Many forced air furnaces have throw-away type fabric filters. It still surprises me how often these filters are installed so that they can be drawn out of place by the suction created by the furnace blower. When this happens the filter can be sucked against one side of the blower. This condition allows all the air returning to the furnace to by-pass the filter and go into the other side of the blower where it is blown back through the supply air registers and into the house.
In my opinion these filters should never be installed "inside" the plenum immediately attached to the furnace----unless there is some means of "positively" holding all four edges in place. This is rarely the case. Usually there is some assortment of wires that hold it in place (or nothing at all) and the filter, under suction, deforms to allow air to pass unfiltered around the edges.
As the filter becomes clogged with dust, the blower has to work harder and shortens the blower's life as well. This picture shows a filter that has been sucked against the blower and the circular discoloration is dust that has been pulled through the filter. This is the "clean" side of the filter.
While I could go into how these things "should" be installed, a better solution is to upgrade to an electronic type air-cleaner. These cloth type filters, even when "ideally" installed, still do not clean the air the way an electronic filter will.
Charles Buell
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PS, for those of you that are new to my blog (or for some other "unexplained" reason have never noticed)
all pictures and smiley-face inserts (emoticons) (when I use them) have messages that show up when you point at them with your cursor.
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