Heat Exchanger

How a furnace heat exchanger works

Inside a gas or oil furnace, fuel burns within a set of sealed metal chambers or tubes — the heat exchanger. Hot combustion gases travel through the inside of these passages on their way to the flue, while the blower pushes household air across the outside. The metal walls get hot and transfer that heat to the passing air, which then flows into the supply ducts. The critical job of the heat exchanger is separation: combustion byproducts, including carbon monoxide, stay inside the metal and exit through the vent, never mixing with the air you breathe.

Over years of heating and cooling cycles, the metal expands and contracts thousands of times. Eventually it can fatigue, crack, or rust through — especially if the furnace was oversized, the airflow was restricted by dirty filters, or condensation corroded the metal.

What a cracked heat exchanger means for your home

A compromised heat exchanger is one of the few furnace problems that is a genuine safety issue, because it can allow carbon monoxide into the ductwork. Warning signs include soot streaks, a yellow or wavering burner flame, unusual odors when the heat runs, or a carbon monoxide detector alarm. In the older housing stock common around Philadelphia, furnaces often serve well past their design life, so annual heating inspections that include a heat exchanger check are worthwhile. A cracked exchanger generally means replacing the exchanger or, in an older furnace, replacing the unit — a judgment call best made with a technician's camera or combustion readings in hand. PJ MAC HVAC inspects heat exchangers as part of furnace repair and maintenance visits.