I keep getting bad sewage odors throughout my house. The vent line seems to be clear and all drains have traps. Should I put deeper traps on the drains or am I missing something?

Some times when you get sewer odor in your house it is because of a drain that has a trap but there is no water in the trap. Do you have a shower that you hardly ever use, or a drain in the floor under the washing machine? The floor drain under the washer never gets water added to it unless you add it. One other place might be close to the water heater that has a floor drain under it, it is suppose to have a trap filler that keeps it filled but sometime they don't work. I advise you just check every where you can think of.

You are right there is a drain under the washing machine. I have in fact flushed with lots of water and actually put a plug in this drain. I will however check under the water heater, I have not noticed one in that area before though. The smell comes more often in the colder weather and seems to coincide with running the washing machine. My house was refurbished before I bought it a year and a half ago. All of the new plumbing has traps and is tied into a vent line. The new vent line goes into the attic and then takes a 90 degree turn and runs about 10' and taps into the old vent line then up through the roof. Is it possible that with the horizontal run in the vent line is not allowing it to work properly?

If the horizontal line is a galvanized pipe it is possible that some of the rust and scale has broken off the wall of the pipe and has stopped up the vent in that area, that would make you traps suck for air and it would break the seal and let odor back into the house.