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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