Indoor Thermal Comfort Calculations: Air and Water Heating

Indoor thermal comfort is a crucial aspect of creating pleasant and productive living and working environments. Achieving optimal thermal comfort is essential for the physical well-being and mental satisfaction of building occupants. To accomplish this, engineers and designers rely on sophisticated calculations and mathematical formulas to determine the heating requirements for air and water heating systems. In this comprehensive 3000-word article, we will explore the principles behind indoor thermal comfort, delve into the mathematics involved in heat loss and heat gain calculations, examine the heating load calculation process, and explore the formulas used for both air and water heating systems.

1. Understanding Indoor Thermal Comfort

Indoor thermal comfort is a measure of the occupants' satisfaction with the thermal environment inside a building. It involves achieving a balance between the heat generated within the space, the heat loss through the building envelope, and the heat gain from external sources. Several factors influence thermal comfort, including air temperature, humidity, air movement, and radiant temperature. The main goal is to maintain a stable and comfortable indoor temperature for the occupants throughout the year.

2. Heat Loss and Heat Gain Calculations

The first step in understanding indoor thermal comfort is to analyze the heat loss and heat gain mechanisms within a building. Heat loss occurs when heat escapes through the building envelope, which includes walls, windows, roofs, doors, and floors. On the other hand, heat gain results from external factors such as solar radiation, internal heat sources (occupants, lighting, equipment), and air infiltration.

a. Conductive Heat Transfer

Conductive heat transfer occurs through solid materials, such as walls and roofs. The rate of heat conduction is influenced by the material's thermal conductivity, thickness, and the temperature difference between the indoor and outdoor environments. The formula for conductive heat transfer is:

Q = (k * A * ΔT) / d


  • Q = Heat transfer rate (W)
  • k = Thermal conductivity of the material (W/m·K)
  • A = Surface area (m²)
  • ΔT = Temperature difference between indoor and outdoor (K)
  • d = Thickness of the material (m)

b. Convective Heat Transfer

Convective heat transfer involves the movement of air, either through natural convection (due to density differences) or forced convection (with the aid of fans or blowers). The rate of convective heat transfer depends on the convection coefficient, surface area, and temperature difference. The formula for convective heat transfer is:

Q = h * A * ΔT


  • Q = Heat transfer rate (W)
  • h = Convective heat transfer coefficient (W/m²·K)
  • A = Surface area (m²)
  • ΔT = Temperature difference between indoor and outdoor (K)

c. Radiative Heat Transfer

Radiative heat transfer occurs when heat is transferred between two surfaces without direct contact, through electromagnetic waves. The rate of radiative heat transfer depends on the emissivity of the surfaces and the temperature difference. The formula for radiative heat transfer is:

Q = ε * σ * A * (T₁^4 - T₂^4)


  • Q = Heat transfer rate (W)
  • ε = Emissivity of the surface
  • σ = Stefan-Boltzmann constant (5.67 × 10^-8 W/m²·K^4)
  • A = Surface area (m²)
  • T₁ = Temperature of the warmer surface (K)
  • T₂ = Temperature of the cooler surface (K)

3. Heating Load Calculation

The heating load calculation is crucial for determining the capacity of the heating system required to maintain the desired indoor temperature. It involves analyzing the heat loss and heat gain factors, taking into account the worst-case scenario during the coldest outdoor conditions. The formula for heating load calculation is:

Q_total = Q_heat_loss + Q_heat_gain


  • Q_total = Total heating load (W)
  • Q_heat_loss = Heat loss (W)
  • Q_heat_gain = Heat gain (W)

4. Sensible Heat and Latent Heat

In indoor thermal comfort, both sensible heat and latent heat are significant factors. Sensible heat refers to the heat that causes a change in temperature, while latent heat is associated with heat absorbed or released during phase changes (e.g., evaporation or condensation).

a. Sensible Heat Calculation

The formula for calculating sensible heat is:

Q_sensible = m * c * ΔT


  • Q_sensible = Sensible heat (W)
  • m = Mass flow rate of the air (kg/s)
  • c = Specific heat capacity of the air (J/kg·K)
  • ΔT = Temperature difference between supply air and return air (K)

b. Latent Heat Calculation

The formula for calculating latent heat is:

Q_latent = m * h_fg


  • Q_latent = Latent heat (W)
  • m = Mass flow rate of the moisture (kg/s)
  • h_fg = Latent heat of vaporization (J/kg)

5. Air Heating Systems

Air heating systems are widely used for indoor thermal comfort. Two common types of air heating systems are:

a. Forced Air Heating

Forced air heating systems use a furnace or heat pump to heat the air, which is then forced through ducts and distributed to various rooms through vents or registers. The heating capacity of the system is determined based on the heat loss calculations and the desired indoor temperature.

b. Radiant Heating

Radiant heating systems heat objects and surfaces in a room, which then radiate heat to the surrounding air and occupants. The heating capacity of radiant heating systems is calculated based on the surface area to be heated, the desired indoor temperature, and the thermal characteristics of the heating elements.

6. Water Heating Systems

Water heating systems are also prevalent for providing indoor thermal comfort, especially in larger buildings and industrial settings. Two common types of water heating systems are:

a. Hydronic Radiant Heating

Hydronic radiant heating systems involve circulating hot water through pipes or tubing installed in floors, walls, or ceilings. The heat from the water radiates through the surfaces, providing even and gentle warmth throughout the space. The heating capacity of hydronic radiant heating systems is determined based on the flow rate of the water, the temperature difference between the supply and return water, and the surface area of the heating elements.

b. Steam Heating

Steam heating systems utilize steam generated by boilers to provide warmth. The heating capacity of steam heating systems is calculated based on the boiler's capacity, the steam distribution network, and the condensate return system.


Indoor thermal comfort calculations are a fundamental aspect of designing and maintaining a comfortable and healthy indoor environment. By understanding the principles and using the appropriate formulas for heat loss, heat gain, sensible heat, and latent heat calculations, engineers and designers can select the optimal air or water heating system to achieve optimal thermal comfort for building occupants. Whether it's a forced air system providing warmth during the cold winter nights or a hydronic radiant heating system ensuring a comfortable workspace, these calculations play a vital role in creating inviting and productive indoor spaces. As technology continues to evolve, we can expect further advancements in indoor thermal comfort solutions, promoting energy efficiency and sustainability while enhancing the overall well-being of occupants.