New to forum / Seeking help regarding a bad smell in our home

[OSUCarrieA]

Our home has a bad smell only in the winter months when the air conditioning is not running. We can smell it coming from several vents and in the basement where the heater and hot water tank are located. The smell is similar to rotten eggs, sewage and we had someone mention sulfur. We cannot seem to pinpoint what triggers the odor, except that we do NOT smell it when the A/C is on. It seems to come and go both when the heater is running and when it is turned off.
We moved in a couple of years ago and have smelled the odor both winters. We cleaned the de-humidifier (which was disgusting – we thought was the source) and have turned it off completely, but the odor still returned. We have an aerobic septic system which has been checked out; we’ve asked a plumber about the smell, who didn’t have an answer. He did suggest we fill the traps with water, but that has not made a difference.
From researching online, we have some theories – we’d like to have the ductwork cleaned, but are discouraged after reading that other home owners have spent thousands cleaning their ductwork and without eliminating the odor. My husband has a few theories, including replacing the hot water tank. He’s also tempted to tear down a wall to see if there is a possible leak; however this would not explain why we do not smell the odor in the summer months.
We appreciate any feedback, or suggestions on what professional might be able to help find the source. Thank you!

[paul52446m]

Our home has a bad smell only in the winter months when the air conditioning is not running. We can smell it coming from several vents and in the basement where the heater and hot water tank are located. The smell is similar to rotten eggs, sewage and we had someone mention sulfur. We cannot seem to pinpoint what triggers the odor, except that we do NOT smell it when the A/C is on. It seems to come and go both when the heater is running and when it is turned off.
We moved in a couple of years ago and have smelled the odor both winters. We cleaned the de-humidifier (which was disgusting – we thought was the source) and have turned it off completely, but the odor still returned. We have an aerobic septic system which has been checked out; we’ve asked a plumber about the smell, who didn’t have an answer. He did suggest we fill the traps with water, but that has not made a difference.
From researching online, we have some theories – we’d like to have the ductwork cleaned, but are discouraged after reading that other home owners have spent thousands cleaning their ductwork and without eliminating the odor. My husband has a few theories, including replacing the hot water tank. He’s also tempted to tear down a wall to see if there is a possible leak; however this would not explain why we do not smell the odor in the summer months.
We appreciate any feedback, or suggestions on what professional might be able to help find the source. Thank you!

The sulfur smell is what you get when the rod in your water heater does bad.
When you get a glass of hot water do you smell it there? Do you have a humidifier on you furnace? Also you have drain pans on your air and when they dry up in the cold month you can get bad orders and mold. Paul

[rmw]

If you have a basement does it get damp in the winter a seasonal water issue could cause mold growth that would slow when weather drys out causing only seasonal issues.