Hvac School - For Techs, By Techs

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