Heat Gain Formula

Heat is a form of energy and the power is the rate at which energy is used. The rate at which heat energy is utilized is measured in watts which can also be called as power loss. Heat gain via walls and roofs is affected by solar energy which is absorbed in the outer surface represented by the air temperature. It is the outdoor temperature which would generate the same heat gain through the element without solar energy since it exists with the actual air temperature.

The heat gain formula is given by,

q = 1.10 × cfm × (to – ti)


  • q = sensible heat gain from the outside air in Btu/h
  • cfm = flow rate of outside air entering the building
  • to= outside air temperature in F
  • ti = inside air temperature in F

Example 1

Calculate the sensible heat gain for a ventilation flow rate of 9,000 cfm if the outside air temperature (to) is 88F and the inside air temperature (ti) is 80F.


Given data:

cfm = 9000

to = 88

ti = 80

Substitute in the given formula, we get

q = 1.10 × cfm × (toti)

q = 1.10 × 9000 × (88-80)

q = 79200

Therefore, the heat gain is 79200.

Example 2

Calculate the total heat gain due to the ventilation for 5000 cfm of air when the enthalpy of outside air is 35 and the enthalpy of the inside air is 30.0.


Given data:

cfm = 5000

to = 35

ti = 30

Substitute in the corresponding formula,

q = 1.10 × cfm × (toti)

q = 1.10 × 5000 × (35 – 30)

q = 27500

Therefore, the heat gain is 27500.


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