coming home to sewer gas odor

We have a one story home that is approximately 60 years old. We have lived here 22 years and were disappointed to have a sewer gas odor appear 2 months ago. This odor would only appear around 5 pm and would disappear after a couple of hours. We assumed it was the very old furnace and air conditioner coil and had both replaced (they were quite old). The odor persisted every day at 5 pm, 6pm or 7 pm. We live in a landlocked state here in the midwest and are suffering from very high temps so the air conditioner must be on. The air ducts and returns are clean and the odor seems to come directly through the air vents. Our plumber came and there was no sewer leak under the house (we have a crawl space). We need to figure this problem out but are not sure exactly which way to turn now! new plumber? new a/c person? Thanks for any suggestions ---

odor

ONe common cause is the air conditioner condensate line being connected to an open drain, or hanging into a roof vent pipe, and sucking the fumes into the system. Normally in the summertime there would be enough condensation to seal a trap, but sometimes they are installed in such a way as to bypass the trap.

The condensate drip tray could be getting ripe as well.

hhhmmm

The coils in the furnace have been replaced and the new condensate drain line has been installed and drips outside the house.

If there's no odor at the A/C intake, but it's pronounced at the output, then the A/C is the source. If the A/C is off during the day to save a few bucks, try leaving it on for a week or so and see if that helps.

Thanks!

Thanks, I'll try that!