RV holding tanks are a necessary part of RVing, especially when using your RV for extended periods. You may love the scenic landscapes you’ll experience in your RV, but foul smells coming from your holding tanks can ruin even the most beautiful views. So today we’re talking about the importance of fully functioning air admittance valves for keeping RV holding tank odors at bay.
What Causes RV Holding Tank Odor?
When you smell a mysterious odor in your RV, your black tank may spring to mind quickly, but it’s not always the culprit. While an RV’s gray tank doesn’t contain raw sewage, it collects bacteria-filled dishwater, food particles, body oils, and soap. When this combination sits for extended periods, it’s a recipe for odor.
RVs have plumbing systems engineered to vent odors to the outside. Air admittance valves are often a part of this engineering, so if you’re getting foul odors inside your RV it could signify a faulty valve.
If you smell a foul odor and follow your nose to the area under your RV’s sink, you may have discovered a bad air admittance valve.
What Are Air Admittance Valves and How Do They Work?
Air admittance valves are critical components in plumbing systems… not only in RVs but also in standard residential plumbing. Air admittance valves allow air into the system through a one-way valve, allowing water to flow smoothly and preventing the formation of a vacuum in the plumbing system.
When water flows down a drain, it fills the space previously occupied by the air. But as the water passes through the plumbing, it needs to be able to draw air in as it goes by so that a vacuum isn’t formed. This is where air admittance valves come into play. They are one-way valves designed to allow air INTO the drain pipes, not out of them.
As water flows down the system, the air admittance valve opens, allowing air to enter through it and into the drain plumbing as the water flows into the gray tank. This ensures that the flow doesn’t create a vacuum that could siphon the water out of the p-trap (the North American term for a trap in a plumbing line). The water that stays in the p-trap is supposed to be there to block odors from escaping through your sink drain.
Without air admittance valves, a large flow of water down the drain could siphon that water out of the p-trap. If that happens, odors from the gray tank can escape through the drain in your sink.
Once the flow of water slows or stops, the air admittance valve closes, preventing any gases (a.k.a. odors) from escaping the gray tank or drain plumbing. If there’s not enough water in a P-trap, you’ll experience signs of a faulty air admittance valve – namely a nasty odor.
⬇️ ⬇️ Watch the video embedded further down in the post for an animation showing this at 1:48! ⬇️ ⬇️
How to Know If Your Air Admittance Valve Is Faulty
Odor is frequently an indicator of a faulty air admittance valve. As odors from your RV’s gray tank escape into the living space, the noticeable smell will grow stronger, often to an unbearable level that you can’t ignore.
So, if you smell something foul, check this valve first. You’ll smell the odor throughout your RV, but it will be more powerful the closer you get to your sink.
Many RVs have air admittance valves in the cabinet under their sinks, but your RV’s construction could be different.
If the valve failed and is remaining open when it shouldn’t (after the water has finished draining), the strongest smell will be under the sink, since the valve isn’t closing to prevent odors in the gray tank from escaping into your cabinet.
If the valve failed the other way, and isn’t opening when it should (while water is running down the drain), the odor will be strongest coming out of the sink drain itself, since the air admittance valve didn’t prevent the p-trap from being siphoned. This allows the odor to get past the p-trap and come out the drain.
Either way, the air admittance valve needs to be replaced.
When our RV’s air admittance valve stopped functioning properly, we found our valve located under the galley/kitchen sink and replaced it very easily.
Even if you’re not currently experiencing this issue, it’s worth researching and noting where your air admittance valves are located for future reference. Doing so will save you time and trouble should you experience an issue in the future.
How to Replace RV Air Admittance Valves
Finding the valve is often the most challenging part of replacing it, but your nose is likely to be your greatest assistant in this process. Look underneath your sink(s)… the air admittance valve is usually installed at the top of a length of pipe that extends up, almost to sink level. They may be hard to tell apart from the drainpipes they are attached to, often looking like a cap.
- Compact air admittance valve with PVC adapter; ideal for a single fixture or group of fixtures up to 20 DFUs (drain fixture units) on a branch
- Creates ventilation that prevents the loss of water seal in trap
Air admittance valves are incredibly easy to replace or upgrade and typically require no tools or assistance. The valves have threads on the bottom so they simply need to be unscrewed to remove them (depending on how old they are, you may need a plumber’s wrench or large pair of channel lock pliers to get it loose). Once removed, it’s easy to screw the new air admittance valve on until it’s snug, but don’t overtighten it. And that’s it!
Replacing a failed valve is a great opportunity to upgrade it to a higher-quality one, too (the ones that RV manufacturers choose aren’t usually the best… shocking, we know!). When we replaced our failed air admittance valves, we used these:
- Compact air admittance valve with ABS adapter; ideal for a single fixture or group of fixtures up to 20 DFUs (drain fixture units) on a branch
- Creates ventilation that prevents the loss of water seal in trap
Once replaced, you can 0pen a window or two to clear the foul odor, and you’ll be back to enjoying the comforts of your RV in no time.
Other Ways to Reduce RV Holding Tank Odor
While this can be one cause of RV holding tank odors, it’s certainly not the only one. Following are a few other possible culprits and solutions to research, should you smell a foul odor inside your RV.
Upgrade The Plumbing Vents On Your Roof
Our 360 Siphon roof vents create sufficient negative pressure in the system to keep odors out of our RV.
Your RV’s holding tanks need to be vented as well, ensuring that as water enters and fills them, the air has someplace to go (other than into the RV). So vent pipes are installed into the tops of the tanks, running directly to the roof to provide a route for the air to escape.
Standard plumbing roof vents that are installed at the factory are simple caps designed to prevent debris from entering, and blocking, the pipe. But they can actually cause problems with airflow… in the right (or wrong) conditions, they can allow wind to blow down the pipe, pressurizing the tank and pushing odors out into the RV.
Replacing those standard roof vents with something that’s better designed can eliminate that problem… and even help prevent odors by ensuring there’s a constant flow of air up and out the vent pipe. That’s why we installed 360 Siphon Vents in place of our standard plumbing vents. When the wind is blowing across the roof (in any direction), these clever vents use the venturi effect to draw air up and out through the vent pipe, helping to keep odors flowing in the right direction… away from you!
- ELIMINATES ODOR - Exhausts odors out the roof vent before they have a chance to invade your RV
- COMPATIBILITY - Engineered for a universal fit, the 360 Siphon is compatible with all RV vent caps
Regularly Clean Your Holding Tanks
Think for a moment of all the items that go down your RV drains and into your holding tanks. In addition to the raw sewage that ends up in the black tank, the gray tank accumulates food particles, soap, body oils, and other substances.
If you have a flush valve on your holding tanks (much more common on black tanks than gray), using them regularly will help reduce these unwelcome additions from your tank. If not, you can fill your tanks with clean water to remove unwanted particles. Filling and emptying with fresh water can help rinse 0ut odor-inducing particles from your holding tanks. If you don’t have a built-in tank flush system, a wand that goes down the toilet is a great option for rinsing the black tank.
Use Holding Tank Deodorizer
There are many holding tank deodorizers on the market. These products not only speed up the process of breaking down leftover food particles and bacteria, but they can also leave a pleasant scent. Deodorizers continually release their odor-fighting chemicals as the pod contents break down in the tank.
Use Holding Tank Enzymes to Help Break Down Waste
They’re not just for your black tank! For maximum efficiency in all of your RV waste holding tanks, you can use specially-designed enzymes to break down waste. These products are specifically engineered to target large and small chunks of waste and food. Enzymes attack these waste particles and break them down, so they’ll flush out the next time you empty your RV’s holding tanks.
We’ve been happy with a product called Happy Campers, an organic holding tank treatment.
- Eliminates RV Tank Odors Fast: Our mineral-based formula tackles foul smells in RV, camper, and marine holding tanks. Prevents lingering waste odors...
If you’re in a situation where stubborn odors just won’t go away, or you’re having the common problem of tank sensors that read incorrectly, or not at all, there’s an additional step available — Power washing your holding tanks! We recently shared our experience of having our black and gray tanks blasted for the very first time.
It’s easy to take for granted a fresh-smelling RV until foul odors invade your living space. Conducting regular preventative maintenance and using a combination of cleaning methods is the best way to keep your RV holding tanks clean and odor-free.
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Tom
Monday 11th of August 2025
I'm having a problem where the shower and bathroom sink will not drain, although the grey tank is empty and there isn't a clog in the drain pipes. It makes sense that there could be a vacuum in the drain system which is preventing water from draining. I'm thinking the AAV is bad, but I swapped the AAV from the galley tank (under the kitchen sink) but the shower still doesn't drain. Is it possible to have both AAV's bad -- the galley tank and the bathroom tank? Thanks
TheRVgeeks
Monday 11th of August 2025
Hey Tom! This is a head-scratcher, and we've got questions! LOL 1) Is this trailer brand new, and if so, has it done this since day one? 2) How do you remove the water that's backed up into the sink or shower? In other words, is there something/anything that you're able to do to get it to drain (and if so, what enables that)? Or do you have to bail the water out with a pot? 3) Does your RV have separate gray tanks for the bathroom and galley? 4) You mentioned that the gray tank is empty. Are you determining that from the indicator lights, or have you confirmed it by pulling the gray valve and watching plenty of water run out until it stops on its own?
If it were the AAVs, the moment you removed one to swap in another one, that would immediately release the vacuum and allow the drain to flow. Do you recall if anything like that happened when you removed the old AAVs? Anything more you can tell us will help us get our Sherlock Holmes hats on!
Anna
Monday 7th of July 2025
All of my AAV are located below the P-traps / weirs. They flood and fail. If you have an air vent on the roof, and a P-trap (like a residential home) I do not understand the purpose of the AAV. It would make sense if there was no P-trap or roof vent/ in apartments where the sinks don’t vent out of the roof etc. they seem useless, especially when installed incorrectly.
TheRVgeeks
Tuesday 8th of July 2025
Hi @Anna. That's VERY strange that all of your AAVs would be mounted at points LOWER than the P-traps! Definitely NOT the way they're supposed to be installed. If you have the room above where they're mounted now, you could likely easily extend the plumbing that the AAVs mount to in order to get them above the P-traps. That would at least stop them from being ruined by getting flooded.
As far as their purpose, they're primarily there to ensure that the P-traps don't get sucked empty by sudden movements of liquid in the system... like when you dump the gray tank. That sudden outrush of water from the tank will, of course, pull air IN via the roof-top plumbing vent pipe(s). But because of the size of the drain pipe for emptying the tank(s), it could require more air than the roof vent pipe can allow in. At that point, it would begin pulling against the water in the P-traps. But, since the AAVs are installed between the P-traps and the gray tank, air can get sucked into the plumbing there... stopping it from draining the P-traps (which would then allow tank/sewer odors into the interior of the RV).
The same is true if you drain a large amount of water down the sink drain (like pouring off water from making pasta, as an example). The sudden rush through the plumbing could create a siphon that would pull all the water out of the p-trap at the sink. But, as the water goes past the pipe for the AAV, the AAV opens and allows air in, breaking the siphon and stopping the water flow from pulling the water from the p-trap.
Hope that helps explain.
Gay Travel Enthusiast (Jason)
Friday 20th of December 2024
Hey guys. Happy Holidays! I love the video you did regarding foul tank odours and how to deal with it when it gets bad. I loved Johnny's expression when he opened the doors under the sink. He had that look like "Ewwww!" and "Ugh!" I don't know why, but I remember falling on the floor laughing! 🤣
robert
Wednesday 14th of June 2023
I have 3 anti-siphon valves(Grand Design calls them anti-siphon on design sheets) on my front gray tank for my GD rv 3740. The highest in physical elevation is on the clothes washer drain line in master closet (no washing machine connected), then the master bathroom gray water drain line (probably pretty close to same height as washer drain), then the lowest is kitchen sink drain line. from the design sheet, looks like master shower drains into tee for front gray vent line to roof and into tank?? have had odors that appear to originate from master sink and/or master shower, appreciate your insights on if you think anti-siphon and air admittance are the same and if I need to replace only one (or more) of the anti-siphon (I was thinking the master sink but could be easily convinced to replace one under kitchen sink) thanks, I have design sheet if that would help but not sure how to post so be happy to send via email
Robert
TheRVgeeks
Wednesday 14th of June 2023
Hi Robert. First... yes, in effect, "anti-siphon" valves and "air admittance valves" are the same thing (there may be technical differences between the two, but their purpose is, generally, the same). The easiest thing to do to test is to put a plastic bag over the top of the valve under the master sink, using a rubber band to secure it in place. See if the odor goes away. If it does... you likely have a faulty/defective anti-siphon valve. Easy-peasy to replace it and be back to normal.
But, of course, if that DOESN'T stop the odor, you'll have to look further. But, at least you'll know it's NOT the anti-siphon valve being the culprit.
Another possible issue is if your tank vents come out on the roof too close to the vent fan we assume is in the master bathroom. If so, if you leave the vent fan open, but not on, wind can blow odor coming up out of the tank vent pipe down and into the roof vent opening. So, if you normally leave the bathroom vent open and not on, try closing that for a few days (except when its needed and running) to see if that solves it.
Hope one of these two are your issue!
Chris B
Sunday 7th of May 2023
You didn't post a link to the replacement RV Air Admittance Valve. Curious what you used. Thanks.
TheRVgeeks
Tuesday 9th of May 2023
You're right, Chris! Not sure how we missed that! We just added an embed with the AAV's we used... you can get them on Amazon here: https://amzn.to/3HWCDDd