How to calculate temperature rise
Temperature rise = supply air temperature − return air temperature
- Run the furnace in heating for 10–15 minutes so temperatures settle.
- Measure return air temperature before the furnace.
- Measure supply air temperature in the supply plenum, out of line of sight of the heat exchanger.
- Compare the rise with the range on the furnace nameplate (for example 35–65°F).
Worked example
Return air is 68°F and supply air is 118°F. The rise is 118 − 68 = 50°F, inside a 35–65°F nameplate range.
What a high or low rise means
High rise: not enough air is moving across the heat exchanger. Check the filter, closed registers, and return sizing, or the furnace may be overfired. A high rise can overheat and crack the heat exchanger.
Low rise: too much airflow or underfiring, often from low gas pressure.
Questions
Where do I find the temperature rise range?
It's printed on the furnace rating plate, usually inside the blower door, as a range such as 35–65°F or 30–60°F.