My Ducane gas furnace is 6 or 7 years old (I bought the house a year ago). We had no problems last winter, but now the jets only light about 3/4 of the time -- so when the blower comes on, it blows cold air. It has an electric pilot that works fine. The jets either don't come on or they blow out after a few seconds.
Whoever installed the furnace reduced the return air duct from 15 or so inches to 6 or 8 inches right before it enters the furnace. I don't know if this is part of the problem.
We've had a repairman come twice. He changed some valves and the filter, then he put in a new control board. I installed a new digital thermostat at his suggestion. We still have the problem! Can anyone help?