We get a sewer smell in the house only when the ac runs and it's near 90. Cold returns are in the crawl. Help!

Question

There's no specific source of the odor, but it only occurs when it's very warm outside. It's done this for several years and a completely new H/AC system. AC drains outside. It doesn't appear to be caused by open traps and we really can't find a general smell in the crawlspace ( although I must admit, I can't get down there when it occurs because the entrance to the crawl is too small for me). The house is 50 years old but is very tight. There's no smell in the attic. Plumbers report no smell in the crawl, but haven't been down when the smell occured. The mystery is worse than the odor. It smells bad, but not horrible. It's not a moldy smell, although the crawl is typically cool and musty. The only consistencies are the outside hot temperature and the use of the AC being the triggers-no problem in the cooler morning or evening AC or using the heat in the winter. If someone can figure this out, a small army of baffled plumbers and sewer experts (and us) will be greatful!

O.K.,
Here's aperfect example. Today I shut the AC down ( after getting down to73 degrees)
and left it off all day. It hit 92. No smell. I put a fan by the cold air duct to try to draw out any smell down there-NOTHING! At 6:15 pm I turned the AC back on. STILL NO SMELL. So either the AC draws it out when it runs hard in the heat of the day, or it's something in the main duct work which is inconveniently below the attic floor boards. I doubt the second, nothing runs through those ducts and it's not something dead since it's been this way for years.

Our new furnace/ac is a Carrier Infinity. The ductwork simply runs under the house (the floor joists are the sides of the returns, sheet metal on the bottom, and cross rebar in the ducts have made it impossible to clean so far). The outside drain is fine and it has clear p-traps and lines out so you can monitor it. This problem goes back before the new system, but seems to be worse since the install. I'll try running the AC hard when it's not really hot out to try to determine if it's the outside temp or just the increased AC working that contributes to the problem. I never knew condensation could have this smell but I'd describe it as more of a heavy odor than a plain sewage stink.

Answer 1

I don't know if u mean they take return air from the crawlspace or if the ductwork runs thru it but the ductwork should be sealed and insulated if the ductwork is insulated on the inside and some of it is loose when the a/c runs the ductwork sweats on the insulation getting it wet creating a musty,sewer smell,also if your a/c drain runs outside the house and it don't have a p-trap or selfmade trap to keep water in it at all times not allowing hot air or the outside smell in then u have very hot humid air meeting condensation and that's not good.U stated u have a new furnace and A/C i hope that means a new
-A-Coil, has anyone checked to make sure it's draining and not backed up. Also if any mold,algea,dirt has gotten on the coil when it sweats it creates this smell as well,also make sure no ductwork is open or leaking. Cold air,condensation,and a damp crawlspace don't mix if after checking this it doesn't help you'll need an ultraviolet uv lamp to kill any airborne mold,algea,etc. and an electrectronic aircleaner to kill the smell these 2 are expensive i'm sure it's either wet loose insulation or not having a trap close to the a/c this is one of those problems that u have to troubleshoot in person, u also might have to call an Air-Quality Company that Specializes in this GOOD LUCK.

Answer 2

Look to see were the vent pipes are located for the sewer and drain system. I once found a septic vent pipe that terminated in the attic of a small house. When the house got hot, the smell came in and was very noticeable. I then extended the pipe in the attic through the side of the house, in the gable end. That took care of the problem. Also, check to see if anyone drilled a hole into any of the vent pipes. This is an old trick to try and repair an improperly installed vent system. Also, you have got to use your nose to locate the smell. This helps greatly.
Good Luck.

Answer 3

This happens every time an installer connects the condensate to the sewer waste lines with a "hard" connection (no air gap). As soon as the fan starts up, it sucks the water seal in the trap dry, and sucks stink into the air handler for all to enjoy.

Answer 4

there has to be a break in the seal of the duct work as well as some problems with the sewage lines. DUCT Tape every joint again on the AC lines then look for the source of the plumbing problems