I've seen a few reviews on this site and I have to tell you they have an agenda. My wife and I moved into our new house a year ago and immediately installed blocking filters (we had them in our old house too). We don't have a chimney and use our HVAC system year round. We installed two sizes of blocking filters --- 14x14x1 and 20x24x1. You are really great. After a year of use, the front of our filters was dirty (see photo), while the back was completely clean. Don't believe bad reviews. These are the best filters out there, period.
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