Our gas furnace recently stopped working. One day it was fine, the next it doesn't work. A little background info: Thermostat is a Maple Chase, Furnace Door says Rheem Criterion Installed 1992. I have no manuals. I did change the filter but that did nothing. If I turn the fan to On, it blows very well but only cold air. I have noticed the pilot light seems to be out. If I flip the breaker, the furnace will come on for about 5 minutes and blow cold air then shut off. Gas water heater works fine. I am a complete novice at home repair but would like to avoid the cost of calling a pro if I can. So if anyone can help me figure this out, it would be wonderful. We're surviving with space heaters that take all day to heat the house and have doubled our heating bill.

Try cleaning the flame senser. Also clean any drain hoses and ports. Let me know how you make out.

If your pilot light is out try relighting it. Read the instructions on the furnace door.If you manage to light the pilot and it doesn't stay on you probably need a thermocoupler. A thermocoupler is rather easy to install. A thermocoupler senses the heat that is applied to it from the pilot. When the thermostat calls for heat it opens the gas valve and let gas flow to burners where it is ignited. Course there are several things that may cause the furnace not to light, but the most obviuos in your case would be to try and get the pilot lit, once pilot is lit and furnace doesn't lit, there is probably another problem