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Proper Temperature Split on Furnaces

The temperature rise is the temperature difference between the supply temperature and the return temperature. Take temperatures at the supply plenum and the return plenum.  We do not want the temperature rise on our furnaces to be above 60 degrees.


The airflow of a gas furnace can be calculated using the following formula:

CFM = (Input x Efficiency) / (1.08 x Temperature Rise)

The following tables are the product of this formula for an 80% and 90% efficient appliance: