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Old 12-08-2021, 10:03 AM   #1
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Tank Smell Flows in thru A/C units

Good day RV community! I just joined this forum and this is my first post here in iRV2.

I have worked with 5th wheels for about 12 years now so I consider myself somewhat accustomed to managing the needs to maintain a coach.

I just purchased a second 5th wheel as a live in coach a couple of months ago, a 2021 Alliance Paradigm 370FB. After being in the coach for a few days we noticed a sewer odor would linger in the coach. Most of the time it was very dilute so we would neutralize to it after being in there for a short time. But when we would first enter we could tell it was there. Guests too would mention upon first arrival.

However, the smell would be very sharp to the point it would make us open all the windows and exit the coach each time we dumped a load of water down the toilets or the sinks. My initial thought was the AAV's having an issue, or tank fumes were backing up the plumbing, but this wasn't the case. The odor was not coming up the sinks or toilets.

The real sharp odor was being pumped in thru the roof AC units. Each time liquid is added to a tank the gasses inside are displaced and come out the vents on the roof. When there is little to no wind outside, and this is heavily dependent on wind direction as well, the strong odor will come in thru the closest down wind AC from the respective roof vent we are adding liquid to.

The kitchen sink tank vent is at the rear of the coach (for some reason). When I pull the drain plug on the kitchen sink, the rear most AC will pump in the odor. When flushing the guest bathroom toilet, the middle AC will pump in the odor.

When we take a shower in the front master bathroom, the front AC will pump in the odor.

If I leave the gray tank valves open, the link between the tanks (dump line plumbing) creates a breathing condition because all 4 tanks tie into one common dump line header. When the wind blows against the coach, small pressure differences that are created at the two gray tank roof vents that are at opposite ends of the coach causes the gasses in one tank to flow below and up into the other, where that tank emits the fumes from the lower pressure roof vent and the closest down wind AC pumps the smell into the coach.

I have studied the cause and effect heavily these past few weeks and this is the theory I have determined. Keeping all the valves closed eliminates the breathing issue and limits the odor pumping to only when a generous addition is made to a tank.

Now I know this is the correct function of the vent system. So now I have to ask, are the AC units not sealed internally to maintain a separation of outside air and inside cabin air?

The RV has the Coleman Mach V (or could be Mach IV) AC units. The middle and rear AC's are dump vent styles, while the front AC is ducted, but this doesn't seem to matter.

Has anyone had issues with these AC's pumping in outside smells?

I have had my Forest River Salem Hemisphere with the Dometic AC for 12 years and it has never had this issue....ever.

It's embarrassing to have a $100K coach smell like it needs a constant stick up while my old coach still smells new after all these years.

Any words of wisdom is certainly welcome. Thanks!
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Old 12-08-2021, 10:29 AM   #2
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It seems you are not alone with this issue

Sounds like the Wast tank vents are below roof line/not sealed to inside of roof and odors are going into the 'attic' area ----A/C duct not sealed off or they are using the new improved ducting. IE: raceways cut into the solid insulation to be used as ducting.

https://www.irv2.com/forums/f59/blac...ng-555167.html



A/C Unit on roof not properly sealed

https://www.irv2.com/forums/f54/blac...-c-497224.html
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Old 12-08-2021, 10:41 AM   #3
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I gather your air conditioning is creating negative pressure within the living space and that whenever a drain is opened the noxious fumes enter the space. Perhaps you could start using an IR gun to trace cold air loss to the outside.
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Old 12-08-2021, 11:01 AM   #4
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Quote:
Originally Posted by Old-Biscuit View Post
It seems you are not alone with this issue

Sounds like the Wast tank vents are below roof line/not sealed to inside of roof and odors are going into the 'attic' area ----A/C duct not sealed off or they are using the new improved ducting. IE: raceways cut into the solid insulation to be used as ducting.

https://www.irv2.com/forums/f59/blac...ng-555167.html



A/C Unit on roof not properly sealed

https://www.irv2.com/forums/f54/blac...-c-497224.html

Agree, these are your two most likely causes^^.


Follow up on sealing the AC ducts from "attic" space and roof vent penetrations. AC should only draw in air from the ceiling air return vent, but the weak link is in the ducting to the exhaust (cooling) vents.


There is always the possibility that the vent pipes themselves are compromised or have dropped below the outer roof level, and odors are seeping into the attic.
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Old 12-08-2021, 11:03 AM   #5
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The roof ACs should not be blowing outside air into the coach, ever. They should circulate outside air thru the condenser to cool the refrigerant, and circulate inside air thru the evaporator to cool the inside air, and never the two should meet. Therefore, you have leak likely at the roof seal for the AC unit, or within the AC itself between the condenser and evaporator. OR...

Does your unit have a washer dryer? Even if it has the WD prep but no WD, it has a p trap to the gray tank, and without use, that p-trap may dry out. Some cap it, some fill it with veggy oil that never evaps away.
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Old 12-08-2021, 07:36 PM   #6
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Hmmmm, I agree with the sentiment that the AC's are jut spreading it around.

When a load of liquid enters the tank the air above the fluid has to be displaced, and should go out the vent. Apparently it is finding it's way into the envelope, or better: it escapes the plumbing.

I would expose the vent and plumbing as much as possible and see if I can sniff it out. If not, I have used a theatrical smoke maker, a card board box, a small fan and a lot of duct tape to put low pressure smoke on the system at the vent.......
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Old 12-08-2021, 09:50 PM   #7
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Something so basic but...

Pardon me for mentioning something pretty basic, but nobody appears to have mentioned this yet.

Why do your tanks smell SO bad? Aren't you using either tank deodorizer and/or enzymes to break down the waste? I learned that tanks should never get to smell that bad.

RV tanks and vents are nothing like the sewage you have at home. The vents cannot be sticking up from the roof 2' nor can they be 5m to 10m from any openings. A bad smell WILL find its way into the RV, most likely backdraft through a cracked open roof vent, an open door or window,etc.

Check your vents for cracks, breaks, or dry and uncapped P or S traps, but don,t be quick to say there's a defect if you use nothing to control the waste smell. How often do the tanks get drained anyway?
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Old 12-09-2021, 07:36 AM   #8
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Quote [Pardon me for mentioning something pretty basic, but nobody appears to have mentioned this yet.

Why do your tanks smell SO bad? Aren't you using either tank deodorizer and/or enzymes to break down the waste? I learned that tanks should never get to smell that bad.

RV tanks and vents are nothing like the sewage you have at home. The vents cannot be sticking up from the roof 2' nor can they be 5m to 10m from any openings. A bad smell WILL find its way into the RV, most likely backdraft through a cracked open roof vent, an open door or window,etc.

Check your vents for cracks, breaks, or dry and uncapped P or S traps, but don,t be quick to say there's a defect if you use nothing to control the waste smell. How often do the tanks get drained anyway?
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Yes, we have been using tank treatments. At first I though maybe I had not been adding enough. My old RV has smaller tanks and I was putting in the same amount of treatment into larger tanks. I have since quadrupled the amount. I add an entire 1/2 gallon of black tank treatment to the black tanks, and a 1/2 gallon of deodorizer to the grey tanks. Even flushed a dilute bleach mix thru them after draining them, and getting them about half full and draining them again.

This has battled the odor so it isn't constant, but when copious additions to the tanks are done the smell still gets pumped in.

Thanks for the input!
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Old 12-09-2021, 07:46 AM   #9
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Quote:
Originally Posted by Old-Biscuit View Post
It seems you are not alone with this issue

Sounds like the Wast tank vents are below roof line/not sealed to inside of roof and odors are going into the 'attic' area ----A/C duct not sealed off or they are using the new improved ducting. IE: raceways cut into the solid insulation to be used as ducting.

https://www.irv2.com/forums/f59/blac...ng-555167.html



A/C Unit on roof not properly sealed

https://www.irv2.com/forums/f54/blac...-c-497224.html
Thanks for those threads. I read thru them and fear that there is an internal leakage of fumes. But to prove this, I plan on sealing the roof vents to a hose and allow them to hang down and away from the RV and test dumping and filling to see if the smell is still getting pumped in the coach thru the AC units.

If no smell, then AC units are not sealed well. If smell, then gas leakage under the roof surface.

Will report back what I find.
Cheers!
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Old 12-09-2021, 07:48 AM   #10
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Originally Posted by TandW View Post
I gather your air conditioning is creating negative pressure within the living space and that whenever a drain is opened the noxious fumes enter the space. Perhaps you could start using an IR gun to trace cold air loss to the outside.
I have an FLIR cam. Hadn't thought of that. The front most AC is the only one with ducting, the other two are direct dumps.

Thanks for the idea!
Cheers!
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Old 12-09-2021, 07:53 AM   #11
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Originally Posted by TonyMac View Post
The roof ACs should not be blowing outside air into the coach, ever. They should circulate outside air thru the condenser to cool the refrigerant, and circulate inside air thru the evaporator to cool the inside air, and never the two should meet. Therefore, you have leak likely at the roof seal for the AC unit, or within the AC itself between the condenser and evaporator. OR...

Does your unit have a washer dryer? Even if it has the WD prep but no WD, it has a p trap to the gray tank, and without use, that p-trap may dry out. Some cap it, some fill it with veggy oil that never evaps away.
You spawned an idea I need to look into. Coach is prepped for WD but we do not have them yet so I will need to look at that drain line.

I do fear there is an internal leak in the roof space though. And it is pretty disappointing that for all the vents up there, it seems all of them are doing this. Could be the assembly line overlooked some critical steps?

ExploreUSA RV informed me that Alliance had been having some issues with the tank vent piping not being sealed.

Thanks!
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Old 12-09-2021, 07:56 AM   #12
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Originally Posted by oscarvan View Post
Hmmmm, I agree with the sentiment that the AC's are jut spreading it around.

When a load of liquid enters the tank the air above the fluid has to be displaced, and should go out the vent. Apparently it is finding it's way into the envelope, or better: it escapes the plumbing.

I would expose the vent and plumbing as much as possible and see if I can sniff it out. If not, I have used a theatrical smoke maker, a card board box, a small fan and a lot of duct tape to put low pressure smoke on the system at the vent.......
That is a good idea as well. This is a procedure that is used for tracking emission leaks on cars. I will stop by the party store and see if they have a smoke machine for rent.

Thanks!
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Old 12-09-2021, 08:00 AM   #13
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BTW - I forgot to reply to the latter questions. I try to dump the black tank weekly. Normally I like to leave the gray tanks open, but if I do and the wind is blowing, it creates pressure difference at the roof vents and the gray tanks "breath" down thru the dumpline up into each other. So now I keep the grays closed and dump them one at a time daily.

I stopped using deodorizer in the grays becasue I dump them daily and the cost of the treatment was beginning to mount up.
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Old 12-09-2021, 09:06 AM   #14
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You spawned an idea I need to look into. Coach is prepped for WD but we do not have them yet so I will need to look at that drain line.
If you have the WD prep and have never used the WD, that p-trap is dry. It should be easily accessible -- fill the p-trap first with a cup or 4 of water followed with a dash of veggy oil to stop the water from evaporating, followed by good ol' duct tape.
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