipality's list of accepted testers.
What happens if we skip testing?
Municipalities can issue orders and, in some cases, suspend water service until devices are tested. More importantly, an untested device may not protect the water supply when it is needed.
Why is our RP assembly dripping or discharging?
Occasional small discharges can happen with pressure fluctuations, but a steady drip or a large discharge usually means a check valve is not sealing, often because of debris or wear. Do not plug the relief port. Have a certified tester or licensed plumber inspect and repair it, then retest.