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.
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Charles Buell, Seattle Home Inspector
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Good post agreed on all points EXCEPT. The reason why these filters are pulled in is because they are clogged. The air can’t move freely through the filter and the negative pressure becomes great enough to pull on the filter. The higher the filter quality, such as micro fiber, the faster they clog because they catch more dirt as designed. I remove these clogged filters during inspection because if the air is moving across the heat exchanger or evaporator to slowly it can give you a false positive temperature reading. These filters must be changed as often as needed. A forced air system is all about the air movement which does much more than just push the conditioned air through the house. System performance, efficiency and longevity also depends on it’s designed air flow. A clogged filter can do more damage than no filter and it is the number one cause of compressor or heat exchanger death. It all comes down to the M word maintenance.
Daniel the whole business of how these filters get pulled out of place is an interesting phenomenon. A filter that is held in place VERY well, as it clogs, the blower will pull on it less and less until when it is completely blocked the blower will not really be pulling on it at all. The blower will just sit there and spin and actually use less electricity than it would if there was no filter there at all. Not so good at all for the heat exchanger when that happens. 🙂
Sound like the clogged filter is causing drag but if the resistance increases, so should the amperage. With these new electronically controlled units I’m just not sure any more. I just know it’s a bad thing 🙂