Floods are destructive. In 2013, floods in Alberta caused $3.5 billion in damage. One underrated—yet dangerous—flood risk is sewage.
When sewer water backs up or floods people's homes, diseases can spread fast. Without clean water, natural disasters can quickly become public health matters.
That's why many Canadian cities mandate flood protection measures on all homes. Depending on where you live, you might need to upgrade your home to stay legal. Specifically, you might need to install—or upgrade—a backwater valve.
What Is A Backwater Valve?
In plumbing, backflow is the term for water flowing in the wrong direction. Backflow poses a serious health risk when sewage water flows back into a system. It can contaminate drinking water.
Backwater can also create a hazardous standing water pool. When unclean water floods and pools in a basement, it becomes a fertile breeding ground for diseases.
Backwater valves mitigate this risk. A backwater valve is a device that prevents backflow. Plumbers attach these valves to the main sanitary sewer in a building or home.
A backwater valve is, essentially, a one-way door for water.
How Does A Backwater Valve Work?
A home's plumbing system directs unclean water away from the house. The water flows to the city's sanitary sewer system or into germ-neutralizing septic systems.
When the water passes from the home's plumbing system to the broader system, it leaves through an open door: the backwater valve. When everything is running properly, the backwater valve is simply an open exit gate.
But, sometimes, something blocks the home's used water from flowing into the broader system. A pipe may be clogged or damaged. Or, there may be more water flowing into the broader system than it can handle.
This blockage increases pressure on the sewage water leaving the house. Because liquid fluids are non-compressible, the buildup pressure increases the water's kinetic energy.
The increased kinetic energy can eventually become a significant force—a momentum greater than the gravitational or mechanical forces that pushed the water forward. At that point, the water flows backward.
The backflow triggers the backwater valve. When the backwater valve senses sewage water flowing back to the home, it closes the door.
The closed door is a solid, physical barrier. It blocks unclean water from re-entering the system.
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Types of Backwater Valves
Different backwater valves use different blocking mechanisms. Backwater valve mechanisms fall into three general categories:
This overview summarizes how each type of backwater valve works. Talk to a licensed plumber to determine which option is best for your home or building.
Check Valve (Swing Type)
All check valves block backflow automatically in response to a trigger. The check valve senses backflow, and that sense triggers the door or flap.
A swing-type check valve has an internal, swinging flap. It's attached to the top or bottom of the valve.
When sewage water flows outward through the device, to exit the system, the flap swings with the current. It doesn't block the flow. The outward flow may even press the swing flap flat against the ceiling or floor of the device.
When sewage water flows inward through the device, due to backflow, the flap catches. It swings with the backflow current, but only up to 90°. Then, it stops and holds its position.
A "seat" on the interior side keeps the door closed against outside (backflow) water pressure. The upright, vertical flap becomes a solid barricade against the backflow.
Check Valve (Ball Lift Type)
Some plumbers call ball lift check valves "ball check valves."
Ball check valves block backflow with a metal sphere. The ball covers or blocks the entrance of the valve to prevent backwater re-entry. The entrance is narrower than the exit, so the ball can't accidentally block exiting sewage.
When water flows forward, to exit the system, it lifts the ball or pushes it forward. When there's no water flowing, the ball rests in the blocking position. So, the "closed door" is the default state.
When backflow threatens to re-enter the system, the ball stays in place. It does not budge in response to pressure from the outside. Some engineers incorporate springs to strengthen the ball's resistance to backflow pressure.
Ball check valves are common, reliable backwater valves.
Check Valve (Dual Plate Type)
Plumbing engineers may refer to a dual plate valve as a "dual-disc type check valve" or a "butterfly valve." A dual plate valve blocks backflow with two spring-loaded metal plates. The plates are attached to a central hinge pin.
The dual plate valve uses a torsion spring. A torsion spring twists along an axis, and it stores mechanical energy. Functionally, the torsion spring exerts an oppositional force (torque) in proportion to the degree of its twist.
In a dual plate check valve, the torsion spring exerts force on the plates. The default position of the plates is closed. Like the ball check valve, the plates block water flow in their default state.
In the default state, the plates completely seal off entry. It's nearly impossible for even high-pressure backflow to break through the plates.
When water flows forward, to exit the valve, it pushes open the "doors." The plates open in exact proportion to the force of the exiting water. The torsion springs store that mechanical force.
Then, the springs return the force to the doors, pushing them closed. The doors close at the same rate (by the same degree) as the decreased rate of outflow.
Check Valve (Plug Type)
Engineers typically only design plug-type mechanisms for vertical check valves.
The "plug" in a plug-type backwater valve is rubber or synthetic polymer. Like the ball-type and plate-type valves, a plug valve's default position is closed.
When no water flows, the plug corks the interior opening of the valve. In a vertical check valve, this is the bottommost opening.
When used water flows out to exit the system, it lifts the plug. Some valves enable the water to lift the plug directly.
Others incorporate mechanisms that use rotational motion to unplug the opening. The mechanisms react in response to interior water pressure.
Once the water is done exiting the system, the plug drops back into place. When backflow attempts to re-enter the system, the plug blocks its path.
Unfortunately, this design doesn't often work as intended. Too often, once exiting water uncorks the plug, the plug does not return to the closed position successfully. So, the Insurance Bureau of Canada warns against plug valves.
Manual Gate Valve
Plumbers might call manual gate backwater valves "sewer gate valves" or "flood gate valves." These valves do not automatically react to backflow.
Instead, a manual gate valve completely cuts off the sewage water flow in either direction. A person must use a manual wheel or crank to shut the gate. Typically, the crank is on the ground floor.
You can close a gate when you know there's a high risk of backflow. So, you might close the gate during a flood, or if you hear about a sewer main break.
You must also re-open the gate manually to let your system's water exit.
Manual gate valves are typically used in industrial settings rather than personal homes.
Automatic Gate Valve (aka Combination Valve)
Combination valves block backflow with two mechanisms. One is automatic. This is typically one of the check valve functions described above.
The second is a manual gate. This combination gives the valve stronger resistance against high-pressure backflow. The automatic function also buys you time to shut the gate, if you don't initially realize there's a risk.
An electro-pneumatic backwater valve is a new invention. The device uses digital sensors, a computer chip, and pressurized air bladders.
The digital sensors scan for water surcharge and sewage water backflow. When they detect these dangers, they trigger an air pump and deploy an inflatable bladder.
The bladder blocks the backflow's passage. In the default, deflated state, the electro-pneumatic valve lets water flow freely. The bladder automatically deflates once the sensors report no danger for at least thirty minutes.
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Mainline Backwater Valve
A mainline backwater valve serves two functions. Like all backwater valves, a mainline valve prevents sewage backflow from re-entering a home's plumbing system.
On top of that function, a mainline backwater valve vents gas. The default state of a mainline backwater valve is open. Sewage gas can flow through the pipes freely, so it doesn't build up dangerously in one space.
A plumbing system that uses check valves needs more than one. Check valves block backwater re-entry at every point of egress in a branching pipe system.
In contrast, a plumbing system only needs one mainline backwater valve. This valve works with the main sewer drain. It's easier to repair a single, hefty valve than several smaller ones.
Backwater valves and backflow preventers have distinct (but similar) functions.
Backwater valves protect a home's plumbing system from backflow. Essentially, they prevent sewage from pushing up through the pipes, ultimately backing up and visibly overflowing in shower drains and toilets.
In contrast, backflow preventers specifically protect stores of potable water. These devices keep clean water free from contamination.
Backwater Valve Installation
Backwater valve installation is a complex process. It's not a DIY project. If you need a backwater valve—or if you need a replacement—please contact a licensed plumber.
For information on backwater valve installation rebates, scroll down to "Basement Flooding Protection Subsidy Program."
Floor Drain Backwater Valve
A floor drain backwater valve is another term for a check valve. Specifically, a plumber would install this check valve in the floor drain in a basement. Typical floor drain backwater valves use the ball or plug mechanism to block backflow.
Best Way to Maintain Your Backwater Valve
Please hire a professional plumber to maintain your backwater valve.
But, if you are determined, you can learn to maintain your backwater valve yourself. The Ontario Water Works Association is a non-profit research group. OWWA polishes standard practices for flood prevention system maintenance.
To learn how to maintain your own backwater valve, read OWWA's Manual For the Maintenance and Field Testing of Backflow Prevention Devices. You can also purchase manuals to read their other standard practices.
Basement Flooding Protection Subsidy Program
Installation doesn't have to break the bank. There are programs in many cities and provinces that subsidize backwater valve installation. If you live in Toronto, you can apply online for flooding protection subsidies.
Backwater valve installation costs range from $1000 - $2500. Fortunately, you can get a subsidized rate almost anywhere in Canada.
To find rebates that apply to you, you can talk to a local plumber. Or, you can search for programs on a directory site, like the Flooding Protection Program page on Show Me The Green.
Backwater Valve FAQs
Below are the commonly asked questions online:
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