We have had a sewer smell in the house for a couple weeks which is getting worse. The smell is not coming directly from any toilet or sink drain. The smell comes and goes. It can be very strong or weak. The basement will smell, and then the smell will work it's way through the walls (the return air space for the heating/air conditioning system) and come out in one of the second floor bedrooms. Nothing is clogged and nothing drains slowly. There is no gurgling in any toilets or sinks and nothing seems unusual. We went as far as the attic but did not smell anything around the three vents. We tried to look around in the basement, but we don't see a floor drain and all the pipes disappear behind a cinder block wall. Any suggestions??

speedball1

Since you say the smell originates in the basement that's where I'd start checking. The average basement, if the drainage's under floor, is plumbed with a floor drain. This is standard, we provide a floor drain for cleaningand any washer overflow that might occure. if your's is covered up or hidden then the water seal in the trap has evaporated and that's the cause of your smell. Are there any other fixtures in the basement? Washer? laundry tray? that might have lost the water seal out of the trap? Let me know what you find. Tom