Calculate air temperature rise from CFM and Btu/h, or find the CFM needed for a heat load and the Btu/h from airflow, using standard air you can adjust.

Air properties and entering temperature

Prefilled with 0.075 lb/ftยณ, standard air near sea level and 70 ยฐF. Lower it for hot air or high altitude.

Prefilled with 0.24 Btu/(lbยทยฐF), the usual value for dry air.

CFM Temperature Rise Formula

For approximate standard dry air, temperature rise equals sensible heat delivered to the air (Btu/h) divided by 1.08 times airflow (CFM). With adjusted properties, replace 1.08 with 60 ร— density (lb/ftยณ) ร— specific heat (Btu/(lbยทยฐF)).

TR = Q / (1.08 ร— CFM)

Variables:

  • TR is the temperature rise in degrees Fahrenheit (ยฐF)
  • CFM is the cubic feet per minute
  • Q is the sensible heat rate delivered to the air in Btu/h
  • 1.08 = 60 ร— 0.075 ร— 0.24, using approximate dry-air density and specific heat

To calculate temperature rise, divide the sensible heat rate in Btu/h by 1.08 times CFM. Use output heat delivered to the air, not unadjusted furnace fuel input or BTU energy. This sensible-heat model excludes humidity changes and latent cooling.

What is CFM Temperature Rise?

Air temperature rise is the increase between entering and leaving air during sensible heating. It depends on heat delivered to the air, airflow, density and specific heat. The 1.08 shortcut assumes approximately 0.075 lb/ftยณ density and 0.24 Btu/(lbยทยฐF) specific heat. Adjust these properties for the actual air conditions and flow-volume basis. Temperature rise alone does not establish system efficiency or equipment suitability.

How to Calculate CFM Temperature Rise?

The following steps outline how to calculate the CFM Temperature Rise.

  1. First, determine the cubic feet per minute (CFM) of the air.
  2. Next, determine the sensible heat rate delivered to the air in Btu/h.
  3. Check the air density and specific heat; use 1.08 only for its approximate standard-air assumptions.
  4. Finally, calculate TR = Q / (1.08 ร— CFM), or use 60 ร— density ร— specific heat in place of 1.08.
  5. After inserting the values and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

CFM = 500

Sensible heat delivered to air = 10,000 Btu/h

At the standard-air assumption: TR = 10,000 / (1.08 ร— 500) = 18.5185ยฐF rise (10.2881ยฐC difference).