I'm experiencing strong "sewer odors" coming from one of the basement drains. This is an old house (meaning the basement floor drains don't have an elbow bend and instead run straight down / there is no p-trap), but the strong odor is only present at one of three drains--which happens to be right by the furnace.

The rest of the plumbing in the house is new (just over 1 year), and the smell hasn't been an issue until the past month or so. Temperatures are still in the 30-50s range.

I'd gotten some "septic helper" at Home Depot, and was looking at ordering some active enzymes, but was not sure if they'd remain in the area long enough, or would be quickly whisked through.

Pouring water into the drain helps a little, but it's a daily exercise to even keep pace.

Basically, I'm not sure what to do to treat the root cause of the issue, and keep the odors from making me gag.

Any help would be appreciated!

  • Don't floor drains "dry out" after a while? I've heard you need to pour water down them on a regular basis to keep the trap full and fresh.
    – JPhi1618
    Mar 21, 2016 at 20:39
  • 1
    Is your house sealed tightly? Pouring water down the trap should last for at least weeks. If you have negative pressure in the house or basement the gasses can be pulled through the trap. try opening a window slightly and see if the problem stops. If opening a window solves the problem you might want to add a "make up air vent" into the furnace intake (this will keep the basement warmer than a open window).
    – Ed Beal
    Mar 21, 2016 at 21:37
  • Is there visible water in the drain or does it seem empty?
    – DMoore
    Mar 22, 2016 at 4:55
  • Near-duplicate: diy.stackexchange.com/questions/74792/…
    – Ecnerwal
    Apr 29, 2016 at 3:11

3 Answers 3


I'd be surprised if there's really no trap on that line. Humans have known how to block sewer gas for a very long time.

Anyway, consider adding a drain trap or a trap above floor level if you don't need that drain as a flood relief. Chances are you can raise the furnace condensate line to match.

Also consider making a small box cut in the concrete and fixing it properly with a new trap. You can rent a diamond saw and open up maybe 18" square, then patch it back in with a bag of concrete mix.

  • I'm sure it was in your head, but I'm not seeing an explanation of what the OPs problem is. Two seemingly good fixes, but what are you saying is wrong?
    – JPhi1618
    Mar 21, 2016 at 21:23
  • 1
    No trap, apparently.
    – isherwood
    Mar 21, 2016 at 21:31
  • @JPhi1618 - If there's no trap then there's nothing to keep the sewer gas out.
    – Hot Licks
    Oct 10, 2022 at 21:30

Just cover or seal it since it's not being used for anything. If it has a screw-down grate you can take that up & punch the screws through even just a plastic lid or its portion & screw the old cover back down for a solid seal...coffee container lid, peanut can lid, etc.

The Home Improvement stores also have plumbing plugs for most sizes of pipe, which are more permanent & better sealing. They expand as you turn the wing-nut to lock into the pipe fairly well. Plug all 3 drains so the house can't fill with either methane or just stink at any time.


If it is an evaporation issue, the water evaporated opening the drain, try some RV winterizing fluid a) won't freeze b) slower evaporation rate.

You said "Pouring water into the drain helps a little, but it's a daily exercise to even keep pace." This leads me to believe that you do have a P trap and it is losing water. cause a)evaporation, not likely to fast. b) the trap froze at some point in the past and has only a slow leak. The RV antifreeze will prevent this in future down to -50 after you fix the possible broken pipe.

As a temporary fix/test

Parts: 1gal. Screw top jug, 1/4"-1/2drip line 3"-1',sharp objet to make hole in lid slightly smaller than tubing , 1)attach pice of tubing through the lid,make it a air/water tight fit must not leak.2) add water to p trap 3) turn over and put on top of the drain the tubing should be in the water 3)pull it backout and cut tubing at the water line 4)filliwth water5) poke the tube down the drain invert jug once the trap is full (stops flowing)mark water level time and date. it should stay put until the water in the p trap dropps below the tube allowing air to enter the jug and water to refill the trap. This should take months to empty a gal jug through evaporation or hours for a cracked pipe with your previous daily chore of refilling the p trap my guss is that you need a new p trap

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.