Ask Dave: Where are the black tank odors in RV coming from?

Dear Dave,
When I dump my black tank I get an odor in the main living space in my fifth wheel. Do you think the smelly black tank odors are coming through the air conditioner from the vent on top? —Bud

Dear Bud,
First of all, make sure it’s not a propane leak. You can check with a portable leak detector.

Where are black tank odors coming from?

I doubt that the black tank odors could be coming from the roof black water vent and into the roof air conditioner. The roof A/C draws warm, moist interior air in through the inside air return, pulls it through the evaporator coil and pushes it back down into the rig.

The only outside air that comes into the unit is through the back compressor coil to help cool the coolant tubes and is pushed out the sides of the top unit. The system should be completely sealed from any outside air coming in the coach.

To provide more specific information on the odor coming into your fifth wheel, I’ll need the make, model, and year of your rig.

Both the black and gray water tank have vents that typically go from the top of the tank up through walls and vent out the top of the rig.

Original vents have a cap over the pipe and look like this. However, some are now installing a Cyclone version that turns with the wind and provides a better venting method.

A “cheater” or siphon vent

However, since many fifth wheel floor plans have a lot of countertop space, it could be a “cheater vent” that has gone bad. A cheater or siphon vent cap comes up from the top of the tank. If the tank is under a countertop, there is no wall for the pipe to hide in going up to the ceiling. So manufacturers stop the pipe just below the countertop and use a siphon cap.

Here is one from a Raptor Toy Hauler that we worked on and it really did smell! The vent is supposed to draw air when the tank is dumped to eliminate a vacuum in the lines and close tight so odors do not come into the rig. Check to see if this is where the odor is coming from and replace it.

Black tank odors could be from cracked pipe or loose connection

It could also be a cracked pipe or a loose connection. Keep in mind, this vent might not be easy to find, or it might just be sitting out in the open underneath the cabinet. We had to tear out an access panel under the pantry to find ours on the Raptor. Once we did that, we removed the vent cap and used an air hose to verify it was not holding air on the back pressure.

A weak toilet seal could also create black tank odors

Another issue could be a weak toilet seal. Put water in the toilet bowl and make sure it holds. If not, odors can come up through there as well, especially if the ceiling fan is on.

If the other troubleshooting options do not find the problem, you can get up on the roof and take off the cover to see if the insulation is intact.

Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club.

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