Furnace filter art.

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.

Furnace filters that move out of place

Furnace filters that move out of place

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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Whole house ventilation

As homes have gotten tighter and tighter to conserve energy, we have created indoor air quality issues.  Stale air can carry all kinds of particulate as well as allow for elevated carbon dioxide levels.  Furnishings, pets, molds, dust mites and hundreds of other elements in the home can make for poor indoor air quality and affect the way we breathe and thus our over all health.

This post is about whole house ventilation and how we use it to improve indoor air quality in new and remodeled homes—often required in many jurisdictions and certainly in Washington State.

In some homes the whole house ventilation is achieved by integrating it with the furnace operation.  With this approach air is pulled into the duct work by the furnace fan and distributed to the home through the duct-work the same way heat is distributed.  This is a good way to do it because it results in fresh air being distributed evenly throughout the home.

Other homes have exhaust fans (perhaps laundry or bathroom exhaust fans) that pull the air into the home.  In these homes there may be little vents in the window units that create an actual opening in the wall to pull air in.   This picture shows the air intake at the top of the outside of the window.

Window air intake

Window Air Intake

There are other means of doing this as well, including HRV’s (Heat Recovery Ventilation units).  These units have the ability to pull fresh air into the home while the outgoing air passes over the incoming air.  The heat of the outgoing air transfers to the cooler incoming air reducing heat loss dramatically.  These units are very effective and even required in some jurisdictions.

But back to this story.

I did a one year warranty inspection next door to a home that I had done a purchase inspection on.  On the home of the one year warranty inspection I found the same defect that was on the other house—and the whole development may have been this way.

This first picture shows the duct-work of the air intake coming off the side of the furnace return air duct and the location of the electronic damper that shuts off the duct-work when the furnace is in heating mode.

Air intake damper

Air Intake Damper

This next picture shows the intake pipe termination at the roof.  What is wrong in the picture?  Note that the cap has a back draft damper in it.

Air intake

Air Intake With Damper Installed

This is not the correct cap.  When the furnace is trying to pull in fresh air it is just pulling the damper tighter—-no fresh air is drawn in.  A simple enough fix—-but not functional the way that it is.

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