A Second HVAC Air Filter Is Not Recommended

A second HVAC air filter is not recommended.   In fact, it is one too many.

Some furnace/AC systems have filters beside the unit.  Others, when the system is in the attic, have return grills in the ceiling to conveniently be able to replace the filter without having to climb into the attic each month.

Those ceiling return grills are specially made with clips and hinges to be able to lower the grill, replace the filter, and clip the grill back into place.

The fixed return grills on the wall are not intended to be removed so often, and are screwed in place.

This homeowner decided he was really going to filter that air in the house!  So he installed a second filter in the return grill just outside the kitchen.

This is not recommended for two reasons:

1.  It puts a strain on the system as it restricts air flow, and makes the blower actually spin faster due to less air volume to try to equalize the in and out air needs.  According to my buddy Charlie Buell that super spinning is called cavitation, with the blower essentially inside an air bubble, and he proved that when the return air is like this and cavitation is happening the system is actually using less electricity.   Modern heat systems, especially heat pumps, utilize a lot of air flow to manage temperatures efficiently.

2.  Since the grill is screwed into place it is more difficult to remove and the filter would be easier to forget.

The grill in this wall is much larger than the return duct.  As a result the filter in this photo can be seen to have two colors.  It was a little difficult to photograph, still the lesson can be seen.

The blue portion of the filter on the right side, much larger than is shown here, covers the section of the return where no air passes. 

The left side, which is gray with collected dust, has been filtering the passing air for quite some time.  It was completely clogged, blocking the passage of air.  You should know that restricted air flow is a great way to kill a heat pump.  This air flow was so restricted you could hear the whine of the blower as it was spinning faster and faster.

My recommendation:  HVAC air filters do not handle enough air to keep the household air clean in any house.  Not enough volume passes through the system each hour to be able to do that.  So the real intention of the filter is to keep the system clean.  A second filter is liable to do more harm than good, and is NOT recommended.  An air filter is not a place and forget product.  Replace or clean them regularly, and each month during seasons of high use.