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What is Air Exchanges Per Hour?

When talking about ventilation the term Air Exchanges Per Hour is probably going to come up, but what is it? The simple answer if how many times the air inside your home is replaced by either outside or filtered air per hour. Think of it this way – if you have a glass full of water and you pour it all out, then fill it back up with water you have exchanged the water in the glass one time. The same holds true with air but it is a little more difficult to see.

How is the air exchanged?

Air exchanges take place in one of three main ways.

Exhaust Fans: Exhaust fans are one of the most effective ways to exchange the air in your home. They take the stale indoor air and exhaust it to the outdoors allowing fresh air to replace it. National building codes now require a continuously running fan to be installed in all homes for this very reason.

HVAC system: Your HVAC system also helps provide air exchanges but there are some challenges with using this as the only method. The biggest one is that the system has to be running in order to provide air exchanges. You can set your system to just run the fan, but this is a very inefficient way to exchange air in your home. Another challenge is for this to be effective, the system needs to be drawing fresh air into the home or have filters installed that will take out the “bad” stuff. A good amount of HVAC systems do not draw fresh air in – you can check with your technician. As for the filters, most systems utilize a MERV 6 or MERV 8 filter. These are good for capturing dust and larger particles but when it comes to the stuff we are really trying to remove, they are not as effective. It is recommended that a MERV 13 or above be used, but not all HVAC systems can handle that (see our blog on Filters).

Natural Exchange: It sounds so nice – Natural, but there are also some issues with this method. Most of the time this means opening a window or door and letting the air come in. If you have a nice breeze blowing on a moderate temperature day with good outdoor air quality (low pollen, smog, etc.) this is a good way to exchange the air. The big issue is you can’t control how much air is coming in or what is in the air. A second and significant issue it temperature, think Texas in the summer or Minnesota in the winter. Opening doors and windows probably is not an option.

Why are air exchanges important?

A key component to Indoor Air Quality is air exchanges. When we are not properly exchanging the air we allow all kings of contaminates to linger in the air. Think of a smoky bar room. Without any ventilation or air exchanges the smoke just lingers in the air. While hopefully our homes are not full of smoke, they are full of contaminates in the air caused by breathing, cooking, pets, and just general everyday living. Having a plan to continually remove these contaminates is key to better living.

How many air exchanges per hour should take place?

This is where we come to the great debate. Home many air exchanges are needed, what is too little, what is too much. There is always a balance and as we learn more about our indoor air quality the numbers can change. As of this posting ASHRAE, who sets most of the building standards when it comes to indoor air quality recommends that a minimum of 0.35 air exchanges per hour occur in your home. Other organizations recommend up to 5 per hour. Our thought is that 5 per hour might be a bit extreme and will definitely have a negative effect on the efficiency of the home in that you have to condition all that air coming in to replace the air going out. With that being said, lets examine what 0.35 exchanges per hour looks like. For a 1,000 square foot home with 8-foot ceilings you have 8,000 cubic feet of space. Divide 8,000 by 60 (minutes) to get 133. Now times 133 by 0.35 to get 46.67. This is the amount of CFM you would need to provide the recommended air exchanges per hour. ASHRAE also provides a calculator that is a bit more involved to determine how much continuous ventilation your home needs. You can access the calculator on the Air King site by clicking here.

For more information about the indoor air quality of your home and how you can improve it, please read through the many articles we have written over the years. For information regarding Air King exhaust fan and fresh air intake solutions click on the links in the menu at the top of the page.

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