Short Answer:

Infiltration load is the amount of heat or cooling energy required to condition the outdoor air that enters a building unintentionally through openings, cracks, or gaps. This outside air brings heat, humidity, or coldness depending on the weather conditions. It increases the total cooling or heating load of a building. In air conditioning systems, infiltration load must be considered carefully because it affects both temperature and humidity control.

The infiltration load depends on factors like wind pressure, building construction, number of openings, and air tightness. When hot and humid air enters a cooled room, it increases the cooling and dehumidification requirement. Hence, proper sealing and insulation help to minimize infiltration and maintain energy efficiency in buildings.

Detailed Explanation:

Infiltration Load

Infiltration load refers to the unwanted heat gain or heat loss that occurs due to the uncontrolled entry of outdoor air into an indoor space through leaks, cracks, or openings in the building structure. It is an essential component in the calculation of the total heating or cooling load of a building. In mechanical engineering and HVAC (Heating, Ventilation, and Air Conditioning) design, infiltration load is considered a significant factor because it directly influences energy consumption, comfort level, and indoor air quality.

Infiltration occurs naturally because of differences in air pressure between the inside and outside of the building. It can carry both sensible heat (temperature difference) and latent heat (moisture content), affecting the performance of air conditioning systems.

1. Meaning of Infiltration Load

The term “infiltration load” combines two effects: air infiltration and thermal load. Air infiltration is the unwanted entry of outside air into a conditioned space through cracks in walls, doors, windows, or ventilation ducts. The load refers to the amount of energy the HVAC system must use to maintain the desired indoor conditions after infiltration happens.

Infiltration brings in outdoor air that may be warmer, colder, or more humid than the indoor air. Therefore, the air conditioning system must remove or add heat and moisture to maintain a stable temperature and humidity. This extra energy requirement due to infiltration is the infiltration load.

2. Causes of Infiltration Load

Several factors contribute to infiltration in buildings and rooms. Some of the common causes are:

• Pressure Difference: Difference between indoor and outdoor air pressure due to wind or temperature difference.
• Poor Construction: Gaps around windows, doors, walls, and ceilings allow outdoor air to leak inside.
• Openings and Cracks: Air can pass through small cracks, joints, or around electrical and plumbing fixtures.
• Frequent Door Openings: In commercial buildings, doors opening frequently allow unconditioned air to enter.
• Ventilation and Exhaust Fans: Improperly designed exhaust systems may create negative pressure, pulling outdoor air inside.

All these causes make infiltration a common but often overlooked source of extra load on air conditioning systems.

3. Components of Infiltration Load

Infiltration load has two main components:

• Sensible Heat Load: It is caused by the temperature difference between the outdoor and indoor air. For example, when hot air enters a cooled room, it increases the sensible heat load.
• Latent Heat Load: It is caused by the moisture content difference between outdoor and indoor air. Humid outdoor air entering a dry, air-conditioned room adds moisture, increasing the latent heat load.

Thus, infiltration affects both temperature and humidity levels, and both must be managed by the HVAC system for proper comfort.

4. Calculation of Infiltration Load

The infiltration load can be calculated using the following equations for both sensible and latent heat loads:

• Sensible Heat Load (Qₛ):

where,
= air leakage rate in m³/s
= outdoor temperature (°C)
= indoor temperature (°C)

• Latent Heat Load (Qₗ):

where,
and  = humidity ratios of outdoor and indoor air respectively.

These formulas help determine how much energy is needed to remove heat and moisture brought in by infiltration.

5. Effects of Infiltration Load

Infiltration load affects several aspects of building performance and indoor comfort:

• Increased Energy Consumption: The HVAC system must work harder to maintain indoor conditions, leading to more energy use.
• Poor Humidity Control: Humid air entering increases latent load, making rooms feel sticky or uncomfortable.
• Temperature Instability: Hot or cold air entry causes uneven temperatures inside.
• Dust and Pollutants Entry: Outdoor air may carry dust, pollutants, or odors, reducing air quality.
• Reduced Equipment Efficiency: The system runs longer and more frequently, shortening its lifespan.

Because of these effects, minimizing infiltration is important for both comfort and energy savings.

6. Methods to Reduce Infiltration Load

Several techniques are used to minimize infiltration load in buildings and air conditioning systems:

• Proper Sealing: Close gaps around doors, windows, ducts, and joints using sealants or gaskets.
• Weather Stripping: Install rubber or foam strips around moving parts like doors and windows.
• Air Curtains: Use air curtains at entrances to reduce air exchange between indoors and outdoors.
• Double Door or Vestibule Design: Create a small buffer area between outside and inside doors.
• Positive Indoor Pressure: Maintain slightly higher pressure indoors to prevent outdoor air entry.
• Regular Maintenance: Check for leakage in ducts, vents, and filters to maintain efficiency.

These measures not only reduce infiltration but also help maintain better control over temperature and humidity.

7. Importance of Considering Infiltration Load

In air conditioning design, infiltration load must be accurately estimated because it affects both the size and efficiency of the system. If the infiltration load is underestimated, the system may not be able to maintain comfort levels. If it is overestimated, the system may become oversized, leading to high installation and operating costs. Hence, proper balance is necessary for both comfort and efficiency.

Conclusion:

Infiltration load is the extra energy required to cool, heat, or dehumidify the unintentional outside air entering a building. It includes both sensible and latent components, depending on temperature and moisture differences. The load arises due to cracks, openings, and air pressure differences. Managing infiltration through proper sealing, insulation, and ventilation control helps reduce energy loss and maintain comfortable indoor conditions. Therefore, understanding and controlling infiltration load is essential for efficient HVAC design and operation.