Galley Sink Vent issue

Actually, I disagree with this statement. Holding tank pressure is EXACTLY what the AAV should do. It is supposed to let air INTO the tank, and keep air (and smells) from exiting the tank. What is supposed to let air (and smells) out of the tank is the roof vent, which is exactly why everyone keeps saying to check to see if it is blocked. If you are getting hissing air (and smells) when you loosen the AAV, the tank is pressurized meaning the roof vent is not working.

Another cause for this, rarer than merely a blocked vent, is the vent pipe has come unfastened and slipped down into the tank. That means the bottom of the roof vent pipe is under the water line of the tank, and not venting.

All of your tanks should have a vertical vent that exits the roof.
Buzzer! Nope!

Hey, I'm not a technical writer for a plumbing distributor. Both of you guys would agree with the basic stuff I wrote. Maybe I should have written "drain pressure/vacuum" instead of "drain pressure" and explained that a single pipe can handle two drains. The information remains the same but I'm about done being scolded for not appealing to the sensitivities of some readers.

Instead of scolding, help the guy out.

Sorry, westend, but the valve under the sink is not supposed to drain anything. It's job is to let some air IN when the sink drains so the P-trap doesn't siphon dry and it is likely working just fine, Like others said, if it hisses when loosened the tank vent is blocked.

The vent is where the tank pressure escapes as the drain water and air enters. It's also where the air enters when the tank is drained.

The valve under the sink and the tank vent on the roof serve different purposes.

I once had a smell problem from my black tank. Took me a long time to figure out it was a blocked roof vent.

I replaced the AAV under the bathroom sink. The problem went away. I thought life was good, until I dumped again, then the problem returned.

This sequence was repeated a few times.

Finally, after replacing the AAV yet AGAIN, the next time I dumped, I heard a loud sucking noise. I finally figured out that the roof vent was blocked, the sudden outflow of "stuff" from the black tank created a strong enough vacuum in the black tank to destroy the AAV, and the tank then vented through the AAV. Air in, smells out!

Explained why the problem recurred every time I dumped my black tank.

westend wrote:

Bobbo wrote:
Actually, I disagree with this statement. Holding tank pressure is EXACTLY what the AAV should do. It is supposed to let air INTO the tank, and keep air (and smells) from exiting the tank. What is supposed to let air (and smells) out of the tank is the roof vent, which is exactly why everyone keeps saying to check to see if it is blocked. If you are getting hissing air (and smells) when you loosen the AAV, the tank is pressurized meaning the roof vent is not working.

Another cause for this, rarer than merely a blocked vent, is the vent pipe has come unfastened and slipped down into the tank. That means the bottom of the roof vent pipe is under the water line of the tank, and not venting.

Hey, I'm not a technical writer for a plumbing distributor. Both of you guys would agree with the basic stuff I wrote. Maybe I should have written "drain pressure/vacuum" instead of "drain pressure" and explained that a single pipe can handle two drains. The information remains the same but I'm about done being scolded for not appealing to the sensitivities of some readers.

Instead of scolding, help the guy out.

I did help the OP out. Read the last sentence of my quote above.

Also, you may not be a technical writer, but misinformation still needs to be corrected.

westend wrote:
An air admittance fitting under the sink should not hold drain pressure,i.e. if it hisses when you twist it partially off, it is not working correctly.

This statement is just plain wrong. If it hisses when you twist off the AAV, it is the roof vent that is not working correctly.

And, in a section that you did NOT quote, I also pointed out that tanks may "Y" into the same vent so the number of vent pipes through the roof may not match the number of tanks.

Bobbo wrote:
I agree that you may have just 1 or 2 vents through the roof, with "Y" connections to the other tanks, if they are close together.

Most of the vents look like this:

My black tank vents to the roof. The galley and bathroom tanks vent through the side, about 6 feet up.

The AAV may be "hissing" from the vacuum within the drain pipe when loosened. If the OP unthreads it enough, it will pass some gas out of the system. Keep arguing but we're all pretty much on the same page. There is a problem in the drain system relating to pressure/vacuum.

And, in a section that you did NOT quote.....

I was addressing two different posters. That is why I made two quote boxes. Now I'm on the hook for Forum etiquette, sheesh.:(

westend wrote:
All of your tanks should have a vertical vent that exits the roof. Identify which vent pipe services the galley sink and make sure it is not blocked.

An air admittance fitting under the sink should not hold drain pressure,i.e. if it hisses when you twist it partially off, it is not working correctly.

Hi westend. I think where we (all) are mis-communicating could come down to the meaning of "it".I agree completely with your first sentence. I will preface this by saying I'm not a plumber either and/or could have typo'd something here. But here is my opinion/understanding:


If by "it is not working" you mean "the plumbing system", I agree.
If by "it is not working" you mean the AAV, I disagree.


Granted, the OP's old AAV was probably not working (i.e. leaking gas out, causing the smell). The new one likely is just fine but we can't tell so long as the tank vent is not open and pressure is building.


What complicates this discussion is the fact that home venting is similar, but also different than a typical RV system. In a home system, there are roof vents plumbed to near every P-trap that allow incoming air when something is flushed down the drain. As the water travels down the drain, through the P-trap and then down toward the sewer or septic tank air is pulled IN from the vent. This keeps the final slug of water from vacuuming the P-trap dry as it continues down the pipe. Once the water stops moving, the vent then becomes an outlet for any built-up gasses to escape.


In an RV, the AAV (air admittance valve) is a check valve that is closed until the vacuum of the last slug of water pulls it open, allowing air IN to the plumbing to keep the P-trap full. Once everything stops, the valve closes to prevent gasses from escaping inside the RV where you don't want them. It only serves to keep the P-trap full and does not vent the system.

The RV roof vent need only vent the tank itself and can only serve as an air admittance device when it is connected near a P-trap. Since RV's prefer to have minimal holes in the roof and since routing vent pipes through the walls is not practical it's usually easier to use an AAV for most if not all of the P-traps and then vent the tank directly.

Thanks for all the advice, it really helps having good folks to bounce things off of. I'm thinking it probably is a roof vent problem. Won't have a chance to look at it until next week. But thanks for all the help.

Mark

So, you guys that have had a blockage in the venting system could you please tell the rest of us what it was (blockage) and how it was corrected?

This way the OP will know what to look for and how to correct it.

AND, get this thread back on track .... =)

Cee

Don't have a clue what it was.

Ran a plumbing snake down 3 or 4 times till it hit bottom, then put a garden hose in it and flushed.