Definitions (from CSA B64.10.11)


The reversal of the normal direction of the flow of water.

Backflow preventer:

A device that prevents backflow.

Backflow prevention device tester:

A person who is certified to test backflow preventers by an accredited organization acceptable to the regulatory authority.


Any actual or potential connection between a potable water system and any source of pollution or contamination.

Note: Bypass arrangements, jumper connections, removable sections, swivel or changeover devices, or any other temporary or permanent connecting arrangements through which backflow can occur are considered cross connections.


Any person, firm, or corporation having control over property to which this Bylaw

Potable water:

Water safe for human consumption.

General Questions

Question: What is “backflow prevention”? 

Answer: Backflow prevention, is a plumbing strategy aims at averting the flow of contaminated water into a drinking water supply.

Question: What is a cross-connection? 

Answer : Any actual or potential connections like bypass arrangements, jumper connections, removable sections, swivel and changeover devices or any other connection, permanent or temporary, between a potable water system and any source of polluted or contaminated water. Such systems create situations where backflow is possible. Even an outdoor garden hose that feeds an irrigation system could pose a hazard.

Question: What are the most common causes of contamination of drinking water?

Answer: a) Back siphonage: the reversal of the normal flow of water caused by a depression in the supply system and b) Back pressure: the reversal of the normal flow of water caused by a pressure higher than the supply pressure.

 Question: What can cause back siphonage? 

Answer : Back siphonage can be caused by an abnormality in the public waterworks such as the interruption of service, pipe breakage or the use of many fire hydrants, or other external appliances.

Question: What can cause backpressure? 

Answer : Backpressure refers to a situation when the downstream pressure in a plumbing system is greater than the pressure upstream. For example, when a circulator or boost pump is used in a heating or sprinkler system AND there is a simultaneous drop in pressure in the public water supply, the high pressure generated by the pump may reverse the normal flow of water.

Practical questions

Question: When I turn the water back on after a field test, do I pressurize the backflow device or the piping downstream first?

Answer : To avoid causing a water hammer, first pressurize the backflow prevention device then the piping downstream.

Question: True or false? I installed a backflow prevention device at the main water entrance of the building. So now my work is done. I protected the city’s public waterworks from contamination due to backflow. 

Answer : False. The backflow prevention device installed at the main water source protects the public waterworks from backflow, HOWEVER cross-connections inside the building must also be protected with an appropriate device.

Question: True or false? After completing repairs on the piping that supplies a backflow prevention device, there is no need to test the device since testing is only required on an annual basis.

Answer : False. A backflow prevention device must be field tested in EACH of the following cases and BEFORE turning the water back on:

  • Upon installation
  • When relocated to another location in the system
  • After changes to the supply piping
  • After every maintenance including cleaning,
  • After it has been repaired
  • At least once a year

Question: Can I use the #1 shut-off valve of a backflow prevention as a main shut-off valve for the entire building?

Answer : No, the purpose of the #1 shut-off valve of a backflow prevention device is for the tester to properly field test the device. A main shut-off valve must be installed on the main water connection of the building, in accordance with section 6.1.3 of the Canadian National Plumbing Code.

Reduced Pressure (RPA)

Question: I am currently doing a backpressure test on the #2 check valve of a reduced pressure assembly (RPA). The relief valve is open and water flows constantly. What does that mean?

Answer : The check valve #2 is not watertight; check for leakage.

Question: Without doing any tests, I arrive on a job and the relief valve of a reduced pressure assembly opens and closes from time to time. What can cause this to occur?

Answer : Extreme fluctuations in the supply piping will cause the relief valve of an RPA to open and close periodically.

Question: When testing the opening of the relief valve of a reduced pressure assembly, I noticed that I have to open the low-pressure valve of my gauge more than one-quarter turn. What does this mean?”

Answer : There is a leak in the # 2 shut-off valve because water is still flowing inside the device.

Question: Which valves of the differential pressure gauge must be opened to test the #2 check valve of a reduced pressure assembly.

Answer : The high pressure and by-pass valves.

Question: True or false? Another way to test the opening of a relief valve is to close the #1 and #2 shut-off valves and open test cock # 2. Water should then flow from the relief valve.

Answer : True. The water in the device is static. By opening the # 2 test cock, the pressure of the water upstream of the relief valve is reduced. The reduced pressure zone will then have greater pressure than the zone upstream of the relief valve. It’s like simulating a back siphonage scenario.

Question: True or false? The # 2 shut-off valve is closed but before I can start my test, water is pouring from the relief valve. It is probably due to leakage on the # 1 check valve.

Answer : True. If check valve # 1 leaks, the pressure of the zone upstream of the relief valve and the pressure of the reduced pressure zone will become equal, which will inevitably cause the relief valve port to open.

Double Check Valve Assemblies

Question: I am testing the #1 check valve of a DCVA on normal flow and my gauge reads 0.7 psig. What does that mean?

Answer : A failure of the # 1 check valve.

Question: While testing backpressure of the # 1 check valve of a DCVA, I usually slightly loosen the low-pressure hose plugged on the # 2 test cock. Is it a good idea? Why does the pressure climb when I do this?

Answer : Loosening the low-pressure hose will bleed water from the backflow device, which increases the differential pressure between the two zones upstream and downstream of the # 1 check valve. Since pressure rises on the dial, it becomes easier for you to read.

Pressure Vacuum Breaker / Spill Resistant Pressure Vacuum Breaker PVB/SRPVB

Question: True or false? While testing the check valve of a pressure vacuum breaker, the flexible hose of the gauge should be plugged on the # 2 test cock.

Answer : False: on the #1 test cock.

True or false? The atmospheric vent of a pressure vacuum breaker must be completely opened before the pressure at the gauge reaches 1 psig?

Answer : True, that is the minimum set point, and complies with CSA B64.10 standards.

Question: Water is pouring from the atmospheric vent of a pressure vacuum breaker before I can even do any tests. What does that mean?

Answer : A failure of the atmospheric vent.