Before choosing a hot air generator, it is necessary to calculate the thermal power needed to warm up a given environment, once this data is known, you can choose the right heat generator.

How do I calculate thermal power?

To calculate it we need three data:

  •  V: The volume of the room to be heated in m³
  • ΔT: The difference between the external temperature and the desired internal temperature
  • K: Dispersion coefficient

Calculating the volume is very simple, just multiply width, length and height:

Example: width 4 m, length 10 m, height 3 m = 120 m3

For the temperature difference just add the outside temperature to the desired temperature


  • + 5ºC outdoor temperature, + 20ºC internal temperature = 15ºC
  • -5ºC outdoor temperature + 20ºC internal temperature = 25ºC

The dispersion coefficient is a value based on the materials used in the construction:

  • K = 0.6 -0.9 Well Insulated Construction: Double Walls, Ceiling Insulation Material, Wall and Floor, Double glazed windows and Insulated Doors
  • K = 1,0-1,9 Discreetly constructed construction: double walls, ceiling insulating material, few windows with single glazing
  • K = 2.0-2.9 Little isolated construction: simple walls with glazed parts and uninsulated roofs
  • K = 3,0-4,0 Non-insulated construction: wood, sheet or plastic cover

Example of thermal power calculation

The formula for calculating the required heating power is:

V x ΔT x K = [kcal / h]

Suppose we want to heat an uninsulated structure of 20 m x 6 m x 4 m and the outside temperature is -10 ° C and the desired temperature is + 24 ° C

The volume will be V = 20 m x 6 m x 4 m V = 480 m 3

The temperature difference will be ΔT = -10 ° C T est., + 24 ° C T int. ΔT = + 34 ° C

The dispersion coefficient K = 4.0

480 m3 x 34 ° C x 4.0 K = 65280 kcal / h

Now we know that to heat this kind of room we need a hot air generator with a thermal power that reaches 65280 Kcal / h.

How to calculate in kw

If the technical data sheet of the equipment has the thermal power expressed in kW, it will be useful the equivalent of 1 kW = 860.61 kcal / h

Then in our case 65280 kcal / h = 75.92 Kw