A gas furnace actually works very simply: cold air is brought in from outside the home, run through a filter, and then heated as it is pushed over a steel burner. The warm air is then blown through the home's ductwork and vents and returned to the furnace; it is then reheated and circulated again, until the temperature in the room matches the desired temperature on the home's thermostat.
If your home's gas furnace is not working properly, note a few quick troubleshooting tips. This will tell you if you can fix the problem yourself or what you might expect by way of repairs.
Pilot light is out and won't return
The pilot light or pilot is a small flame that heats the steel burner that warms the air in the furnace. If your furnace's pilot light is controlled by a flow of propane to the pilot, and the flame refuses to reignite and stay lit, the valve that controls this flow of gas may be broken and the pilot is not getting any fuel. Some gas furnaces actually have an electric ignition to the pilot light so that it gets restarted automatically; if the ignition has gotten damaged or stopped working, it won't restart the pilot. Check the valve or the ignition switch for signs of damage and have either of these replaced as needed.
There's not enough heat
If the furnace blows air into the home but it doesn't feel warm enough, this often means the filter is dirty and needs replacing. If the filter is clean and new, the combustion air chamber may need cleaning—this is the chamber that circulates air around the heating coil or burner. If this chamber is very dirty, the metal coil won't get hot enough to heat the air around it. A technician can clean this chamber and ensure the metal burner is working properly as well.
Furnace cycles on and off too frequently
If the furnace comes on and blows warm air but then turns off too quickly, you might need a new thermostat. If the thermostat is not sending the correct temperature reading to the furnace, the unit will shut down while rooms are still cold.
You can also check the belt inside the furnace; this belt connects the motor and blower that push air through the furnace and then into your home. If this belt is loose or frayed, it won't provide enough tension to keep the motor and blower working, and the unit shuts down. You can usually see this belt right inside the front cover of the furnace and check it yourself; have it replaced if it doesn't seem tight enough or looks worn.