By Mark Polk
RV EDUCATION 101®
RV holding tanks are designed with a vent pipe going from the top of the holding tank to the roof of the RV, to vent the tank and the tank odors. The problem is, holding tank odors accumulate in the tank and can’t really be vented outside because there is no air pressure in the tank to force the gases up and out of the vent pipe.
Another problem occurs whenever wind blows across the vent cap on the RV roof. The higher air pressure forces air down the vent pipe, pushing the tank gases to the only other way out of the system, the RV toilet. Whenever the air pressure is higher inside the holding tank than it is inside the RV, the odor escapes into the RV by way of the toilet, when it is flushed.
There are aftermarket RV sewer vent caps you can install (or have installed) that are designed to pull the odors from the tank up and out of the RV. [Editor: Here’s the 360 Siphon Roof Vent Cap as pictured above.]
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Mark Polk’s tech tips are posted every Saturday in the RV Travel Newsletter and every Wednesday on the RV Daily Tips Newsletter.
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This assumes everything is correct in the coach. We bought our current rv used. Someone before us must have replaced the bathroom sink, and when the drain didn’t line up they used a flexible drain connection (https://www.homedepot.com/p/Everbilt-1-1-4-in-PVC-Form-N-Fit-Slip-Joint-Tailpiece-C3526945/205153937) that didn’t seal properly. The kicker that upped the stink is the bathroom sink drains into the black tank. It took a little creative thinking to properly connect the sink to the plumbing but the stink has been contained.
I have had two of the 360 siphon vents pictured and ended up putting a metal vent cap on. I think a tree branch could have killed the first one but the second one succumbed to the sun. There apparently is no UV protection built into the plastic because the base (the top was missing again) was very brittle. My RV winters in Florida and spends most of the year inside a shed protected from the sun. Not worth the money for what you get.
We’ve had 3 black 360 siphon vents installed for 3 years. Our RV has been outside in the elements about 12 months during that time. The vents look like new and work well.