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Building Science

How Often Should You Change the Filter?

Every 30 days, when it looks dirty, or according to some other criteria?

When is a filter dirty enough to change? [Photo credit: Energy Vanguard]

One of the topics I’ve hammered on for years is the inadequacy of using rules of thumb for sizing air conditioners. The standard one ton of air conditioning capacity for each 400 to 600 square feet of conditioned floor area just doesn’t work.  We know that a load calculation based on how a particular house is likely to perform is much better than a rule of thumb.  Another area where a rule of thumb dominates is in replacing the filter in your air conditioner (or heat pump or furnace).  Change it every thirty days, right?

When you buy the standard one inch fiberglass filters for your HVAC system, the instructions on the package usually say to change it every 30 days.  This advice gets repeated dutifully by a lot of people and organizations trying to help.  And the truth is, you won’t cause any problems by changing your filter every 30 days and you might even prevent some.  A few years ago, I spent a day going on service calls with Phil Mutz of Moncrief Heating & Air Conditioning here in Atlanta.  As you might expect, some of the problems we found were low air flow due to dirty filters.

Changing the filter every 30 days is a hassle, though.  In spring and fall, the filter generally won’t get as dirty because there’s not as much need for heating and cooling, so you can get by with not changing it as frequently.  And in a house that doesn’t have much infiltration or duct leakage and a low level of dust overall, the filter can stay clean longer even when being used a lot (unless your return vents are in the floor).

And then there’s the issue of the type of filter.  If you’ve been following this blog over the…

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