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!