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morlach · 08-11-2018, 03:03 PM

Here are the conditions and what I know so far.

With the rear AC running, when the toilet is flushed, sewer odor immediately comes from the AC.

It doesn’t not happen with the front AC is running.

The odor is not coming from the toilet.

The odor is not coming from the sink.

Somehow, sewer odor is getting drawn into the AC when the toilet is flushed creating a positive pressure in the black tank. The smell ONLY appears when the AC is running. The smell does not appear in the bathroom, but in the main cabin coming from the AC.

I posted the issue on a Facebook page and got inundated by “answers” that were focused on the odor coming from the toilet and the sink, not properly conditioning my tank, and attributing the problem to the hot weather.

Baloney. None of these “answers” hold water - they are all useless.

Does anyone have any experience with a similar issue? My current thinking is that there is an air leak at the AC gasket that is somehow allowing the AC to draw gas from the black tank roof vent stack. I have not inspected the gasket closely, but if there is a leak it is only an air leak - there is no water leak. Is this even plausible? I can’t explain the phenomenon any other way.

SCOTTinNJ · 08-11-2018, 03:18 PM

Maybe try running the ac and then going up on the roof with something particularly smelly. See if that smell comes through the ac when running.

morlach · 08-11-2018, 03:20 PM

Quote:

Originally Posted by SCOTTinNJ

Maybe try running the ac and then going up on the roof with something particularly smelly. See if that smell comes through the ac when running.

That would be a good test! And for my really smelly thing, I think like peppermint or something. LOL!

DRPREECE · 08-11-2018, 03:23 PM

I provided this answer to your dilemma on FB. I agree. It is a seal in the outdoor AC unit. Time to get on the roof if you are up for the adventure. Good luck!
———————————-

I would guess that if the black tank vent is close to the rear ac, the ac is drawing it in from outside. The clamshell inside the rooftop unit may be separated and need resealed.

SCOTTinNJ · 08-11-2018, 03:23 PM

Quote:

Originally Posted by morlach

That would be a good test! And for my really smelly thing, I think like peppermint or something. LOL!

Yes. Peppermint sounds perfect. Good luck. Report back.

AtomicNo13 · 08-11-2018, 03:40 PM

Ick is being sucked in the AC from the black tank vent pipe which I'll assume is a mere foot or less away from the AC unit.
The seal inside the AC unit is at fault.

morlach · 08-11-2018, 03:42 PM

Quote:

Originally Posted by AtomicNo13

Ick is being sucked in the AC from the black tank vent pipe which I'll assume is a mere foot or less away from the AC unit.
The seal inside the AC unit is at fault.

Yeah, it’s right next to it. That’s what I’ve been thinking.

rowiebowie · 08-11-2018, 04:22 PM

I currently have a new Coleman Mach 8 sitting in my garage pending installation on Monday when my son-in-law can help provide some muscle.

I removed the outer cowl and then the blower shroud so I could pre-install the control module from my old unit. Turns out I didn't need to, but I rather have found out now than once the unit is on the roof.

As to your problem, I found the blower compartment to be fairly well sealed. However, not hermetically so. The plastic shroud fits tightly down on the framing, but without any tape or weather stripping like you'd expect in a home HVAC system. There is also 1" foam stuffed between the blower motor and the opening in the shroud were the fan shaft enters. Your model may be different.

It summary, it would not seem too hard to take just the outer cover off and look for gaps to tape/foam the fan shroud down tightly. I should involve removal of no more than 4 screws to take a look.

Good luck.

morlach · 08-11-2018, 04:24 PM

Quote:

Originally Posted by rowiebowie

I currently have a new Coleman Mach 8 sitting in my garage pending installation on Monday when my son-in-law can help provide some muscle.

I removed the outer cowl and then the blower shroud so I could pre-install the control module from my old unit. Turns out I didn't need to, but I rather have found out now than once the unit is on the roof.

As to your problem, I found the blower compartment to be fairly well sealed. However, not hermetically so. The plastic shroud fits tightly down on the framing, but without any tape or weather stripping. There is also foam between the blower motor and the opening in the shroud were the fan shaft enters. Your model may be different.

It summary, it would not seem too hard to take just the outer cover off and look for gaps to tape/foam the fan shroud down tightly. I should involve removal of more than 4 screws to take a look.

Good luck.

Cool. Thanks for the info!

rowiebowie · 08-11-2018, 04:35 PM

I meant removal of no more than 4 screws. Went back and corrected my post.

Some manufacturers have cover screws on top, others around the lower edge. But I haven't seen a cover that's hard to take off. In fact, I'm surprised more do not flap in the wind or blow off given that they seem minimally secured.

morlach · 08-11-2018, 04:36 PM

Quote:

Originally Posted by rowiebowie

I meant removal of no more than 4 screws. Went back and corrected my post.

Some manufacturers have cover screws on top, others around the lower edge. But I haven't seen a cover that's hard to take off. In fact, I's surprised more do not flap in the wind or blow off given that they seem minimally secured.

I see AC shrouds laying on the side of the road all the time! LOL!

Mollysdad · 08-11-2018, 05:42 PM

Temporarily plug the black tank roof vent and see if the smell goes away.
Then look how outside air is getting into the intake of the AC.

In addition to the smell, you're drawing in hot outside air and humidity making your AC work harder.

BillB44 · 08-12-2018, 12:07 PM

Try a simple "smoke test." Do as suggested by others but get on roof and light a smoke-generating device like an oil-soaked rag, gopher smoke bomb, etc. (Obviously, be careful with the fire and don't breath in a lot of the smoke!)
You should be able to move the device around and see visually where the smoke is being "inhaled" by the AC or roof vent, etc.

Al and Missy · 08-12-2018, 12:58 PM

There is another possibility, if you have a factory black tank flush system. If so, there is a siphon break valve inside somewhere near the flush inlet. On my trailer it is under the bathroom lavatory. When the rear air conditioner turns on it pulls a slight vacuum on the inside of the trailer and draws black tank odors into the trailer through the valve. I know of at least two trailers where this has been a problem. Further I never had odors, but my black tank flush didn't work. When I had to change the dump valves I removed, cleaned and re-installed the flush nozzle in the black tank. Now, when driving down the road I get black tank odors into the trailer. I don't have a rear AC so I don't observe that problem, but I think there is a slight vacuum at the rear of the trailer which is sucking air out through the rear storage doors and/or windows. Find the valve and try the smoke test near it.

BTW, if this is your problem the fix others have implemented, and I plan to, is to replace the valve with pipe.

Al

Whitakerml23 · 08-12-2018, 01:08 PM

Sounds like your a/c may be set-up to drain through black water tank. That’s being said, I’m wondering if some type of check-valve could be installed on drain to prevent fumes from entering a/c ventilation area.

rowiebowie · 08-12-2018, 01:21 PM

Quote:

Originally Posted by Whitakerml23

Sounds like your a/c may be set-up to drain through black water tank. That’s being said, I’m wondering if some type of check-valve could be installed on drain to prevent fumes from entering a/c ventilation area.

Mmmm. Sounds like worth looking into.

guskmg · 08-12-2018, 06:29 PM

Flushing the toilet is allowing a draft to develop and push fumes out of the black tank vent. The only way those fumes can enter the trailer is through a poorly sealed A/C.
the only other option is to install 360 Syphon vents. They vent any time there is air movement over the TT. That would prevent build up of sewer gases in the tank.

77walter · 08-12-2018, 07:04 PM

take it to a dealer !!

3wd67rg · 08-22-2018, 04:58 AM

Quote:

Originally Posted by 77walter

take it to a dealer !!

We are the kind of people who shop thrift stores and pick up stuff other people have left out for trash. We believe in purchasing gently used old stuff and fixing things. Oh, and we’re not rich an that’s Ok.

REDUCE
REUSE
RECYCLE
UPCYCLE
FIX
or PASS ALONG

saving the world one broken thing at a time.

truxpin · 08-25-2018, 08:16 AM

Another possibility....

Have you recently had any work done involving the black tank?
A few years back I dropped the black/grey tanks for valve replacement. After reinstallation we had kind of the same issue your having. To make a long story short, the black vent tube had pulled out of the vent cap assembly at the roof area on removal & on re-installation didn't go back together correctly, leaving the top of the vent pipe basically venting in between the inner & outer skins.
Pulled off the old vent cap & installed a new one with new sealant & making sure the vent pipe made proper connection.

Just another thing thats relatively easy to check....