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Old 09-23-2021, 01:02 PM   #1
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Black Tank fumes via A/C vents when flushing.

I know it's long, I'd ask for folks to read the whole thing to avoid guesses or suggestions that are clearly out of scope. The subject rig is a 21 Jayco 5er which has not exhibited this behavior in 12,000 miles and 70 nights camping.

Suddenly, when I flushed the toilet this week while the A/C is running, distinct black tank odor comes immediately and directly from the A/C ducted vents, even the farthest vent from the bathroom.

Grey Tanks are empty. All sewer connections are closed. All windows are closed. All vents are closed. Batteries are Lithium. All the other guesses folks have (P traps, and boiling batteries, and condensor coils, and air admittance valves and fantastic fans) do not apply here. Black tank odor is getting in to my AC system during tank flushes and being immediately distributed via the ducted vents. (As a test I opened the door and repeated the test with the same result)

If I turn the AC off, no odor seemed to be noticeable during a flush. Likewise, no odor is noticeable until you flush.

I triple rinse-flushed my black tank, and do not notice the smell after doing that. Never have I seen such clear water .... This of course treated the symptom, but I fear the problem will return.

The only PD I was able to do before sending the trailer to off site storage for a few weeks was to get on the roof and quickly visually inspect the black tank vent. It appears well lap sealed, and I could smell black tank gas from it. I did not have a chance to remove the cap (not sure how either).

Note, there is front and rear AC units, both connected to the duct. I frankly did NOT test them independently. The black tank vent is slightly closer to the front A/C unit, but close enough that either unit could be part of the problem.

This thread from another site is the best match to what I am describing, and I am thinking taking the vent caps off and examining whether the stubs are too short and are letting air in to a punctured duct (!) is what I have to do next. I assume the flush disrupts the gas, sends it up the vent where it unfortunately does not completely exit the vent, but rather is introduced directly in to the ductwork:

https://www.forestriverforums.com/fo...ts-168511.html

There are plenty of threads here on irv that are tangential or not matches to the problem described in this thread. Here's to hoping someone here has resolved this very specific problem and can let me know if they think I am on the right track. Being away from the rig makes PD difficult so I like to get armed with ideas so when I have time to diagnose I am prepared.

Thanks!
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Old 09-23-2021, 02:07 PM   #2
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Perhaps you finally caught a shift in the wind where your vent opening is blowing towards your AC and the AC is sucking the lovely odor into your RV.
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Old 09-23-2021, 02:14 PM   #3
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Originally Posted by Pointerman View Post
Perhaps you finally caught a shift in the wind where your vent opening is blowing towards your AC and the AC is sucking the lovely odor into your RV.
An AC doesn't take in outside air, at least not one that's functioning correctly, that's the purpose of the inside air return. That unit up on top is simply blowing outside air across a radiator removes the heat from the 'freon', there is no air exchange outside-to-inside going on up there.

The first thing that comes to mind is to check to make sure the AC unit is properly bolted down so that it's nice and tight. It's conceivable that it's loose up there and is pulling some outside air, and as Pointerman suggested it's pulling in some air from the vent on the roof.
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Old 09-23-2021, 04:15 PM   #4
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As noted, the A/C does not take in outside air unless you have a pretty good leak. You may well have a vent pipe leaking from the tank into the RV.

Also make sure it is not the air admittance under one of the sinks leaking back.

Ken
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Old 09-23-2021, 05:39 PM   #5
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As noted, the A/C does not take in outside air unless you have a pretty good leak. You may well have a vent pipe leaking from the tank into the RV.
I understand the AC does not take in outside air. I kind of think it is a leaking or low stubbed vent pipe, and for the speed it happens I fear it is leaking literally directly in to the ductwork.

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Also make sure it is not the air admittance under one of the sinks leaking back.

Ken
If you have read the OP, you know it's obviously not related to AAVs connected to gray tanks.
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Old 09-23-2021, 05:41 PM   #6
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Perhaps you finally caught a shift in the wind where your vent opening is blowing towards your AC and the AC is sucking the lovely odor into your RV.
This would only make sense if the vent discharge is entering the coach directly, and with the speed it happens, I fear it is literally entering the duct work.
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Old 09-23-2021, 05:44 PM   #7
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Quote:
Originally Posted by Podivin View Post

The first thing that comes to mind is to check to make sure the AC unit is properly bolted down so that it's nice and tight. It's conceivable that it's loose up there and is pulling some outside air, and as Pointerman suggested it's pulling in some air from the vent on the roof.


This is an interesting thought. I find it more likely that I have a leak from the vent pipe in to the attic or in to the ductwork but you are absolutely right that it is possible the whole unit is loose. I'd say it is It is doubtful ... but possible and I will add this to the list of things to check, before I attempt to remove the vent cap and examine the vent pipe.
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Old 09-23-2021, 05:48 PM   #8
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Roof top A/C Unit
Stink as soon as you flush toilet

Vent leaking into attic area....A/C duct leaking would be under positive pressure...not negative pressure and sucking stink in

Check the A/C hold down bolts...if roof gasket is not sealing off then vent stink could be sucked back in to RV interior being pickup up by the A/C Return

Everything else on A/C is under Positive pressure when running
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Old 09-23-2021, 08:56 PM   #9
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I think your ac is just sucking it in the intake and distributing it into the ducts.
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Old 09-23-2021, 09:02 PM   #10
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Originally Posted by Old-Biscuit View Post
Roof top A/C Unit
Stink as soon as you flush toilet

Vent leaking into attic area....A/C duct leaking would be under positive pressure...not negative pressure and sucking stink in

Check the A/C hold down bolts...if roof gasket is not sealing off then vent stink could be sucked back in to RV interior being pickup up by the A/C Return

Everything else on A/C is under Positive pressure when running
Thanks. I will check em out.

I do think whether positive or negative pressure, if the stink has access direct to the duct system that could be a problem. In the link I posted up thread the manufacturer punched a hole through the duct to run the vent pipe itself. Unbelievable.
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Old 09-23-2021, 10:35 PM   #11
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Thanks. I will check em out.

I do think whether positive or negative pressure, if the stink has access direct to the duct system that could be a problem. In the link I posted up thread the manufacturer punched a hole through the duct to run the vent pipe itself. Unbelievable.
Didn't occur for 12,000 miles/70 nights camping

A/C hold down bolts....new installation/vibration---bouncing etc
Bolts are NOT real tight
Just finger tight then SNUG.....(SNUG is scientific term for 40 INCH Pounds of torque, INCH pounds/Not Foot pounds. Very few folks have an Inch torque wrench)
**New roof gasket is ~1" thinck. When installed it S/B compressed to 1/2 of thickness---roughly.
SNUG
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Old 09-24-2021, 08:28 AM   #12
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I think your ac is just sucking it in the intake and distributing it into the ducts.
Yes, but the mystery is how, because air is not introduced to the A/C fro outside the coach. It has to be infiltrating ad tat is the mystery.
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Old 09-24-2021, 08:34 AM   #13
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I suggest as TXiceman did to check the air admittance valve. What you report is not uncommon with a sticking air admittance valve. ~CA
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Old 09-24-2021, 09:28 AM   #14
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Do your a/c units have a ducted return or do they draw in from directly below the unit? Return ducts would be under negative pressure and draw in odour from the ceiling space through any cracks or gaps. Any chance you have rodents that may have chewed a hole in the ducts?
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