There could be several reasons why you have a burning smell coming from your heat vents. The answer could be something as simple as burning dust to a faulty heater component. I will discuss these in depth below.
The most common reason you might experience a burning smell when you first turn on your heat for the season is that dust has settled on your furnace heat exchanger. The heat exchanger of your furnace gets extremely hot when it operates and this heat will “burn” any dust that has accumulated on the parts over the summer months. This is a good indicator of the buildup of dust that is occurring inside the system. The dust in the heat exchanger will burn off but in order to remove the dirty buildup inside the ducts you must have them vacuumed out by a professional air duct cleaner. Be sure to call a reputable company that is certified by the NADCA (National Air Duct Cleaners Association) which sets industry standards and cleaning procedure guidelines.
Though the most common reason is dust in the duct work, there are several other reasons why you might notice a burning smell when your heat comes on. Be sure to check the top of the heat exchanger for debris that may have fallen into the duct work during construction or at another time when your registers might have been off: these items could be smoldering and cause an odor. Also, if you have a heat pump, check the electric heat kit for a wiring short, debris that may have fallen on top of it, or other problems. When electrical shorts occur in your heat pump or furnace, the wiring can melt the insulators or wires. This has a smoky smell to it and may signify a problem. Be sure to call a qualified technician to check your system.
The blower section of your furnace collects dust over the summer while it is not in use. This should be cleaned and the meter should be oiled (if applicable to the particular unit) so it runs smoother, quieter and more efficiently. Additionally, make sure that the chimney of your furnace is free and clear; sometimes during the warmer summer months birds or rodents make nests in the chimney. Leaves or twigs might also make their way in and cause a blockage. It is important that this is cleared out by the homeowner or a professional chimney sweep for the furnace to function properly. While you are checking the other parts of your system, don’t forget to check your furnace filter. See if it is blocked. If the airflow is restricted enough it can cause resistance heaters, motors, and controls to overheat and possibly burn out.