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Calculating Indoor Temperature and Humidity Loads

Calculating sensible and latent heat from persons, lights, electric equipment, machines, evaporation from water surfaces, polluting fluids and miscellaneous loads

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Indoor climate is influenced by

  • sensible and latent heat from persons, lights, machines and electrical equipment and industrial processes
  • pollution and gases from persons, building materials, inventory and industrial processes

The most important sources influencing the indoor climate may be summarized to

  1. sensible and latent heat from persons
  2. sensible heat from lights
  3. sensible heat from electric equipment
  4. sensible heat from machines
  5. latent heat from evaporation from water surfaces
  6. evaporation from polluting fluids
  7. miscellaneous loads

1. Sensible and latent heat from persons

Sensible heat from persons are transferred through conduction, convection and radiation. Latent heat from persons are transferred through water vapor.

The sensible heat influence on the air temperature and latent heat influence on the moisture content of the air.

The heat transferred from persons depends on activity, clothing, air temperature and the number of persons in the building.

2. Sensible heat from lights

Heat transferred to the room from the lights can be calculated as

Hl = Pinst K1 K2                                        (1)


Hl = heat transferred from the lights (W)

Pinst = installed effect (W)

K1 = simultaneous coefficient

K2 = correction coefficient if lights are ventilated. (= 1 for no ventilation, = 0.3 - 0.6 if ventilated)

The table below can be used to estimate heat load from lights. (The manufacturers datasheets should be checked for details)

Installed effect (W)Illumination (lux)
Incandescent lamp 38 75 110 145 180
Fluorescent tubes 15 25 36 48 60

Normal illumination of rooms:

Office ActivityIllumination (lux)
Normal work 200
PC work 500
Archive 200
Drawing work, normal 500
Drawing work, detailed 1000

3. Sensible heat from electric equipment

Heat transferred from electrical equipment can be calculated as

Heq = Peq K1 K2                                           (2)


Heq = heat transferred from electrical equipment (W)

Peq = electrical power consumption (W)

K1 = load coefficient

K2 = running time coefficient

4. Sensible heat from machines

When machines runs, heat may be transferred to the room from the motor and/or the machine.

If the motor is in the room and the machine is on the outside - the heat transferred can be calculated as

Hm = Pm / hm - Pm                                          (3)


Hm = heat transferred from the machine to the room (W)

Pm = electrical motor power consumption (W)

hm = motor efficiency

If the motor is belt driven and the motor and belt is in the room and the machine is on the outside - the heat transferred can be calculated as

Hm = Pm / hm - Pm hb                                      (3b)


hb = belt efficiency

If the motor and the machine is in the room - the heat transferred can be calculated as

Hm = Pm / hm                                      (3c)

In this situation the total power is transferred as heat to the room.

Note! If the machine is a pump or a fan, most of the power is transferred as energy to the medium and may be transported out of the room.

If the motor is outside and the machine is in the room - the heat transferred can be calculated as

Hm = Pm                                          (3d)

If the motor is belt driven and the motor and belt is outside and the machine is in the room - the heat transferred can be calculated as

Hm = Pm hb                                            (3e)

5. Latent heat from evaporation from water surfaces

Evaporation from open vessels or similar can be calculated as

qm = A (x1 - x2 ) ae                                      (4)


qm = evaporated water (kg/s)

A = surface area (m2)

x1 = water content in saturated air at water surface temperature (kg/kg)

x2 = water content in the air (kg/kg)

ae = evaporation constant (kg/m2s)

The evaporation constant can be estimated

ae = (25 + 19v)/3600                                        (5)


v = air speed close to the water surface (m/s)

The temperature in the water surface will be lower than the temperature below the surface.

The temperature can be calculated as

t1 = t2 - (t2 - t3) / 8                                 (6)


t1 = temperature in water surface (oC)

t2 = temperature below the surface (oC)

t3 = wet bulb temperature in the air (oC)

The heat for evaporation can be calculated as

He = qm / (x1 - x2) (h1 - h2)                                      (7)


h1 = enthalpy in saturated air (J/kg)

h2 = enthalpy in air (J/kg)

6. Evaporation from polluting fluids

The flow of a polluting fluid can be calculated as

qf = 22.4 qe / M T / 273                                        (8)


qf = flow of the fluid (m3/s)

qe = evaporated fluid

M = molecule mass of the fluid at 0 oC and 101.3 Pa (kg/mole)

T = temperature (K)

7. Miscellaneous loads

Carbon dioxide - CO2

Carbon dioxide (CO2) concentration in "clean" air is 575 mg/m3.

Huge concentrations can cause headaches and the concentration should be below 9000 mg/m3.

Carbon dioxide are produced by persons during the combustion. The concentration of carbon dioxide in the air can be measured and used as an indicator of air quality.

ActivityRespiration per person
CO2 generation per person
Sleeping 0.3 0.013
Sitting, relaxed 0.5 0.02
Working, moderate 2 - 3 0.08 - 0.13
Working, heavy 7 - 8 0.33 - 0.38


Ammonia Sticking 0.5
Carbon disulphide Aromatic, little sticking 2.6
Chlorine Sticking 0.06
Chlorate phenol Medical 0.18
Ether Geranium 0.069
Prussic Acid Bitter almond 1
Hydrogen sulphide Rotten egg 0.26
Ozone Little sharp 0.096
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  • Engineering ToolBox, (2003). Calculating Indoor Temperature and Humidity Loads. [online] Available at: [Accessed Day Mo. Year].

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