This is a video of Greg Wayman conducting a hydro test on a Lennox G11 natural gas forced air furnace. There is no audio, but what you are seeing is the exterior of the heat exchanger being sprayed with a soapy water solution and the test being recorded by our high-tech furnace camera from the inside of the heat exchanger at the cracks. We're 100% certain this furnace has a cracked heat exchanger.

As the burner flames heat up the metal of the heat exchanger and the flue gases flow through the inside of the exchanger up and out the flue pipe, the air from the cold air return is pushed by the blower past the heated metal of the exchanger on the outside. The air is warmed and blown throughout the home. If a crack develops in the metal of the heat exchanger, there is a serious threat of carbon monoxide and other flue gases leaking into the supply air and being blown throughout the home as well.