Define infiltration

Assumptions about outdoor air infiltration rates can have a significant impact on results, especially for buildings in cold climates. There are different calculation methods used by different simulation programs, and some programs offer more than one approach.

One common method is to specify a peak infiltration airflow rate, often in air-changes per hour, along with a schedule. In those cases, the infiltration rate depends solely on those inputs. Other methods modify the infiltration rate based on conditions such as wind speed and outdoor air temperature.

In commercial buildings, which typically are provided with constant outdoor air ventilation, the building indoor air pressure is usually slightly higher than outdoor pressure, and the infiltration rate will be low when the HVAC system is running. Therefore, typical infiltration schedules are low or zero during operating hours and higher at night when the system is off. Building types that warrant more attention to infiltration inputs are residential buildings, which may not have constant mechanical ventilation, and tall buildings, which can experience significant infiltration airflow during cold weather due to stack effect pressure.

Additional Resources

• Infiltration Modeling Guidelines for Commercial Building Energy Analysis - A report by PNNL providing detailed modeling instructions to account for infiltration

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