Sewer smell...this one has me stumped!
 

Hoping someone can help me on this one. I have a sewer smell occasionally in my house. Only when there is high water usage. Running washing machine, dishwasher, etc. at the same time. It is a two story with three baths. All toilets and sinks are fine. No gurgling from drains, no odors, no leaks. No wet or soft spots in the lawn to indicate a bad line. I have an ejector pump on the furnace for the ac unit. It is piped to drain into the laundry tub, so does the washing machine. No drains have dried to allow gases in. My stack pipe is not cracked from what i can visually see.my roof sewer vent is free of any debris. The smell is the strongest in the furnace room, the sewer stack pipe is located there. But when I sniff around the stack pipe I can't smell any off gases. The whole room just seems to smell, and of course then the furnace unit kicks on and sends the smells through the entire house.what am I missing here? I know whatever it is, will be the solution to my problem! Thanks for the help! I can post pics if you need them!

could there be a hidden floor drain in the basement? that might be where your smell is coming from. they didn't by any chance connect the condensate drain from the furnace to the plumbing drains did they?

Any chance of a dead animal in the wall?

The condensate drain empties into the utility tub, along with the washing machine. We have covered every inch of the basement floor looking for a hidden drain and have found nothing. This is definitely a sewer smell, not a dead animal. The thing that has me scratching my head is this is a sporadic event, does not happen everyday. It can be summer or winter, day or night. But only when our water usage is high. Some type of siphoning must be happening from my vent stack. But is this even a possible scenario? And what would cause this?

Get the system smoke tested, no more guessing. if its sewer gas the smoke test will show the source,

smoke test
 

How much does smoke test cost?

Sure, it's possible. When you have fast-moving water that fills or nearly fills a pipe, it creates vacuum above it that can suck water out of nearby P-traps. Normally, venting prevents this vacuum from forming, but if a vent is clogged (birds nest etc) or your house is older construction back when code allowed more "wet venting" scenarios, the vents may not be adequate.