Ask Dave: Why does my roof AC unit have a rotten egg smell?

Dear Dave,
We are living in a Coachmen in Wyoming while our house is being built. The air in the bedroom when turned on smells like rotten eggs or sewage. We are first-time owners of a camper. We Google and read the books. There is no animal in there. The air is cold. The bedroom has no thermostat. The living area does. No problem in the living area. What do think it could be? Please help us. Thank you. —Cindy S.

Dear Cindy,
If there is a lot of condensation dripping off the unit, this could indicate the drain holes are plugged.

Let’s start with how the system works

This unit has a thermostat directly on the unit rather than on the wall. You can see the two black knobs in the photo, so you control the temperature and fan speed there. When the AC unit turns on, the motor spins the front squirrel cage and the back condenser fan. The squirrel cage pulls moist inside air up through the vented area located on the back part of the bottom unit, and draws that air through the evaporator coil which has copper lines with fluid pumped from the compressor. This flashes and removes heat and moisture and pushes that conditioned air back down into the rig through the front vents. In a roof-ducted model there is a diffuser plate or baffle that directs the air to the roof vents.

The coolant travels back to the condenser and the rear fan pulls in outside air from the back that blows over the lines and exhausts to the side of the unit. This is a completely sealed unit and no outside air ever enters the coach. The photo below shows what the evaporator coil looks like with the drip pan underneath and drain holes.

To access this you need to remove the outside or upper shroud of the air conditioner unit from on top of the rig and then remove the evaporator metal cover. Here you can see the return air vent which is the opening area on the bottom of your rig. This photo also shows the drip pan and it angles on each side back to the drain holes.

If you don’t want to get up on the roof, you can also see this from the inside of the rig by twisting the tabs on the vent cover and removing it. There should be a thin foam filter on it and the opening will have the evaporator coil there. You might be able to vacuum it from inside?

Check the drain holes of the AC unit

Since you have so much condensation, I would check the drain holes to make sure they are clean and open. Since you are pulling in air from inside the rig, there can be accumulation of dust, pet dander, and other things that can clog these drains. Also, check the evaporator coil and make sure it is clean and the fins are straight to allow proper air flow. Since you are experiencing a rotten egg or sewage type smell and there is so much condensation, my guess is there is a buildup on the evaporator coil and maybe standing water in the drip pan which has now started to mold.

Here is a photo of an AC unit that quit working for an owner. When we went to diagnose it, we found the evaporator coil covered in body powder! There was not much condensation so the rotten egg smell was not present. However, the blockage of the airflow through the evaporator coil made the motor work harder and increased the amp draw and eventually ruined the unit.

Smell the air coming out of the vent

All that being said, if you check the drain holes and evaporator and they all look good, get up on the bed and smell the air coming out of the vent. If it smells OK and you still have the foul smell in the bedroom, it could be an issue with a holding tank vent pipe nearby. The black and gray water system needs a vent pipe that typically goes up to the roof and vents out the top. This photo shows the typical vent cover to the lower left.

This eliminates a vacuum in the system and allows water/liquids to flow to the tank. These pipes also allow the smells in your holding tanks to follow up the pipes and exhaust at the vent. That is typically not a problem with the vent being at the top of the rig so you normally do not smell it. However, if a pipe or connection pulls away or the system is not vented properly, the smell can make its way inside the rig. You might not smell it when the roof AC unit is not running as the airflow or draw in the rig is relatively low and the warm holding tank air and smell will naturally rise and make its way out the top. But when the roof AC unit kicks on, it changes the airflow dynamics and mixes the air. That will pull inside air from everywhere and create what you are experiencing.

What a “cheater vent” is

One situation we found was a toy hauler unit that had a vent tube from the black water tank coming up into the kitchen area. Due to the kitchen countertop being directly above the tank, they could not have a pipe go all the way up to the roof so they installed what is called a “cheater vent.” The pipe comes up from the tank into the base cabinets under the sink. It is capped with a one way vent cap to allow air to come in, but not let air from the tank to escape out. Sounds good in theory, but it did not work well. You never see this in a residential plumbing system!

Hopefully this gives you some steps to take to eliminate the smells in your coach.

We originally posted this question and answer in September 2021.

Read more from Dave here. 

Dave Solberg worked at Winnebago for 15 years developing the dealer training program, as marketing manager, and conducting shows. As the owner of Passport Media Creations, Dave has developed several RV dealer training programs, the RV Safety Training program for The Recreation Vehicle Safety and Education Foundation, and the accredited RV Driving Safety program being conducted at rallies and shows around the country. Dave 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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