I have a Carrier gas-pack Heating/Airconditioning system (about 12 years old). A few weeks ago I noticed the heating cycle was not working. I ran some checks to determine the cause and discovered that the main circulation fan was never coming on when the fan was set to the "Auto" position. The gas burners would come on, but since the main fan never came on the high temp switch would eventually shut the burners down and I get no heat. However, if I turn the fan to the "On" position (on all the time) the burners do come on and stay on until the preset temperature is reached. As long as I leave the fan in the constant "On" position the heating cycle appears to work normally. But it (the fan) won't work in the auto mode. Anyone seen this problem before?

Most furnaces have a delay built in so that the fan doesn't come on until the air is warm. The timer could be bad, or the relay it controls. Unfortunately when I had the same problem 2 years ago, the relay was part of a $300 board.