Sewage Odor

Question: Everytime that we turn on the heating furnace, there is an awful sewage odor that comes through the vents. We have plenty of sewage treatment chemicals in the tank. My husband even went on the roof & checked the ventilation caps on everything & all was normal. We just can’t figure out what is wrong. We bought the RV used & just have been living in it for about a month. It is an 2008 Big Horn 5th wheel. Hope you can give us some answers.

Answer: We have several ideas on what might be wrong, without knowing the specifics of your fifth wheel furnace system. So we offer our thoughts on the subject and welcome other comments.

  1. One possible reason for the trouble is that sewer gases are being drawn in.
    • Your return air duct on the furnace may be pulling sewer gases from the sinks, toilet, shower. This could be happening whether your gray or black water tank drain valves are closed or open, but odors are more likely with the drain valves open. You should always keep the valves closed unless you are emptying/flushing the tanks.
    • Another reason that sewer gases could be being drawn in is that there is a problem with your sewer vent pipes that are used for your sinks and shower. Some sink drains use a one way vent that is supposed to open while water is flowing and close when it is not flowing. Other sink/shower drains have a sewer vent pipe that runs up to the roof. In either case, you could have a problem with the sewer vent pipe – the one way vent could be stuck, there could be a break or blockage in the vent pipe, etc.
    • You might also want to verify that your toilet bowl is still well attached. A slipped of broken wax ring could be the cause of all your problems.


  2. A second possible cause is a problem in your furnace duct work itself. It is possible something has gotten into the furnace duct work and is sitting there rotting or moldering away.


So…you will need to determine if the odor comes from the furnace itself, the duct work or some other source. If this were our rig here are some steps we would take to figure out what is going on. Most of them will involve using your nose, a mirror and a flashlight:

  • Before you do anything make sure that your dump valves on both your black and grey water tanks are closed. If they are open they will definitely allow sewer gasses into your 5th wheel.
  • Run water into all of your drains to make sure all of your P traps are full of water.
  • You mentioned that you have gone up on the roof to look at the sewer vents. But did you take the caps off to peer down the pipes? You have to do this to determine if there is a blockage. Also while you are up there to try gently press down on the pipe. If it moves it may be the source of the problem. The connection may be broken where it goes into the tank. Be very careful – you do not want to create a new problem like a leak in the roof or a broken pipe or fitting.
  • Go to your plumbing bay or on the outside of your rig and inspect the holding tanks. Look for any obvious signs of leakage especially around the top of the tanks. If they are enclosed then the problem will be more difficult. Just look for leaks.
  • Figure out where the furnace is drawing its air (It should be from outside of the 5th wheel). In this step turn the furnace on and go outside to the fresh air intake. Is it blocked?
    Use a flashlight to look into the air intake/grate, make sure nothing is in there. If the air intake is clear, do you smell the sewage smell near the access door to the furnace? Open the access door and smell around it. Can you smell the sewage? If no smell is detected, then the problem is in your duct work.
  • Next draw a picture of all of the vents and ducts from the furnace to the inside of the 5th wheel. This will give you a reference point to start the process of elimination.
  • Now for the really hard part. Start on the inside with the furnace on. Do any of the vents seem more odorous than the others? Try to look down the vents (here is where the mirror and flash light will come in handy.) What do you see? If nothing, start with the next vent and go through all the vents until you have inspected all of them.
  • Don’t overlook the air return vent – do the same inspection you did for all the others.

If all this fails to uncover the problem I suggest you contact the place that sold you the unit and see if they can provide some help. If you ask nice, they might be motivated by the possibility of a return sale and how you will tell everyone how caring and helpful they were with your problem. If they provide no help then it is time to seek professional help by having your duck work cleaned. If that does not fix the problem then you will be forced to take your rig to an RV repair facility to get the problem fixed.

Comments for Sewage Odor

Apr 15, 2016 Another possible cause
by: Kirk L

I get sewer odor when the furnace is on. My toilet has a overflow hole that is always open. When I have the furnace on I believe some of the heated air is being blown out of the system to the outside, creating a partial vacuum inside the RV, thus pulling odor from out the toilet overflow hole. This doesn’t happen when I crack open a window though.

Jan 23, 2013 sewage odor
by: Anonymous

Thank you for all the info. Hopefully we will have it conquered.