| The metal heat exchanger of a furnace, expanding and contracting hundreds of times each year as the furnace cycles “on” and “off”, will eventually produce hairline cracks.
Combustion (gas or oil) takes place inside a sealed heat exchanger. The only outlet for the products of combustion is the chimney.
Air to be heated, is forced around the exterior of the heat exchanger (by the blower) where it absorbs heat from the hot surface of the exchanger and delivered into the house.
Fine hairline cracks in the exchanger permits some of the products of combustion (including carbon monoxide) to be absorbed into the air stream and enter the house.
As AGA (American Gas Association) considers only 15 parts per million to be hazardous, the furnace should be immediately turned off.
Click here to see detailed cutaway of furnace and air flow.
Click here to see animated version of furnace and air flow.
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