Check valves

May 19, 2025

By Steve Pando

This article does not replace, in whole or in part, the current regulations, i.e. the Quebec National Plumbing Code.

A check valve is an essential mechanical device in plumbing systems, designed to allow liquid or air to flow in one direction only, while preventing any backflow upstream. This mechanism plays a crucial role in protecting plumbing systems and their users. Backflow can lead to contamination of the fluid upstream, damage property, compromise system efficiency, increase maintenance costs and create problems of compliance with current standards.

Chapter III of the Quebec Construction Code, Plumbing section, strictly regulates this protection through Article 2.4.6.4 entitled Backflow, which specifies the cases where the installation of non-return valves is mandatory.

The aim of this article is therefore to explore in detail how non-return valves work, their technical variants, their importance for the safety of installations, and the standards requirements governing their installation.

How does a Check Valve work?

A check valve allows fluid to flow in only one direction and automatically closes to prevent backflow. Its operation is based on three key principles:

Opening Pressure: The valve opens when the upstream pressure exceeds a minimum threshold (cracking pressure), which varies by model.
Automatic Closure: If the pressure drops or reverses, a spring, gravity, or back pressure forces the valve to close, using an internal component like a disc, ball, or membrane.
Installation Direction: An arrow on the valve body indicates the correct flow direction. Installing it the wrong way blocks flow and can damage the system.

The National Plumbing Code of Canada defines thwo types of check valve operation:

Normally Open Valve: This valve remains open under normal conditions and closes only in the event of backflow. It is allowed only on building drains serving a single-family dwelling (article 2.4.6.4. 2)(b)) and must be CSA-certified.
Normally Closed Valve: This valve stays closed to block frequent backflows and only opens when wastewater flows through. It does not allow air circulation and is therefore not permitted on main building drains. It must also be CSA-certified.

Where to install Check Valves?

The situations where the code requires or strongly recommends the installation of a check valve apply to any building covered under Chapter III and are enforceable inside any building, as defined by Article 1.1.1.1. of Part 1 – Division A of the Construction Code.

Code Article 2.4.6.4 (1)

"Every main drain or building sewer shall be free of check valves that would prevent the free movement of air."

Explanation:
It is strictly prohibited to install a normally-closed check valve on the main drain or building sewer. These parts of the drainage system must allow the unrestricted movement of air for proper venting.

Code Articles 2.4.6.4 (3) and 2.4.6.4 (5)

"Sanitary fixtures, sump pits, floor drains, or trap drains located below the level of the adjacent street must have a check valve installed on the drainage pipe, if connected to a main drain at risk of backflow."

Any fixture located below street level that could be affected by municipal sewer backflow must be protected with a backwater valve on its drainpipe—unless the entire main drain is already protected according to Article 2.4.6.4 (2).

Septic Tank Considerations

In the case of a septic tank, if any fixture is located below the overflow level of the tank or its access hatch (manhole), then that fixture must be protected by a backwater valve—similar to situations involving street-level overflow risks.

Buildings Without a Basement

Even in buildings without basements, the sanitary fixtures on the ground floor may be below the overflow level of the municipal manholes. In the event of a sewer blockage, water can rise in the pipe and backflow into these fixtures.

Best Practice:
While Chapter III does not explicitly require protection in this case, it is strongly recommended to install check valves on fixtures below municipal manhole overflow levels.

Shared Drain Branch

When several fixtures on the same floor are connected to a common drainage branch, a single check valve can be installed on that branch.

Benefits:

Cost-efficient: Protects multiple fixtures with one device.
Maintenance-friendly: Only one valve to clean or replace.

Floor Drains and Insertion Valves

More and more municipalities now allow the use of squeeze-in insertion-type check valves in floor drains.

Note:

These are not prohibited by Chapter III.
However, local municipal regulations may restrict their use.
Always check local codes before installing.

Code Article 2.4.6.4 (4)

"Where the fixture to be protected is a floor drain, a screw-type cleanout plug installed upstream of the trap is permitted instead of a check valve."

This exception is mostly applied to floor drains, particularly in certain installations where check valve installation is difficult or not feasible.

Accessibility Requirements – Code Article 2.1.3.2 (1)

"Every check valve must be installed in an accessible location for use, maintenance, repair, or cleaning."

The property owner is responsible for maintaining access to all devices (valves, shut-off taps, etc.).
This is also enforced by Article 4 of Chapter I – Plumbing of the Quebec Safety Code.
It is good practice for the contractor to inform the client before completing the work.
If the owner later makes renovations that block access to the valve, they alone are liable for any resulting issues.

Special Case: Single-Family Home with Combined Sewer and Flat Roof

Context

Despite Article 2.1.2.1 (2) of Chapter III prohibiting combined sewers, many existing buildings in older neighborhoods still use a combined sewer system (rainwater + wastewater).

Challenge:
If a normally-open check valve is installed on the main drain of such a home with a flat roof or surface runoff leading to the sewer, heavy rain may overload the system. If the city network is flooded, the valve will close to block backflow—but it will also block rainwater and wastewater inside the building, causing backup and possible damage.