HVAC School - For Techs, By Techs
Heat Exchanger Crack Comment - Short #280
· Summarized
Key takeaways
- Heat exchanger cracks can significantly influence the operation of the inducer motor and combustion dynamics.
- Understanding the differences between 80% and 90% seal combustion furnaces is crucial for accurate diagnostics.
- Proper combustion analysis is essential to diagnose heat exchanger issues accurately.
- Geographical differences can lead to variations in heat exchanger crack occurrences and HVAC challenges.
Summary
Discussion on Heat Exchanger Cracks
In this episode, Bryan addresses a listener's comment regarding heat exchanger cracks that were discussed in a previous podcast. He elaborates on how cracks can counter the effects of the inducer motor and affect air pulling dynamics within systems, especially when blower motors are activated.
Inducer vs. Draft Pressure in Furnaces
Bryan explains the operational differences between 80% open-combustion furnaces and 90% sealed combustion furnaces. He clarifies how air is pulled through the heat exchangers and how cracks can influence combustion air dynamics, potentially leading to issues like flame rollout and CO measurement spikes.
Combustion Analysis for Safety
The episode dives into the importance of proper combustion analysis when diagnosing potential heat exchanger issues. Bryan highlights how fluctuating readings during blower activation may indicate the presence of cracks and the need for thorough troubleshooting.
Regional Differences in HVAC Issues
Bryan mentions how geographical factors, particularly in Florida, affect the frequency and seriousness of heat exchanger cracks. He contrasts this with areas that may have older constructions and corresponding issues.
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Show notes
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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