How Do You Calculate Air Changes Per Hour

How to Calculate Air Changes Per Hour (ACH)

Air changes per hour (ACH) is a crucial metric in ventilation design and building performance assessment. It quantifies the rate at which the air within a space is replaced with fresh air. Understanding how to calculate ACH is essential for ensuring proper indoor air quality, energy efficiency, and occupant comfort. This comprehensive guide will walk you through various methods of calculating ACH, explaining the concepts, formulas, and considerations involved.

Understanding Air Changes Per Hour (ACH)

ACH represents the number of times the entire volume of air within a specific space is completely replaced within one hour. A higher ACH indicates faster air exchange, which can be beneficial for removing pollutants and maintaining fresh air but might also lead to increased energy consumption if not managed efficiently. A lower ACH signifies slower air exchange, potentially resulting in poor indoor air quality but potentially lower energy costs. The optimal ACH varies significantly depending on factors such as building type, occupancy, and intended use. For instance, a hospital operating room requires a much higher ACH than a residential bedroom.

Methods for Calculating Air Changes Per Hour

There are several ways to calculate ACH, each with its own applications and limitations. The most common methods include:

1. Using the Airflow Rate and Volume of the Space

This is the most straightforward method and is generally used for simple spaces with a single air supply and exhaust.

Formula:

ACH = (Airflow Rate (cfm) * 60 minutes/hour) / Volume of Space (cubic feet)

Where:

• Airflow Rate (cfm): This represents the volume of air moved in cubic feet per minute (cfm) by the ventilation system. This can be obtained from the ventilation system's specifications or measured using an anemometer.
• 60 minutes/hour: This conversion factor transforms the airflow rate from cfm to cubic feet per hour (cfh).
• Volume of Space (cubic feet): This is the total volume of the space being ventilated, calculated by multiplying its length, width, and height.

Example:

Let's say we have a room that measures 10 ft x 12 ft x 8 ft. The volume of the room is 960 cubic feet (10 ft * 12 ft * 8 ft). The ventilation system delivers 240 cfm.

ACH = (240 cfm * 60 minutes/hour) / 960 cubic feet = 15 ACH

This indicates that the air in the room is completely replaced 15 times per hour.

2. Using the Supply and Exhaust Airflow Rates

In more complex systems with multiple supply and exhaust points, it's essential to consider both the supply and exhaust airflow rates. Ideally, these should be balanced for optimal ventilation; however, this is not always the case in practice.

Formula:

ACH = (Supply Airflow Rate (cfm) * 60 minutes/hour) / Volume of Space (cubic feet) OR

ACH = (Exhaust Airflow Rate (cfm) * 60 minutes/hour) / Volume of Space (cubic feet)

Important Note: If the supply and exhaust rates differ significantly, using the lower of the two values for the calculation provides a more conservative estimate of the actual ACH. This accounts for potential leakage and ensures the calculation reflects the effective air exchange rate. Significant differences between supply and exhaust rates may indicate system imbalances requiring attention.

3. Considering Infiltration and Exfiltration

Infiltration (uncontrolled air leakage into the building) and exfiltration (uncontrolled air leakage out of the building) significantly impact the actual ACH. These are often difficult to quantify precisely but can be estimated using various methods like blower door tests or simplified building simulation software.

Formula:

ACH_Total = ACH_Mechanical + ACH_Infiltration + ACH_Exfiltration

Where:

• ACH_Mechanical: ACH calculated using the mechanical ventilation system's airflow rate (as described in methods 1 and 2).
• ACH_Infiltration: ACH due to air infiltration, estimated through building performance testing or empirical data.
• ACH_Exfiltration: ACH due to air exfiltration, also estimated through testing or empirical data.

Estimating infiltration and exfiltration accurately requires specialized tools and knowledge. Often, these values are estimated based on building characteristics and climate data. Ignoring these factors can result in significant inaccuracies in the calculated ACH.

4. Using the Air Exchange Rate and Occupancy

In occupied spaces, the air exchange rate is often specified in terms of air changes per person per hour. This approach considers the occupancy load to determine the necessary ventilation rate.

Formula:

ACH = (Air Changes per Person per Hour * Number of Occupants) / (Volume of Space (cubic feet))

This approach is particularly relevant for designing ventilation systems in densely occupied spaces like classrooms or offices. Building codes and standards frequently stipulate minimum air changes per person per hour for various occupancies.

Factors Affecting Air Changes Per Hour

Several factors influence the effective ACH in a building:

• Building Envelope Tightness: A tightly sealed building with minimal air leakage will have a lower infiltration and exfiltration ACH, relying more heavily on the mechanical ventilation system.
• Wind Pressure: Wind can significantly influence infiltration and exfiltration rates, leading to fluctuations in ACH.
• Temperature Differences: Temperature differentials between the indoor and outdoor environments drive air leakage.
• Mechanical Ventilation System Design: The capacity, efficiency, and design of the ventilation system directly impact the mechanical ACH.
• Building Materials: Porous building materials can contribute to air leakage.

Importance of Accurate ACH Calculation

Accurate ACH calculation is critical for several reasons:

• Indoor Air Quality (IAQ): Adequate ACH ensures the removal of pollutants, moisture, and odors, improving IAQ and occupant health.
• Energy Efficiency: Over-ventilation can lead to increased energy consumption for heating and cooling, while under-ventilation can compromise IAQ. Optimal ACH balances these factors.
• Building Code Compliance: Many building codes specify minimum ACH requirements for various building types and occupancies to ensure compliance with safety and health standards.
• Building Design and Commissioning: Accurate ACH calculations are fundamental to designing effective and efficient ventilation systems and verifying their performance during commissioning.

Tools and Resources for ACH Calculation

While simple ACH calculations can be performed manually using the formulas provided, more complex situations may require specialized tools:

• Blower Door Tests: These tests measure the airtightness of a building and can provide estimates of infiltration and exfiltration rates.
• Computational Fluid Dynamics (CFD) Simulation Software: CFD software can model airflow patterns within complex spaces to accurately predict ACH.
• Building Simulation Software: These tools help predict building performance, including ventilation rates, under various conditions.

Conclusion

Calculating air changes per hour (ACH) is an essential aspect of building design, operation, and maintenance. While simple formulas exist for basic calculations, accurate estimation often requires consideration of multiple factors, including mechanical ventilation, infiltration, exfiltration, and occupancy. Using appropriate methods and tools, and carefully considering all relevant factors, allows for the design and operation of healthy and energy-efficient buildings. Understanding and utilizing this knowledge allows for improved indoor air quality, better occupant health, and responsible resource management.