QUESTION: I have a sink drain that includes a vertical pop-up vent after the trap, and just before the one-and-a-half-inch pipe connects into the two-inch drain. It’s been in place about six years. Do these devices wear out? I thought I had a clog in the pipe below it, and as soon as I unscrewed it, the slow-running drain emptied right out. I screwed the vent back into place, but not super tight. I’d call it moderately hand-tight and now the drain works fine. So, either the vent was screwed in too tightly before, or it’s not working properly and should be replaced.
— Howard Freed
Answer: There are lots of low-cost, manufactured items sold for home renovations which are poor quality and should not be used. Depending on the type and quality of mechanical vent you have, it may have a relatively short lifespan, or indefinite if it is the proper device. Determining which type you have installed under your sink will determine whether you have to replace it.
Sink drain traps and vents seem like simple items to design and install, but are often done wrong and often by inexperienced homeowners. The drains are normally constructed of ABS plastic, which is inexpensive and easy to assemble. Parts are joined by using ABS cement or with various types of compression fittings. Pre-constructed traps can be easily found in various configurations, often with screw-on compression fittings that can be adapted to various angles and pipe sizes.
All drains in a home, with the exception of self-trapping fixtures like toilets, must have a trap installed in the drain, connected to a proper vent. The vent is usually just an extension of the drain pipe that runs vertically above the horizontal section of the drain, downstream of the trap. This pipe may run all the way through the walls and attic and terminate above the roof, or they may connect to other vent pipes, which are open to the exterior of the building. The function of these vents is twofold. Firstly, the vents allow sufficient air into the drains to allow the water to properly flow though the pipes. Secondly, the vent allows sewer gas inside the pipes to exit the building, without entering the living space through the plumbing fixtures.
This second element is the most important, from a safety perspective, as improperly vented fixtures may allow dangerous sewer gas into the home, which can make occupants very ill.
In a nutshell, there are two basic types of mechanical vents made for this application. One is an item that plumbers refer to as a "cheater vent," because they know use of this device is not allowed in most jurisdictions. These are small, inexpensive, cylindrical devices that are normally black or silver in colour, which screw into a simple fitting on the end of a typical ABS pipe. While they may work properly for a few years, they are prone to failure due to deterioration in the spring mechanism inside. When these fail, sewer gas may escape the vent, which is why they are not allowed.
Taking a shortcut when installing a mechanical vent, whether for cost savings or through ignorance, can cause the poor drainage issue you have, or even more serious health issues. Simply replacing it with an approved air-admittance valve should improve the drainage and ensure you have a safely venting sink, which will not allow noxious sewer gas to enter your home.
Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the past president of the Canadian Association of Home & Property Inspectors — Manitoba (cahpi.mb.ca). Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at trainedeye.ca.