Air changes per hour
Encyclopedia
Air changes per hour is a measure of how many times the air within a defined space (normally a room or house) is replaced.
Air changes in a confined space are important for a variety of reasons, mainly though, we need fresh air to live. Without sufficient fresh air exchange, moisture is trapped in a room/home/building, molds can feed, and other allergens and excessive dangerous gases (e.g. Carbon monoxide, Carbon Dioxide, urea-formaldehyde
), can remain in the home. "Stale" air is unhealthy and, since humans and pets add to it by breathing, sweating, washing, showering and drying, we need to ventilate the home, increasing the number of times the air 'exchanges' in the home with outside fresh air. Number of 'air changes per hour' were less of a problem before 'air sealing' came into play, because construction practices and products were not geared to energy efficiency (in the USA, with lower energy costs).
With a new focus on energy efficiency, reducing carbon footprints, and reducing dependence on fossil fuels, consumers try to seal their homes from air transfer in and out of their homes in winter and summer. Double edged-sword.
The importance of fresh air intake cannot be understated.
An air change does not represent a complete change of all air in the enclosure or structure unless it can be considered plug flow
. The actual percentage of an enclosure's air which is exchanged in a period depends on the airflow efficiency of the enclosure and the methods used to ventilate it. The actual amount of air changed in a well mixed ventilation scenario will be 63.2% after 1 hour and 1 ACH. In order to achieve equilibrium pressure, the amount of air leaving the space and entering the space must be the same.
ACH equation in Imperial units
Where:
Air changes in a confined space are important for a variety of reasons, mainly though, we need fresh air to live. Without sufficient fresh air exchange, moisture is trapped in a room/home/building, molds can feed, and other allergens and excessive dangerous gases (e.g. Carbon monoxide, Carbon Dioxide, urea-formaldehyde
Urea-formaldehyde
Urea-formaldehyde, also known as urea-methanal, named so for its common synthesis pathway and overall structure, is a non-transparent thermosetting resin or plastic, made from urea and formaldehyde heated in the presence of a mild base such as ammonia or pyridine...
), can remain in the home. "Stale" air is unhealthy and, since humans and pets add to it by breathing, sweating, washing, showering and drying, we need to ventilate the home, increasing the number of times the air 'exchanges' in the home with outside fresh air. Number of 'air changes per hour' were less of a problem before 'air sealing' came into play, because construction practices and products were not geared to energy efficiency (in the USA, with lower energy costs).
With a new focus on energy efficiency, reducing carbon footprints, and reducing dependence on fossil fuels, consumers try to seal their homes from air transfer in and out of their homes in winter and summer. Double edged-sword.
The importance of fresh air intake cannot be understated.
An air change does not represent a complete change of all air in the enclosure or structure unless it can be considered plug flow
Plug flow
In fluid mechanics, plug flow is a simple model of the velocity profile of a fluid flowing in a pipe. In plug flow, the velocity of the fluid is assumed to be constant across any cross-section of the pipe perpendicular to the axis of the pipe...
. The actual percentage of an enclosure's air which is exchanged in a period depends on the airflow efficiency of the enclosure and the methods used to ventilate it. The actual amount of air changed in a well mixed ventilation scenario will be 63.2% after 1 hour and 1 ACH. In order to achieve equilibrium pressure, the amount of air leaving the space and entering the space must be the same.
ACH equation in Imperial units
Where:
- N = number of air changes per hour
- Q = Volumetric flow rate of air in cubic feet per minute (cfm)
- Vol = Space volumeVolumeVolume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....
L × W × H, in cubic feet
Air Change Rate
| Commercial kitchens | 20–60 |
| Public bathrooms | 6 |
| Class rooms | 3–4 |
| Laboratories | 6–12 |
| Smoking rooms | 10–15 |
| Warehousing | 1–2 |