There are some scents that are always welcome in the house—like the smell of cinnamon sticks, cookies being baked, or fresh peppermint. There are other scents, like those of rotten eggs and burning plastic, that you definitely don't want in your home. What if one of those nasty odors is coming from your furnace? Your actions should depend on the actual odor you notice.
If you smell rotten eggs coming from the furnace, this means you probably have a gas leak. The gas companies scent the natural gas with sulfur to ensure gas leaks do not go unnoticed. Gas leaks are really dangerous and can lead to explosions. For your own safety, leave your house, then call the gas company. They can come by to turn the gas off and make sure your home is safe. You might need to have the HVAC (heating, ventilation, and air conditioning) contractor come to inspect the heating system and make some repairs before you turn the heat back on.
A slight smoky odor could just mean that there is some dust on the furnace burner and that it is burning when the heat turns on. You might notice this odor when you turn the furnace on for the first time in the fall. It's no reason for concern as long as it dissipates quickly and you don't actually see any smoke.
If it's a stronger smoky odor you notice, or if you actually see smoke, you should leave the home and call your fire department. If possible, turn the heat off at the thermostat before you do. The furnace might be overheating and burning something near it. Even if there ends up not being a fire, you want the fire department to check and ensure it's safe before you have your HVAC contractor head down to the basement to work on the furnace.
What if the smell reminds you of a strong cleaning solution or a preservative you might use in a biology lab? This could mean your heat exchanger is broken. A crack in the heat exchanger could lead to a carbon (CO) monoxide leak, so it's a big deal. Make sure your CO detector is working. If it has good batteries but is not ringing, it's safe to stay in the home until your HVAC contractor arrives to make repairs. But, if the CO alarm goes off, you need to find somewhere else to stay.
Don't ignore nasty smells coming from the furnace. They could be an early indicator of a bigger problem. Contact an HVAC contractor near you to schedule a furnace repair.