In this short podcast episode, Bryan responds to a comment we received about cracked heat exchangers and how they were characterized in a recent combustion safety podcast episode.

In reality, heat exchanger cracks counter the effects of the inducer motor. The inducer pulls air through the heat exchanger, but a crack causes the inducer to pull the air from elsewhere, especially once the blower motor comes on. That is a cause of flame rollout and a higher CO measurement.

Induced draft (80%) furnace venting is under negative pressure near the equipment (WRT the air in the room around it). The inducer fan pulls the air through the single heat exchanger, but air isn’t “pushed” out of the flue. That’s because the draft in the flue is greater than the inducer itself.

80% furnaces are also open-combustion units that pull combustion air in from the air around the furnace, and they require access to fresh air. When you have gaps and cracks in the ductwork, the blower comes on and may cause the combustion analysis readings to change briefly. In a 90%+ sealed combustion furnace, the change is very minimal and doesn’t last long; if the numbers vary a lot and are sustained, then that’s an indicator of a cracked heat exchanger.

In any case, if you suspect a cracked heat exchanger and want to quote it, you must be able to find it. Proper combustion analysis and combustion analyzer use and care also matter for accurate cracked heat exchanger diagnosis.

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