When the temperature outside is chilly, feeling the warm air from your furnace can be quite wonderful. Unfortunately, it's possible for the air that your furnace produces to have a bad odor. This may make you concerned about a possible gas leak or that your furnace is now a fire hazard. You'll want to fix the problem immediately to alleviate any concerns, which is why it's important to know what could be causing that foul odor.
When your furnace is not being used during the summer, it can gather dust inside the unit where the heating element is. When the heating element gets hot after being off for a while, you may detect a burning smell coming from your vents. The odor will typically go away on its own in about 45 minutes as the furnace burns off any dust that has collected on the heating element. You should only be concerned if the odor is consistent beyond that point.
If you do not have central air conditioning, dust can even build up in the ductwork during the summer. This can cause the air to have a musty odor to it, but it is nothing that is harmful. After years of dust build up, you may need to get professional duct cleaning performed.
When you have gone a long time without changing the air filter on the furnace, it will act as a barrier that contains all kinds of dirt and debris that could have an odor to it. The air will force its way through the filter and take the odor with it. If the filter is packed, the dust will eventually find its way onto the furnace's heat exchange, which can cause that dust burning side effect again. Changing the air filter is a simple fix that will take care of this problem. Old humidifier filters need changing as well, which can form mold as they get old.
You should be very concerned if you smell an odor similar to rotten eggs. That is the clear sign of a gas leak. The smell is added to natural glass so that you can easily identify leaks in situations like these. You should leave your home and have an HVAC professional (such as one from http://www.bishopplumbing.com) come fix the problem for you. Doing this could save the lives of those living in your home from the fumes or a possible explosion.