September, 2018

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Air Conditioner Filters – Penny Wise & Dollar Foolish

AC systems are designed to run with filters, but filters come in several grades.


If we really wanted to cool the home down quickly, we would use no filter at all, but then we’d have dust collecting on the evaporator coil and in the ducts.  Both of which are very expensive to have cleaned. If we didn’t ever want dust in the evaporator coil or in the ducts, we’d put a piece of plywood in the filter slot. But, now we cannot move any cool air out of the system and into the home. Your AC filter is somewhere between nothing and a sheet of plywood.

Cheap fiberglass filters (MERV 5 or less at $1 or $2 each) are very close to the “use no filter” example above. If you can see through them, they won’t do much. They do not stop much in the way of dust, hair, molds or pollens.  Most of that is going to end up in the system and have to be cleaned by a service technician. Cheap filters equals expensive maintenance. Washable filters tend to fall into this category as well.


Expensive allergy rated filters (MERV 10 and up at $10 to $40 each) are close to the “sheet of plywood” example. Yes, they trap mold and pollen, but they plug up quickly. Once they are plugged up, the system has to work much harder to move air through them.  When the AC system cannot move air like it was designed, the house does not cool as well and the system runs longer ($$). If you leave these in too long, you’ll actually see a good AC system physically deform the filter. It will come out all bowed up in the middle.  In Florida, most of this class of filter are full within a week, depending on the time of the year.  Very expensive to use, and actually hurts the performance of the AC system.


Most systems are designed to run with MERV 8 filters. These are the “middle of the road” filters. They are pleated paper and reinforced, generally in a cardboard frame. Typical prices are from $5 to $7 each.  I get mine in packs of 3, but there are also websites that you can order a dozen at time or others will send you a new one every month. Filters should be changed monthly. Don’t believe the “90 days” label. This is Florida and we are sub-tropical, meaning we have more stuff in our air than most other places. Change your filter monthly. Some do it on the 1st of the month, other change the filter when they get the electric bill.  It does not matter, when, just get a system and use it.


MERV 8 is the industry standard, but other filter manufacturers use different standards to measure their filter’s effectiveness.


8 1000 7/Red Dust, Pollen, Mold, Dust Mites, Lint, Bacteria
10 1900 9/ Purple Above +  Pet Dander
13 2200 10/Black Above + Virus Carrier, Odors, and Smoke, 


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