Q: What is an Air Exchange per Hour (ACH)?
A: Many in the Building Science community believe this is the most important measurement to determine a home’s efficiency. The inside of a home has its own environment. Many interrelated components of the home have to align in order to achieve optimum performance.
Think of when you take a breath. You inhale and exhale. One cycle of exhaling and inhaling would be counted as one air exchange. The same is true for a home. The existing air in your home gets exhausted outside while fresh air from outside comes inside. Every time this happens it is called an Air Exchange.
Motor vehicles efficiency is measured by Miles per Gallon (MPG). The home efficiency is measured by ACH. Fresh air from the outside is run through your heating and cooling equipment. If it is 30 degrees outside that temperature of air is run through your furnace, heated then sent to the inside of your home. This takes energy. The more times in an hour that this occurs, the more energy you are using to heat your home. When it is hot outside, the same thing happens. Energy is used to create a comfortable temperature inside of your home.
In extreme hot or cold conditions your equipment can run continuously. Most often your home is leaking air faster than your equipment can treat new air. It is as if your equipment is running a race with no finish line.
We target an ACH number of 2. That means our equipment will cycle off and on 2 times in an hour. This is a very efficient target. A code built home may have an ACH of 8.
Next week I will discuss how to improve the Air Exchanges per Hour (ACH).
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