Air Changes Per Hour (ACH)
Understanding One Of The Most Important Concepts In Industrial Building Performance
Most people understand that opening a door can let heat escape. Fewer understand what is actually happening inside the building when that occurs. Every industrial building continuously exchanges air with its surroundings. Some of this exchange occurs through ventilation systems, some through natural leakage, and some through operational activities such as opening doors.
The rate at which this exchange occurs is known as Air Changes Per Hour (ACH).
Whilst the term may sound technical, understanding ACH provides valuable insight into heat loss, humidity control, contamination control, carbon emissions and operating costs. In many industrial buildings, ACH is one of the most important factors influencing overall energy performance.
What Is An Air Change?
An air change occurs when a volume of air equivalent to the volume of the building is replaced. For example, imagine a warehouse containing 10,000 cubic metres of air. If 10,000 cubic metres of internal air leaves the building and is replaced by external air over a one-hour period, the building has experienced:
One Air Change Per Hour (1 ACH)
If 50,000 cubic metres of air is exchanged during that hour:
Five ACH
If 100,000 cubic metres of air is exchanged:
Ten ACH
The higher the ACH figure, the faster internal conditions are being replaced by external conditions.
In practical terms, ACH measures how quickly a building loses control of its internal environment.
Why Does ACH Matter?
Air carries more than oxygen.
Air carries:
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Heat
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Cold
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Moisture
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Dust
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Insects
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Airborne contaminants
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Carbon emissions
Every time air enters or leaves a building, these factors move with it. This means that ACH influences:
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Heating costs
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Cooling costs
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Humidity levels
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Product quality
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Contamination risk
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Occupant comfort
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Carbon emissions
This is why ACH is often considered one of the most important measurements of building performance.
Our Heat Loss section explains how temperature differences create energy loss. ACH helps explain the mechanism through which much of that loss occurs.
Why Open Doors Increase ACH
Industrial doorways can have a dramatic effect on ACH. Whenever a door opens, internal and external air immediately begin to mix. The larger the opening, the longer it remains open and the greater the temperature difference, the greater the volume of air exchanged.
In busy industrial facilities this can happen hundreds of times per day.
Consider:
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Forklift traffic
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Vehicle movements
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Loading operations
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Personnel access
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Goods transfers
Each opening contributes to additional air exchange. This is particularly important because the doorway often represents the single largest opening in the entire building envelope. Whilst walls, roofs and windows remain largely fixed, industrial doorways may repeatedly expose thousands of cubic metres of conditioned space to external conditions.
ACH And Heating Costs
Every cubic metre of warm air that leaves a heated building must ultimately be replaced. During winter, external air entering the building often needs to be heated from near ambient temperatures to comfortable working temperatures.
The greater the ACH, the greater the volume of replacement air requiring heating. This is why buildings with excessive air exchange often experience:
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Higher gas consumption
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Higher electricity consumption
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Longer heating run times
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Increased operating costs
Many businesses focus on insulation upgrades whilst overlooking the impact of excessive air movement.
In reality, reducing unnecessary air exchange can often provide a significant contribution towards improving overall building efficiency.
Our ROI Calculator demonstrates how reducing doorway exposure time can contribute to measurable operating savings.
ACH And Building Pressure
ACH and building pressure are closely connected. Air naturally moves from areas of higher pressure to areas of lower pressure. This means that pressure differences can increase or decrease air exchange.
Positive pressure environments may push air outward. Negative pressure environments may draw external air inward.
This relationship helps explain why some facilities experience persistent drafts, contamination issues or temperature instability.
Our Building Pressure article explores this principle in greater detail.
Understanding pressure helps explain where air is moving.
Understanding ACH helps explain how much air is moving. Together they provide a powerful understanding of building performance.
ACH And Humidity Control
Moisture travels with air. Whenever external air enters a building, it brings its moisture content with it.
This is particularly important in:
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Food production
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Pharmaceutical facilities
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Cold storage
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Clean manufacturing
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Temperature-controlled environments
High ACH can result in:
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Increased condensation
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Moisture instability
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Product quality issues
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Mould risk
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Ice formation
Many humidity problems are actually airflow problems. Our Humidity section explains why controlling air movement is often one of the most effective ways to control moisture.
ACH And Contamination Control
Air changes influence far more than temperature. They also influence cleanliness.
Every air exchange has the potential to introduce:
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Dust
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Insects
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Airborne pollutants
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Agricultural debris
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Bird ingress
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Vehicle emissions
For food and pharmaceutical operations, contamination control is often a critical operational requirement.
Reducing unnecessary ACH can help support:
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Product quality
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Hygiene standards
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Environmental control
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Regulatory compliance
The objective is not to eliminate airflow entirely. The objective is to reduce uncontrolled air exchange.
Why ACH Is One Of The Most Useful Building Performance Metrics
Many building performance discussions focus on individual symptoms.
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Heat loss.
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Humidity.
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Condensation.
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Contamination.
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Temperature instability.
ACH sits beneath all of them.It helps explain how conditions enter a building, how energy leaves a building and why environmental control can become difficult.
Understanding ACH allows businesses to think beyond individual problems and focus on the underlying cause.
In many industrial environments, reducing unnecessary air changes can improve:
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Energy efficiency
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Environmental control
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Product quality
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Occupant comfort
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Operating costs
Which is why ACH is increasingly recognised as one of the most important measures of industrial building performance.
