Chemicals — A chemical smell can indicate leaking refrigerant. Refrigerant has a specific odor that’s been described as sweet, similar to chloroform. It may be accompanied by other signs like low air conditioner output, ice on the outdoor unit’s parts and a hissing or bubbling noise. Refrigerant leaks should be handled by an HVAC professional, because in addition to fixing refrigerant lines, the refrigerant will need to be recharged.

Rotten eggs — A rotten egg smell is given off by the sulfur additive that’s put into natural gas, which is used to signal a gas leak.

The good news is, it’s unlikely that the problem is with your HVAC system. The bad news is, it’s very likely that there’s a gas leak near your HVAC system’s air handler. If you notice this smell, quickly open the windows in your home, shut off the gas supply, get out of the house and call the gas company.

Mustiness — If the air coming from the air ducts smells like dirty socks or a locker room, and is perhaps accompanied by a moldy smell, this usually means mold and bacteria have built up in the air ducts, most likely when the air conditioner wasn’t being used. In this case, you’ll need to have your air ducts cleaned by an HVAC contractor. To prevent this from happening, make sure that proper heat pump inspection and shutdown procedures are followed whenever turning your heater off for the season. Moisture will enable mold and bacteria to grow in the air ducts unnoticed.

Burning — A burning smell usually indicates electrical trouble, such as a broken capacitor, poorly-lubricated motor or broken fan belt. A gunpowder-like smell usually indicates a shorted-out circuit board or fan motor. Turn off your HVAC system from the panel and call your HVAC technician if you smell this. Leaving the system running can increase the risk of a house fire.