Infiltration (HVAC)

Infiltration, in the context of Heating, Ventilation, and Air Conditioning (HVAC) systems, refers to the unintentional and uncontrolled entry of outside air into a building. This entry occurs through cracks, gaps, holes, and other unintentional openings in the building envelope, such as around windows, doors, electrical outlets, and plumbing penetrations.

Infiltration is distinct from ventilation, which is the deliberate introduction of outside air into a building for the purpose of providing fresh air and removing pollutants. Ventilation is typically controlled and managed through mechanical systems.

The amount of infiltration is influenced by several factors, including:

• Building Envelope Tightness: The more tightly sealed the building envelope, the less infiltration will occur. Older buildings or those with poor construction practices tend to have higher infiltration rates.
• Pressure Differences: Pressure differences between the inside and outside of the building drive infiltration. These pressure differences can be caused by wind, temperature differences (stack effect), and the operation of mechanical systems like exhaust fans and HVAC systems.
• Wind Speed and Direction: Higher wind speeds create greater pressure differentials, increasing infiltration. The direction of the wind also affects which sides of the building experience the most infiltration.
• Temperature Difference: The greater the temperature difference between the inside and outside of the building, the stronger the stack effect, which contributes to infiltration. Warm air rises, creating a positive pressure at the top of the building and a negative pressure at the bottom, drawing in outside air.
• Building Height: Taller buildings are generally more susceptible to stack effect, leading to higher infiltration rates.

Infiltration can have several negative consequences for building performance and occupant comfort:

• Increased Energy Consumption: Infiltration increases heating and cooling loads, as the HVAC system must work harder to maintain the desired indoor temperature. This leads to higher energy bills.
• Reduced Comfort: Infiltration can create drafts and uneven temperatures within the building, making it less comfortable for occupants.
• Moisture Problems: Infiltration can carry moisture into the building, leading to condensation, mold growth, and structural damage.
• Poor Indoor Air Quality: Infiltration can introduce pollutants from the outside air, such as pollen, dust, and vehicle exhaust, which can negatively impact indoor air quality.

Controlling infiltration is an important aspect of building design and energy efficiency. Strategies for reducing infiltration include:

• Air Sealing: Sealing cracks, gaps, and holes in the building envelope to prevent air leakage.
• Proper Insulation: Insulating the building envelope to reduce heat transfer and temperature gradients, which can drive infiltration.
• Window and Door Upgrades: Replacing old, leaky windows and doors with more energy-efficient models.
• Vapor Barriers: Installing vapor barriers to prevent moisture infiltration.
• Balanced Ventilation: Ensuring that the building has a balanced ventilation system that provides adequate fresh air without creating excessive pressure differentials.

Infiltration rates are typically measured using blower door tests, which measure the amount of air leakage through the building envelope. These tests can help identify areas where air sealing is needed.