The Air Exchange

When you hear the term Air Exchange what comes to mind? Some picture a place like where you exchange your gas grill propane tank, some think of a returns department. Even those who have heard the term sometimes don’t fully understand what it really is.

On the surface air exchange is a fairly simple concept. Taking the air inside your home and replacing it (or exchanging it) with fresh outdoor air. Not all that difficult to understand. The complexity comes in on how that is done. Lets first get an understanding of what an air exchange is.

Air Exchange is the number of times the air inside your home is replaced with fresh outdoor air. The American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAR) recommends at least an air exchange every 3 hours. Exchanging air is typically done by having a continuous source of fresh air coming into the home while exhausting the stale air out. So far everything still seems fairly straightforward. Good air in, bad air out – easy. The challenge comes in calculating exactly how much. Bringing too much in and you put unneeded stress on your HVAC system. Not brining enough in and now your indoor air quality suffers. Here is where we need to do a little math. Not to worry, this is fairly simple math. If you have a 1,000 square foot home with 8-foot ceilings you have approximately 8,000 cubic feet of air in your house. Exchanging the air once every 3 hours will mean that you need to move about 2,700 cubic feet of air an hour. Now divide that by 60 (60 minutes in an hour) and you get approximately 45 cubic feet per minute (CFM).  That would be for a home that has zero natural air infiltration. To get a better picture of how much air is actually needed, we need to look at different types of homes.

Older, drafty home. If you have an older home that has drafts, the good news and the bad news is the same news. The good is that you have a natural flow of fresh air coming into your home. Engineers call this natural air infiltration – the rest of the world calls this a draft. The bad part of a drafty home is that you have no way of controlling the air and your utility bills are probably high. For these types of homes it is important that you are properly ventilating the home with bathroom exhaust fans and a kitchen range hood but typically you do not need any additional equipment to ensure fresh air is coming in. The amount of air coming in is going to naturally “push” the stale indoor air out. Adding insulation, sealing doors and windows and so forth can reduce the amount of drafts, which is a good thing, but keep in mind that you may need to add a mechanical solution to bring fresh air in.

The Goldilocks House. Not too drafty but not too tight. These homes are typically a few years old (10 to 20) or an older home that has been renovated. They have a good balance of natural air infiltration but not so much that they would be considered “drafty”. In these houses ventilation is important but you might also need to add some type of system to bring fresh air in. Calculating how much as we have mentioned above gets a little tricky. Using our 1,000 square foot home example we calculated that you need 45 CFM but if you have a natural air infiltration of 10 CFM now you only need 35 CFM. To determine these numbers you will need to consult an HVAC professional. They are able to run tests on your home to see what your true infiltration rate is.

The Tight Home. These are typically homes that have been built in the past 10 or so years to the higher building standards. They are called tight homes because there is little to no natural air infiltration. This is great for the heating and cooling cost, but not so good for indoor air quality and air exchanges. The good news is that building codes include provisions for a mechanical solution to exchange the air – venting the stale air out and brining fresh air in. There are a number of solutions that are able to do this without a noticeable impact on the HVAC system. It wouldn’t make sense to build a home to gain all the advantages of a tight home, then turn around and lose all of that because you need to bring fresh air in. One of the best systems on the market is either an Energy Recovery Ventilator (ERV) or a Heat Recovery Ventilator (HRV). Both “recover” the lost energy by tempering the air coming into the home.

To learn more about air exchanges and indoor air quality visit www.ashrae.org