We have an Inertherm furnace that has been working fine until it got real cold. Now it comes on and stays for less that 1/2 minute, while it's on it blows cold air. If you re-program the thermostat several times it will eventually come on and stay for longer period. Yesterday it finally started blowing warm air after about 3 hours. By the time the serviceman was able to get here it was staying on and was warm. HELP!

Naturally. On newer furnaces, the control board energizes the relay coil a timed delay after the gas comes on. If the blower doesn't come on, it shuts the gas off again. Sounds like a flaky blower relay. Do you have a fan on/auto switch? Usually the fan on uses a different relay and windings from auto/heat. If the furnace doesn't come on, turn the fan to on and see if it runs. If so, the problem is in the blower controls, or possibly the low speed windings of the motor. It is hard to check relays on a control board.

If the furnace still doesn't run on fan on, it could be a limit switch. They are easier to check, and cheaper to replace than the board.