Understanding Air Exchange Rate (ACH)
The Air Exchange Rate, often abbreviated as ACH, is a measure used in building science and HVAC (Heating, Ventilation, and Air Conditioning) to quantify the amount of outdoor air that enters a building or room in one hour, relative to the volume of that space. It's a crucial metric for assessing indoor air quality, energy efficiency, and the overall performance of a building's ventilation system.
Why is ACH Important?
- Indoor Air Quality (IAQ): Adequate ACH helps to dilute and remove indoor pollutants such as volatile organic compounds (VOCs), carbon dioxide (CO2), moisture, odors, and airborne pathogens. Low ACH can lead to a buildup of these contaminants, negatively impacting occupant health and comfort.
- Energy Efficiency: While ventilation is necessary for IAQ, excessive air exchange (high ACH) can lead to significant energy losses. Uncontrolled infiltration of cold outdoor air in winter or hot outdoor air in summer requires your HVAC system to work harder, increasing energy consumption and costs.
- Moisture Control: Proper ventilation helps manage indoor humidity levels, preventing condensation issues that can lead to mold growth and structural damage.
- Building Performance: ACH is often used in conjunction with other building performance metrics, such as blower door test results, to understand airtightness and ventilation strategies.
Factors Affecting ACH:
- Natural Infiltration: Air leaks through cracks, gaps, and openings in the building envelope (walls, windows, doors, foundation).
- Mechanical Ventilation: Intentional introduction of outdoor air through systems like exhaust fans, supply fans, heat recovery ventilators (HRVs), and energy recovery ventilators (ERVs).
- Building Stack Effect: Temperature differences between indoor and outdoor air can create pressure differences, driving air movement.
- Wind Pressure: External wind forces can push or pull air into or out of a building.
Typical ACH Values:
- Very leaky buildings: May have ACH values of 5-10 or even higher.
- Older, average homes: Often fall in the range of 0.5 to 1.5 ACH.
- Modern, energy-efficient homes: Aim for 0.2 to 0.5 ACH, relying on controlled mechanical ventilation.
- Commercial buildings: Standards vary widely based on occupancy and function, but are often higher than residential settings to manage IAQ in denser environments.
Understanding and calculating your building's ACH can help you identify potential issues and make informed decisions about ventilation and energy efficiency improvements.
Air Exchange Rate (ACH) Calculator
This calculator helps you estimate the Air Exchange Rate (ACH) of a space based on its volume and the rate of air change.