As coffee junkie said, do you have a humidifier on your furnace? It usually mounts to the side of the duct chase. If you dont have one or do and it is not solving the problem I would check into an air exchanger, they do come in several differnt sizes depending on how many square footage you have to work with. They can get a bit spendy but in the end I think they are well worth it. They also help with children with alergies and the overall quality of the air. I use to install then several years ago on some of the more high end homes that had tons of square footage or had multiple furnaces.
Here is some info I pulled from a website I found about the air exchangers
"An air exchanger pulls stale air and excess humidity out of your home while allowing fresh, filtered air back into your home.
Some states now require an air exchanger in all new construction.
Enercept SIP houses are so air tight, we have always recommend the use of an Air to Air
exchange system in all of our building projects.
- An air exchanger will control moisture in your home and reduce the opportunity for mold growth.
- An air exchanger will remove stale odors from cooking and smoking, lingering fumes from cleaning products and paint, and organic emissions from carpets and furntiure.
- An air exchanger will remove air contaminants and allergens such as dust mites, pollen, and pet dander.
- And, in most cases, you will also be able to use a smaller more efficient heating system when you use an air exchanger."
This is the models I use to install-VENMAR, there are different brands this is just what I use to install.
Good luck, hope you find a good solution!