Holding tank vent clogged or obstructed?

[RudysWorld]

When emptying the gray tank I noticed that it will flow fairly well then slow, followed by a whoosh sound and then drain completely. Also, after dumping the black tank I am noticing an odor. After reading through the forum I thought it might be a vent clog causing both issues.

How would I clear the vent?

If you think my diagnosis is off what do you think it would be.

Thanks to all!

[donhoward49]

I do not intend to hijack this thread, "But" I do not understand.. Is the whoosh caused by the shower P trap or the Air Valve opening as it should?

[stuplich]

Weldon wrote:
There is a vent most likely under the lavatory sink. Don't recall the name...may be for gray or black....

Most likely for the gray only.
It is called an "air admittance valve".
Here are images of a few:
https://www.google.com/#sclient=psy-ab&q=1.5+air+admittance+valve+image&oq=1.5+air+admittance+valve+image&gs_l=hp.12...11326.15358.2.19681.8.8.0.0.0.0.256.1472.1j2j5.8.0....0...1c.1.19.psy-ab.aEd3uCX7eKE&pbx=1&bav=on.2,or.r_qf.&bvm=bv.48705608,d.aWc&fp=dcbb9b7f9ec108b7&biw=1093&bih=452
Mel

[joeprinter]

Your emptying of slow drain, then hearing of a swoosh if very normal, its either the solids leaving last or else the vacuum balancing out as mentioned.

[Old-Biscuit]

Air Admittance Valve.......

In drain line AFTER 'P' traps.
Used to prevent vacuum in drain lines so 'P' traps don't get sucked dry.

NEVER used on black tank system.

Sounds like yours is not opening and vacuum build up when first draining then water in 'p' trap gets sucked out......whoosh, good draining but now stink comes back into rig cause no water in 'p' trap(s).

Inexpensive......any hardware store.....just screw in to end of stub pipe.

[G_Lewis]

Several years ago, I had a "draining" problem. I ran a 15' sewer snake down from the roof (carefully) in the vent pipe...unplugged something....and all is well.

Most of us do things a little differently, but after draining my black and then my grey I then put a chemical packet and 3-4 gallons of water in the black tank. In hot weather I put two packets...never have a smell problem....

[bigdog2]

Our rig has a vacuum breaker under the bathroom sink and another under the kitchen sink. They are spring loaded rubber flaps that will allow air to enter the tank as the water goes out. That is what makes the sound as the water gets out enough to create a vacuum in the tank. I always dump the black tank first, leave the valve open while dumping the grey tank, then close both. Then I add some liquid fabric softener to the black tank along with a gallon or so of water. Supposedly the fabric softener keeps the seals pliant and is not harmful to the septic system.

[bresdogsr]

I have a 2010 GT and have the same issue. I get the same woosh while draining the grey, then the oder (smells more like grey than black). I will have to look for a vacum breaker nder the sink. Don't think I can access the tank itself. Will fill all p traps after draining maybe that will help as well.

Always great info on this forum.
Sorry for the hijack 🙂

[-Gramps-]

I have found that the gray tank empties a lot faster if I leave the black tank valve open while emptying the gray tank.

[hmknightnc]

RudysWorld wrote:
Thanks for the reply.

The odor started while parked and after dumping tanks, so moving did not cause it. I will have to check for the gray tank vent, I assume both gray and black used same roof vent since there was only 1 on top. 1st time RVer so I am still learning.

If you only have one vent on top it is most likely the black tank vent, don't recall ever hearing of combining the black and gray vents. What you most likely have is a vacuum breaker valve under the sink. If so what you are experiencing is normal, that's what my gallery gray tank does with the vacuum breaker

Black tank oder could be almost anything including a wind direction shift. Are you putting maybe 5 gallons of water back in the black tank after dumping? If not try that so that the small amount of solids remaining stay moist and treated with chemicals

[nelson]

I had a bees nest get in mine and I flushed it by sticking the garden hose down from the top

[Weldon]

There is a vent most likely under the lavatory sink. Don't recall the name...may be for gray or black....

[RudysWorld]

Thanks for the reply.

The odor started while parked and after dumping tanks, so moving did not cause it. I will have to check for the gray tank vent, I assume both gray and black used same roof vent since there was only 1 on top. 1st time RVer so I am still learning.

[PapPappy]

The vent problem sounds like you may be right. It could be that you are actually pulling the water from the shower drain trap, and once you put enough draw on it while draining the gray tank, you hear that woosh sound....of air being sucked in one of the drain traps.

As far as I know (you will have to find your own gray tank vent), the gray tank vent is on the tank itself, not running up to the roof of the RV the way the black tank does. If you can get under the RV and inspect the gray tank, you may find the vent, and see if there is anything that is obstructing it.

Black tank odor may also be coming from the shower drain, if my first thought is correct, and some of the sewage vapor has gotten up into the gray tank line, and then up through a drain without water in the trap.

It could also be that your roof vent IS plugged, but even then, you shouldn't smell the black tank, unless there is a crack in the vent line, or your toilet seal is failing. Will water stay in the toilet, or does it drain down into the tank?

Also, sometimes, if driving down the road, wind will push air down into the black tank, and in effect, "pressurize" the black tank, causing stinky air to be forced up through the toilet seal.
If this is a problem, you can buy items that go on the top of the vent, to help cause the air to be drawn up out of the tank, not down into it.

Understanding the system, will make it a bit easier to trouble shoot.