What is transmission load?

Short Answer:

Transmission load is the amount of heat that enters or leaves a building through its walls, roof, floor, doors, and windows because of the temperature difference between inside and outside air. It depends on the material and thickness of building parts, outdoor conditions, and insulation quality.

In simple words, transmission load represents the heat flow caused by conduction through building surfaces. This load is an important part of the total cooling or heating requirement of a building, as it determines how much energy is needed to maintain comfortable indoor conditions.

Detailed Explanation :

Transmission Load

Transmission load is the heat transfer that occurs through the structural parts of a building such as walls, roofs, ceilings, floors, doors, and windows due to the temperature difference between the outdoor and indoor environments. It is one of the main components of the total heat load in air-conditioning and refrigeration systems. This type of heat gain or loss takes place primarily by conduction and partly by radiation and convection depending on the materials and environmental conditions.

In any enclosed space, the inside temperature is usually maintained at a comfortable level by the cooling or heating system. However, the outside temperature constantly changes during the day and night. When there is a temperature difference between inside and outside, heat tends to flow from the hotter side to the cooler side. This continuous flow of heat through the building structure is called transmission heat load.

For example, in summer, heat flows from the hot outdoor air into the cool indoor space through walls and roofs, increasing the cooling requirement. In winter, heat flows in the opposite direction, from warm indoor air to the colder outside air, increasing the heating load.

The amount of transmission load depends on several factors:

  1. Temperature Difference: Greater the temperature difference between the outside and inside air, higher will be the rate of heat transfer.
  2. Surface Area: Larger wall, roof, or window areas allow more heat to pass through.
  3. Material of Construction: Different materials have different thermal conductivities. Materials like brick, glass, or metal conduct heat faster than insulated materials.
  4. Thickness of Material: Thicker walls or roofs reduce the heat transfer rate.
  5. Insulation Quality: Good insulation materials such as glass wool, foam, or thermocol can significantly reduce transmission load.
  6. Exposure to Sunlight: Surfaces directly exposed to sunlight absorb more heat and increase the load.

Transmission load can be mathematically expressed using the heat transfer equation:
Q = U × A × ΔT
where,
Q = Rate of heat transfer (W or kJ/hr)
U = Overall heat transfer coefficient (W/m²·K)
A = Area of surface (m²)
ΔT = Temperature difference between outside and inside air (K or °C)

This equation shows that the transmission load increases with higher surface area, higher U-value, or greater temperature difference.

Examples of Transmission Load:

  • In an air-conditioned room with glass windows facing the sun, the sunlight and high outdoor temperature cause significant heat transfer through the glass, raising the indoor temperature.
  • In cold climates, if a room has poor wall insulation, heat from the room escapes outside, forcing the heater to work longer to maintain comfort.

To minimize transmission load, engineers and architects use several methods:

  1. Thermal Insulation: Adding insulation layers to walls, roofs, and ceilings reduces heat transfer.
  2. Reflective Surfaces: Using light-colored or reflective paints and roofing materials reduces solar heat absorption.
  3. Double Glazing Windows: Installing double-glazed or low-emissivity glass reduces heat flow through windows.
  4. Proper Wall Construction: Using materials with low thermal conductivity helps minimize conduction.
  5. Air Barriers and Vapor Barriers: These prevent unwanted air movement and moisture, which can affect heat transfer.

In modern building design, controlling transmission load is essential for energy efficiency. High transmission load leads to greater use of cooling or heating equipment, which increases electricity consumption and operational cost. Reducing transmission load helps maintain stable indoor temperatures and saves energy.

Practical Importance:

Transmission load is a key consideration in HVAC design. Engineers must calculate it accurately to select the proper capacity of air conditioners or heaters. Overestimating the load results in oversized equipment, which wastes energy, while underestimating it causes poor comfort and system strain.

In industries such as cold storage or refrigeration plants, controlling transmission load is even more critical. Walls and doors of cold rooms are insulated to minimize heat gain from the external environment. Similarly, in buildings located in tropical areas, proper insulation and ventilation are used to control excessive heat transmission.

Example Calculation:

If a wall area of 20 m² separates indoor air at 25°C from outdoor air at 40°C, with an overall heat transfer coefficient of 2.5 W/m²·K,
Then,
Q = U × A × ΔT = 2.5 × 20 × (40 – 25) = 2.5 × 20 × 15 = 750 W
This means 750 watts of heat enters the room through that wall, which must be removed by the cooling system to maintain temperature.

Conclusion:

Transmission load is the heat flow through the building structure caused by the temperature difference between indoors and outdoors. It directly affects the efficiency and size of cooling and heating systems. By using proper insulation, reflective materials, and efficient construction methods, the transmission load can be minimized, leading to energy savings, improved comfort, and better performance of HVAC systems.