Understanding Backwater Valves
The first rule of plumbing is “sewage flows downhill.” When sewage is not following the rules, you’re going to need a backwater valve. These little understood devices are often overlooked, improperly placed or incorrectly specified in design. They are required by code but should be used only where necessary. They come in many styles and have very specific installation requirements. They can be a source of frequent problems when they are working and when they are not.
A backwater valve is simply a device in the waste pipe that prevents sewage from flowing back into the building. When the city sewer in the street backs up downstream of your connection, there is a potential for the upstream flows to find their way into your building and work back uphill to your fixtures. In these instances, the effluent flows out of the lowest opening it can find, typically a basement floor drain or similar fixture.
Plumbing codes require backwater valves when a fixture is installed on a floor that is below the next upstream manhole. The obvious example is the basement floor drain. Less obvious is a first floor fixture in a lot at the bottom of a hill. In this case, the next upstream manhole is often higher than the first floor slab. A sewer clog in the street downstream of the lot can collect everything from the top of the hill until it comes flowing out of the unprotected first floor fixtures. The next upstream manhole is key, because it is the source of relief. When the street backs up to the next upstream manhole, the sewage will lift the cover and the flow will cascade down the street. It’s not a pretty site and certainly bad for the ocean to have that in our storm drain, but it’s better than having it in your living room.
As much as the codes require backwater valves for lower fixtures, they prohibit backwater valves for higher fixtures. Second story fixtures should not flow through a backwater valve since a valve failure resulting in a clog will divert the second story flows out the first floor fixtures you are trying to protect. Some codes state that backwater valves are required when the flood level rim of the fixture is installed below the next upstream manhole. Taken to the next level, if a first floor slab is six inches below the next upstream manhole, which fixtures should be protected? Certainly the floor drains, mop sinks and showers should be protected, but what about the toilets, lavatories and sinks? Since they sit 15", 30" and even 36" above the floor, they should be plumbed separately around the backwater valve. This results in additional piping and invert challenges where the two lines may cross. Nevertheless, they should be separated.
The first design task is then to separate and route the flows from fixtures that need protection and those that don’t. When doing so, avoid crossing lines where it may lead to lower inverts. If you must cross, route the unprotected line lower than the line going to the backwater valve. You will need the additional height for the proper operation of the valve. Locate the valve where it can be accessed. This too is a code requirement. Consider that it may need frequent access for maintenance and this will be a messy, foul smelling task. Check the inverts in and out of the valve. Additional design requirements will depend on the type of valve selected.
Backwater valves come in both manual and automatic versions. For manual valves, the user notices the sewage flowing into the building and proceeds to the valve access panel. This is typically right in the middle of the spill, about 6 inches under “water.” Lifting the cover, the user then turns the wheel handle until the backflow has stopped. He then waits for the city to correct the problem in the street, opens the valve, and cleans up the mess. In the automatic versions, there are two variations. The original is similar to a check valve. This lift gate prevents the flow from getting back into the building, eliminating the human response time. Unfortunately, as a restriction in the line, they often trap waste flowing out as well and can become a source of frequent maintenance and problems. An improvement on the automatic version incorporates a float with a blade style gate. When the sewage rises, the float lifts and the guillotine like gate rises to close off the flow. These are preferred by some, but have greater installation challenges.
The manual-type works well in retrofit applications because there is typically no invert change aside from the slope of the pipe. Since it will need access in an emergency, its location will need to be obvious and outside of the area that will be flooding. If located below a basement floor, put it at the bottom of the stairs, elevated on a pad or provide a curb around it.
Check valve style devices typically have an invert difference of a few inches from inlet to outlet. (See figure 3.) As such, they usually cannot be cut into an existing line and will need careful planning in most installations. Although they are self operating, they are in fact “valves” and cannot be buried in the structure. These valves come with covers that often look like cleanouts and can be extended deeper if necessary. They need access for maintenance, but should not need to be opened during each and every backflow event. Nevertheless, the servicing of these units is a foul task and they should be located accordingly.
The float-type seems like the obvious choice, but not all authorities would agree. A blade-style gate has less restriction during normal operation, but can be blocked from closing and does not have the self-sealing back-pressure that a check valve has. They also need the sewage to rise up to a foot or more above the valve to close. As such, be sure to locate the valve way downstream and provide sufficient tailpieces to allow for this needed rise. As the most complicated of these devices, maintenance needs are more likely and access should be considered accordingly.
Single-point-of-use valves also include floor drains with integral check valves and tailpieces with ball floats. The latter use a floating ball to rise up and block a smaller hole at the top of the tailpiece. When a single basement floor drain needs protection, either of these may be a good choice. When multiple fixtures are in danger of backing up, a single system valve should be used.
To this point, the topic of concern has been sewage back-up. Storm drains, however, should not be overlooked. It is unlikely that a roof drain is located below the next upstream manhole, but an area drain in a first floor atrium connected to the storm drain can experience the same issues as a floor drain in a toilet room connected to the sanitary sewer. Window well and areaway drains also can see backflow. As before, route drains above the next upstream manhole separately around any backwater valves
Some authorities do not allow emergency floor drains to tie into the storm drain system. Others require backwater valves to separate sub-soil drains from areaways and floor drains. Perhaps the backwater valve is a third level of protection, just in case the duplex sump pump fails. I suspect that more often it is being incorrectly used. I believe the more common problem in these systems is not pump failure, but storm surges in one system overwhelming another.
In the example of a subterranean parking garage, there will likely be a sump pump. It may have a single pipe flowing into the basin. That pipe may have branches coming from air intake shafts open to the sky, emergency floor drains within the parking garage and a perimeter sub-soil drainage pipe to relieve hydrostatic pressure on the outside of the foundation. Each of these flow rates need to be carefully analyzed. The area drain needs to be sized for the 100-year storm. Be sure to include vertical area where appropriate. The sub-soil drain should be sized to handle a continual flow rate as indicated in the geotechnical report. The emergency drains should be sized as required by local codes. The combined pipe must be sized for the total combined flow rate of all of these systems. If not, the common pipe will flow full and the additional water will surcharge up and out of the path of least resistance. Typically, the emergency floor drains become decorative fountains! In most cases, when flows combine, an increase in the pipe size or an increase in the slope will prevent the flows from backing up. Backwater valves should be used sparingly. Remember, it should all flow downhill.
Peter A. Kraut, P.E., CPD is a licensed Mechanical Engineer in 22 states. He founded South Coast Engineering Group, near Los Angeles in 2001. In addition to conventional plumbing and HVAC projects, he has designed more than five million square feet of siphonic roof drainage systems in the United States. He can be reached at (818) 224-2700 or via email at firstname.lastname@example.org.