Heat Exchangers and Temperature Rise - Short #227 · HVAC School - For Techs, By Techs summary

Key takeaways

Temperature rise is a critical factor in furnace efficiency and should remain within the manufacturer's specified range.
Effective airflow management can reduce wasted heat and improve overall heating efficiency.
Cracked heat exchangers pose safety risks; regular inspections are necessary to catch these issues promptly.
Addressing airflow problems through systematic checks can lead to better system performance and longevity.

Summary

Understanding Temperature Rise

Temperature rise refers to the difference between the return air temperature and the supply temperature in heating systems. It's crucial to monitor this parameter in gas furnaces, particularly 80% open combustion types. The range for high-efficiency furnaces is typically lower (30-60°F) compared to mid-efficiency (40-70°F). Bryan emphasizes that maintaining a temperature rise within the specified range ensures peak efficiency, proper airflow, and protects the heat exchanger, crucial for avoiding potential overheating and damage.

Impact of Airflow and Efficiency

Higher temperatures in airflow can lead to wasted heat, as more heat escapes through ducts and surfaces. Conversely, lower temperature returns mean better heat transfer into the living space. Bryan explains the balance needed to avoid condensing in standard furnaces, which can lead to rusting and other issues. Increasing heating airflow can help correct high temperature rises, but it's essential to identify root causes such as restrictive filters or ductwork that might contribute to airflow issues.

Identifying Cracked Heat Exchangers

Bryan discusses the significant risks posed by cracked heat exchangers, particularly in colder markets with fluctuating temperatures. He highlights the causes of these cracks, including improper airflow leading to overheating. Regular visual inspections and awareness of the furnace's operational signs can catch these issues early. Listeners are encouraged to approach any cracks found ethically and responsibly, addressing underlying airflow problems to prevent future occurrences.

Airflow Problems and Solutions

Airflow is a central theme in maintaining furnace efficiency. Problems with airflow may manifest as high temperature rise and are often accompanied by low cooling airflow. The episode advocates for comprehensive checks on duct sizes, filters, and blower wheels to ensure adequate airflow. Listening for abnormal flame movements or inconsistent burn patterns can also indicate issues needing attention to avoid significant heating problems.

Show notes

In this short podcast episode, Bryan talks about heat exchangers and temperature rise in gas furnaces, especially 80% open-combustion gas furnaces.

Temperature rise is the difference between the return air temperature and supply air temperature; in cooling, we usually refer to this difference as a delta T or temperature split. The manufacturer sets a target range on the data plate; high-efficiency furnaces tend to have lower temperature rise targets than mid-efficiency or older furnaces. We want to heat the space and not waste heat on surfaces (like duct surfaces). For that reason, the goal is to stay in the middle of the temperature rise range or slightly on the lower side for peak efficiency, better heat transfer, proper airflow, proper venting, and protection of the heat exchanger.

In mid-efficiency furnaces, we don't want the temperature rise to be too low, as cooler gases can condense. High-efficiency condensing furnaces have a dedicated condensate system, but 80% furnaces do not. That condensation can corrode the heat exchanger, which makes it vulnerable to cracks. Cracked heat exchangers may also happen when the temperature rise is too high, and they are dangerous due to carbon monoxide (CO) leaks; check for abnormal flame movement and color. Lower-temperature air is also uncomfortable if the registers are in locations where blowing air on people is inevitable.

We can reduce the temperature rise by increasing the heating airflow, which can sometimes be done by re-tapping constant-torque and PSC motors or changing ECM settings. However, restrictive filters and ductwork may also cause that problem, and they need to be addressed.

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Heat Exchangers and Temperature Rise - Short #227