How to use the hvac airflow heat calculator
- Choose heat from the airflow or airflow for a heat load.
- For heat, enter the airflow in CFM and the temperature change of the air in degrees Fahrenheit, and add the moisture change in grains per pound if you want the latent heat.
- For airflow, enter the sensible heat to move in Btu per hour and the temperature change.
- Read the sensible, latent and total heat, or the CFM, along with tons and kilowatts.
Formula
Sensible heat (Btu/h) = 1.08 x CFM x temperature change in F
Latent heat (Btu/h) = 0.68 x CFM x moisture change in grains per pound of dry air
Total heat = sensible + latent; tons = Btu/h / 12,000; kW = Btu/h x 0.29307107 / 1,000
CFM for a load = sensible heat / (1.08 x temperature change)
The factor 1.08 is 60 minutes x 0.075 lb per cubic foot x 0.24 Btu per lb per F for standard air. The latent factor 0.68 is the usual rounded value for the heat of vaporization of water.
1,000 CFM and a 20 degree change
1,000 CFM of standard air changing by 20 F carries 1.08 x 1,000 x 20 = 21,600 Btu/h, which is 1.8 tons or 6.3303 kW. If the air also loses 15 grains of moisture per pound, the latent heat is 0.68 x 1,000 x 15 = 10,200 Btu/h and the total is 31,800 Btu/h, or 2.65 tons and 9.3197 kW.
Going the other way, moving 36,000 Btu/h of sensible heat with an 18 F change needs 36,000 / (1.08 x 18) = 1,851.85 CFM, and the load is 3 tons.
Sensible and latent heat
Sensible heat changes the temperature of the air, and latent heat is the energy that moves when water vapor condenses or evaporates. A cooling coil removes both. The moisture change is the difference in grains of water per pound of dry air, which you can read from a psychrometric chart. 7,000 grains make a pound.
For the airflow a room needs for air changes, use the CFM calculator, and for a quick room estimate in Btu see the BTU calculator. The cooling efficiency converter turns tons and Btu per hour into other units.
When the constants change
The factors assume standard air at sea level and about 70 F. At higher altitude or temperature, the air is less dense, so each CFM carries less heat and the constants should be lower. Duct leakage, fan heat and coil conditions also change the real numbers, so treat the page as an estimate and not a system design.
Assumptions and limits
- The air is standard air with a density of 0.075 pounds per cubic foot and a specific heat of 0.24 Btu per pound per degree F.
- The temperature change is the difference between entering and leaving air, in degrees Fahrenheit.
- The moisture change is in grains of water per pound of dry air. If you leave it blank only the sensible heat is counted.
- One ton of cooling is 12,000 Btu per hour.
Frequently asked questions
How do I convert CFM to BTU per hour?
Multiply 1.08 by the CFM and by the temperature change in degrees F. 1,000 CFM with a 20 F change is 21,600 Btu/h.
Where does 1.08 come from?
It is 60 minutes per hour times 0.075 pounds of air per cubic foot times 0.24 Btu per pound per degree F. It holds for standard air.
What is the latent heat factor 0.68?
It is the usual rounded factor for the heat given up when water vapor condenses, per CFM per grain of moisture per pound of dry air.
How many CFM do I need for a heat load?
Divide the sensible heat in Btu/h by 1.08 and by the temperature change. 36,000 Btu/h with an 18 F change needs about 1,852 CFM.