26 year old two storey with a four foot heated crawl space, unvented. The crawl space is completely dry with no signs of moisture, however, every time we turn on our furnace we get a smell of stale air, it is not a smell of mold or mildew. We have also had our heat ducts and cold air returns professionally cleaned should the cause be related to the un-vented crawl space. My thinking is that there is no fresh air exchange and the air is dead. When the furnace is not on, there is no smell. Also, should heated crawl spaces be insulated as ours is not?
This is a very common concern with homeowners that do not have a properly vented crawlspace. Consider having a Humidex Crawlspace Ventilation device installed to vent out the alkaline odours. Call Humidex @ 1-800-416-9111 to find out more.