I just installed a new Nordyne SC Series 92.1% Uplow/Horizontal Condensing Furnace. The problem is that it seems the burner doesn't stay on long enough to heat the Heat Exchanger to a high enough temperature. When the gas turns off and the blower turns on, the air is only slightly warm. After about 5 seconds the air turns cool. The speed is set to M/L and the house is freezing. Could there be a defective part?