We bought a house 2 years ago and have been very good about making sure the drains in the basement do not go dry. We add water every couple of weeks and have even added vegetable oil to prevent evaporation. We've also tried flushing bleach through it. Despite this, we get the bad P-trap smell. Interestingly, it is most likely to occur after rain. Of the 3 drains in our basement, only one has this issue. The only difference between this drain and the others is that the condensation tube from the furnace empties into it. We are on a sewer line in the city. The house is 1896 but has been rehabbed. Not sure age of drain. It doesn't seem like evaporation is the issue. Any ideas why and how to manage?

:confused:

"The only difference between this drain and the others is that the condensation tube from the furnace empties into it."

What kind of a furnace produces condensate? AC units produce condensate, cold water lines in a warm humid basement produce condensate, but furnaces produce heat which will dissipate condensate. Please explain. What am I missing?
As for your smelly floor drain, there might be bacteria growing in whatever's producing the condensate.
I would pour 1/4 gallon of full strength bleach down the floor drain and let it set. Then I would go to the source of the condensate and wash that out with bleach as well. Rain tends to make the air heavier so that smells that normally raise will descend so they can be noticed. Keep me informed of developments. Good luck. Tom

Your plan for the drain sounds like it should work.

As I read the question, I assumed it was A/C condensate that comes out of the A-coil housing attached to the furnace. However, there are condensing furnaces. Haven't heard as much about them more recently, but here in the frigid North, now, the more expensive furnaces wring every possible BTU out of the combustion products, bringing the temperature below the dew point. The water separates out going to a drain, and what is left is vented through a PVC pipe. Yes, a PVC chimney! Don't know if it is DVW grade or not.

I have had a condensing furnace for a while, but went to considerable work to eliminate it this fall. When I installed a new furnace, I ran type ''B'' from it to the old 7'' single wall. In the colder weather, the water was condensing out as soon as it hit the single wall. I ran the 5'' double wall all the way to the top. Hope it works. The condensate is nasty with CO2 and oxides of nitrogen and sulfur dissolved in it. It made short work of the old galvanized. I hope I manage to get it up the chimney now.

"there are condensing furnaces. Haven't heard as much about them more recently, but here in the frigid North, now, the more expensive furnaces wring every possible BTU out of the combustion products, bringing the temperature below the dew point."

Thanks Labman, for teaching this Florida cracker something about furnaces. Down here we heat with reverse air or electric heat strips. Just goes ta show you! You're never too old to learn. Tom