Many forced air furnaces have throw-away type fabric filters—as opposed to electronic filters—or even box-type filters. It 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. They are pulled out of place in spite of the fact that there is even less suction on the filter the dirtier the filter gets. As it moves out of place, if the filter is parallel to the blower, 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 right back through the supply air registers and into the house.
In my opinion these filters should never be installed blower side of 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. This is especially true of the washable mat type filters that do not have any kind of frame to support the edges.
As the filter becomes clogged with dust, the air cannot move through the furnace as designed and can shorten the life of the furnace’s heat exchanger. 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 picture is of 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 or a box type filter. These cloth type filters, even when “ideally” installed, still do not clean the air the way an electronic filter will or the way a box filter will.
Charles Buell, Seattle Home Inspector
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