I have a plumbing question. My house has a bad smell to it, and it is driving me crazy. Not sure where it is coming from. Have checked and there is a trap on every fixture and a 1.5" PVC pipe that goes through the roof for the vent. There is only 1 vent for the house and it is intended to vent 1 toilet, 2 sinks, 1 washer, dishwasher and 1 shower. Is this enough vent?? Also the when the vent comes out it goes straight horizontal for about 3 feet before heading up through a closet and through the roof. There is no consistency to where the smell is coming from. It seems to be coming through the vents of the furnace but I have checked the furnace and haven't noticed any smell in there. It is a high efficience furnace with its own drain system, is there a chance this is clogged?? I am getting very frustrated and have been working on this problem for a year. Initially I thought it was stagnant water laying behind the house so I dug ditches and installed a drain. Then I redid the drain the basement/crawl space. The house is a 1 bathroom house and the kitchen and bathroom are on opposite sides of the small house. The drain for the kitchen picks up the dishwasher, kitchen sink, and clothes washer and hooks in with the other drains for the bathroom including the tub, and sink and go under the house. There is only one vent in the entire house, and it seems no vent on the toilet.
Please suggestions would be great!! Thanks

Is the smell only in the heating season - is that why you suspect the furnace? Describe the smell. Is it a sewer smell? You've had it at least a year, so I think that rules out a dead animal trapped somewhere. What is the size of the one vent going through the roof? I suspect it very well may be enough if large enough. Of course that does not mean it can't be clogged somewhere. Have you ever snaked it?

There is only one vent in the entire house, and it seems no vent on the toilet
I'll bet if you look in the attic you'll see that all your vents are revented back into one vent exiting the roof.
What does it smell like in the attic?
Let me know, Tom