Backflow Prevention: What It Is and Why BC Code Requires It

Backflow is the reversal of water flow in a plumbing system — water moving backward from a non-potable source into the clean water supply. It sounds technical, but the consequences are straightforward: contaminated water can enter your tap or, in commercial situations, the municipal supply. BC Plumbing Code and most municipalities require backflow prevention in a range of situations.

There are two mechanisms. Back-pressure occurs when the downstream pressure exceeds the supply pressure — for example, a boiler system pressurized above the incoming supply. Back-siphonage occurs when negative pressure in the supply line creates a siphon that draws water backward — this can happen during a water main break or heavy system draw nearby. Both can pull potentially contaminated water (from garden hoses in buckets, irrigation systems, cooling systems, etc.) backward into the drinking water supply.

• Irrigation systems: Connections to outdoor irrigation systems must be protected to prevent fertilizer or pesticide-contaminated water from being siphoned back.
• Boiler systems: The connection between potable water fill lines and closed hydronic heating systems requires a reduced pressure zone (RPZ) assembly or equivalent.
• Commercial kitchens and food service: Any commercial dishwasher or equipment with a potential contamination risk requires protection.
• Fire suppression systems: Connections between sprinkler systems (which may contain anti-freeze or stale standing water) and the potable supply.
• Swimming pools and hot tubs: Fill connections require air gaps or approved backflow assemblies.

Reduced pressure zone (RPZ) backflow assemblies — the highest level of protection — are required by BC municipalities and many stratas to be tested annually by a certified backflow tester. The assembly is tested under pressure to confirm it closes correctly and the differential relief valve activates as designed. Failing assemblies must be repaired or replaced before the property can be certified compliant. A licensed plumber with backflow testing certification can perform this test and file the required reports with the municipality.