If your furnace stops blowing hot air and you can feel cold air leaking out of it, you need to check that the pilot light has turned on. Before you check the pilot light, you need to shut off the furnace completely, including the natural gas supply feeding into the furnace. Most newer furnaces do not have a pilot light, and so you do not need to turn them off. Always follow the instructions on the furnace to light the pilot, since the procedure may change from furnace to furnace.
Your furnace may be blowing out cold air around it because of leaks from loose joints on the furnace. Leaks reduce the efficiency of the furnace, meaning it must work harder and use more gas to heat the house. To test that you are feeling leaks from the furnace, hold a burning incense stick or smoke pencil near different portions of the furnace. The smoke will blow around when you get near any leaks. Tighten any bolts or screws on the exterior of the furnace to help close any gaps. Apply foil tape as well to leaking seams, creating an airtight seal.
Cold Air Return
Furnaces do not only blow hot air into the house, which increases the temperature. Air return vents sit in the walls, ceilings or floors and actively draw cold air back to the furnace through ducts, just like the ducts the furnaces pushes hot air through. The furnace heats up this cold air, pushing it back into the house to increase the house’s temperature even more. You should not feel the cold air leaking out of the furnace, however, or the air return ductwork that connects to the furnace. Seal up any leaks by tightening the bolts and screws on the furnace and applying foil tape to leaking seams on the furnace as well as the ducts.
A furnace may also cause cold air to leak into the house through cracks in the basement. When the hot air produced by the furnace naturally rises to the higher levels of the house, this movement of air can pull the cold air through the basement cracks. Leaks lower the temperature of the house, starting at the basement, which in turn can cause you to turn up the thermostat. Stopping the chimney effect takes you sealing any cracks in the basement with silicone caulk or expanding insulation.