FMCA TECH TIP: The Importance of Waste System Venting in RVs

By Gary Bunzer

Proper holding tank venting is required (mandated actually), for both the black and the gray waste systems found on recreation vehicles and the method chosen by most RV manufacturers is to run a length of ABS pipe from the holding tank, up and through the roof of the RV. The importance of proper venting, in both systems, cannot be overstated, especially as it relates to odor control. Additionally, without correct venting, sinks will not drain properly, bacteria can propagate and holding tanks will not drain as quickly or completely.

Keep in mind, as a holding tank empties or a sink drains, air must enter the drainage system. Since RV fixtures as well as holding tanks rely solely on gravity while emptying, having air enter the system as sinks and tanks are drained, results in a faster and more thorough process. There are two types of vents used in recreation vehicle waste systems: direct exterior vents and anti-siphon trap vent devices (ASTVDs).

Direct Vents
Direct vents connect the waste systems (either within the drain piping or directly from the holding tank) to the atmosphere outside. There is one other type of direct vent; a side-mounted vent. Side venting is only permissible in the liquid waste system, (typically from a single fixture), and only found on the smallest recreation vehicles. Clearly, the most common and the most effective waste system vent stacks, however, are those that protrude above the roof.

Anti-Siphon Trap Vent Devices (ASTVD)

Another type of vent is the anti-siphon trap vent device. These handy gadgets are used as a secondary vent to aid in draining sink fixtures. Also called “check vents,” ASTVDs allow air into the drainage system, but prohibit air from passing out of the system. ASTVDs are installed in the liquid drain piping system at or near a P-trap inside a cabinet. Look under the kitchen and lavatory sink area and you should find them. They are mounted at least 6-inches above the P-trap’s horizontal arm. ASTVDs do not allow odors to escape into the living portion of the RV because of an integral atmospheric pressure-controlled, rubberized, one-way valve. In other words, air in, but not out.

In addition to ASTVDs, there must still be at least one vent protruding through the roof to allow sewer gases out of that holding tank; ASTVDs are not primary vents. The better-designed waste systems will have ASTVDs installed at every P-trap as well as a direct vent running from each holding tank up and through the roof.

Direct Vent Maintenance

I’d wager not many active RVers have ever given serious thought to actually performing any maintenance on the waste system vents. But here’s something to investigate if you’ve never done so before. Sometimes coach manufacturers cut a very large hole in the ceiling and roof for vertical vent pipes to pass through; it obviously makes the installation a little easier and a lot faster. Oftentimes this gap is not sealed properly all the way around the outside perimeter of the pipe.

If the area around the pipe is not sealed properly and is routed through an oversized hole, it’s likely the tank odors will pass up the vent, collide with the underside of the common sewer cap and be forced back down, around the side of the vent pipe and into the ceiling area where it eventually migrates to the living area. It would behoove the serious coach owner to remove the top cap of each sewer vent on the roof and ensure the space around the perimeter of the vent pipe is sealed tight.

Better yet, consider the addition of a new roof vent cap. This newer type of replacement vent actually creates a negative pressure inside each holding tank and literally sucks the gases and odors out of the holding tank. Easily installed by any RV handyperson, you might not need holding tank additives ever again! Remember, RVing is more than a hobby, it’s a lifestyle!

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