Calculate Air Changes per Hour

Air Changes Per Hour (ACH) Calculator

(e.g., from an exhaust fan, HVAC system, or air purifier)

function calculateACH() { var roomLength = parseFloat(document.getElementById('roomLength').value); var roomWidth = parseFloat(document.getElementById('roomWidth').value); var roomHeight = parseFloat(document.getElementById('roomHeight').value); var totalAirflowCFM = parseFloat(document.getElementById('totalAirflowCFM').value); var resultDiv = document.getElementById('achResult'); if (isNaN(roomLength) || isNaN(roomWidth) || isNaN(roomHeight) || isNaN(totalAirflowCFM) || roomLength <= 0 || roomWidth <= 0 || roomHeight <= 0 || totalAirflowCFM <= 0) { resultDiv.innerHTML = 'Please enter valid, positive numbers for all fields.'; return; } var roomVolume = roomLength * roomWidth * roomHeight; var ach = (totalAirflowCFM * 60) / roomVolume; resultDiv.innerHTML = 'Calculated Air Changes Per Hour (ACH): ' + ach.toFixed(2) + ' ACH'; }

Understanding Air Changes Per Hour (ACH)

Air Changes Per Hour (ACH), sometimes referred to as air change rate, is a crucial metric used to quantify the ventilation effectiveness of a space. It measures how many times the entire volume of air within a room or building is replaced with outdoor air (or filtered recirculated air) in one hour.

Why is ACH Important?

• Indoor Air Quality (IAQ): A sufficient ACH rate helps dilute and remove indoor air pollutants such as volatile organic compounds (VOCs), carbon dioxide, allergens, dust, and odors, leading to healthier indoor environments.
• Comfort: Proper ventilation contributes to thermal comfort by removing excess heat and humidity.
• Odor Control: High ACH rates are essential in areas prone to strong odors, like kitchens, bathrooms, or industrial facilities.
• Health and Safety: In specific environments like laboratories, hospitals, or workshops, high ACH rates are critical for removing hazardous fumes, pathogens, or airborne contaminants.
• Energy Efficiency: While higher ACH improves air quality, excessively high rates can lead to increased energy consumption for heating or cooling the incoming air. Balancing IAQ with energy efficiency is key.

The ACH Formula

The formula for calculating Air Changes Per Hour is straightforward:

ACH = (Total Airflow (CFM) × 60 minutes/hour) / Room Volume (Cubic Feet)

Where:

• Total Airflow (CFM): This is the total volume of air moved into or out of the space per minute, typically provided by a fan, HVAC system, or air purifier. CFM stands for Cubic Feet per Minute.
• Room Volume (Cubic Feet): This is the total volume of the room, calculated by multiplying its length, width, and height (Length × Width × Height).
• 60 minutes/hour: This conversion factor changes the airflow from per minute to per hour.

Recommended ACH Rates

Recommended ACH rates vary significantly depending on the type of space and its intended use. Here are some general guidelines:

• Residential (General): 0.35 to 1.0 ACH (for general comfort and air quality, often achieved through natural infiltration and minimal mechanical ventilation).
• Bathrooms: 8 to 15 ACH (when fan is operating, to remove moisture and odors).
• Kitchens: 6 to 10 ACH (when range hood is operating, to remove cooking fumes).
• Offices/Classrooms: 2 to 4 ACH (to maintain good air quality for occupants).
• Laboratories/Cleanrooms: 6 to 20+ ACH (depending on the hazard level and cleanliness requirements).
• Smoking Areas: 10 to 30 ACH (to rapidly remove smoke).

It's important to consult local building codes, ASHRAE standards, or specific industry guidelines for precise recommendations for your particular application.

Example Calculation:

Let's say you have a living room with the following dimensions and a ventilation system providing a certain airflow:

• Room Length: 20 feet
• Room Width: 15 feet
• Room Height: 8 feet
• Total Airflow (from an air purifier or HVAC): 300 CFM

First, calculate the room volume:

Room Volume = 20 ft × 15 ft × 8 ft = 2400 cubic feet

Now, calculate the ACH:

ACH = (300 CFM × 60) / 2400 cubic feet
ACH = 18000 / 2400
ACH = 7.5

This means the air in this living room is completely replaced 7.5 times every hour by the ventilation system.