When I'm in Home Depot or Menards, I walk down the heating aisle to look at how different products are marketed. Almost always someone has an "allergen"(these are the more expensive filters promoting clean air) air filter in their cart. Almost always I ask them about why they picked that filter, and always, my embarrassed wife vanishes. Most responses involve wanting better Indoor Air Quality (IAQ) or not minding the cost if it's a better filter.


I wonder how the responses would change if they new that allergen filter was costing 7-8% of the furnace or ac's efficiency and maybe capacity. First, the furnace filter is not designed or installed for IAQ. It is installed to protect the equipment. Dirt and dust travel through the ducts, and any dirt that settles on the fan blades, heat exchanger, in the blower motor, or the wet ac coil will reduce efficiency and equipment lifespan.  Furnace manufacturers design their equipment, mainly the blower motor, to provide a certain cfm against a certain static pressure.


To explain that I need to touch on duct design a little bit. When a duct system is designed and sized several factors are taken into account.

1. CFM.  Cubic feet per minute. This is a measure of volume, similar to gallons, of air. A very specific amount of air must travel through the furnace based on its btu input. Too much variation of this cfm will change the equipment performance.


2. The resistance of the duct. A duct's resistance to airflow is measured in static pressure.


3. A fan chart. A manufacturer's fan chart is available for any furnace a designer might spec. That chart lists the amount of air that can be delivered against a duct systems resistance or static pressure. The fan chart allows for a filter pressure of .1 inches of water column. If that makes no sense don't worry you will never need to know or remember it. The .1 rating is about 20% of the total resistance.



What does any of that mean? It means your homes ducts should have been designed to move 1200 cfm (for a 3 ton ac) with a static pressure in the duct of .5 inches of water column. If tested, it will almost certainly be underperforming. Most houses are running with the SP (static pressure) higher than design standards. Meaning, the airflow is delivered at the edge of the systems limits and any change in resistance can reduce the desired airflow. An hvac system can never perform at full efficiency or capacity without correct airflow.


"Allergen" filters would be any that advertise high"merv", better IAQ, or just cleaner air. They will also cost $5-6 as opposed to less than a dollar for fiberglass. I will agree that the more expensive filters will stop more dust. The filter accomplishes this by being much more restrictive than your fan can overcome. Their static pressures may be as much as 3 times as figured in the design stage. A system that is already
underdesigned  can easily lose 7-8% of capacity with the addition of a restrictive filter. I'm not in the field anymore so I dont test as much as I used to but every summer during a hot streak it was not uncommon to find one house that would be at half capacity, causing an expensive and uncomfortable house. 



If you are concerned about indoor air quality you will have a much more effective and efficient system with a bypass HEPA, EAC or PHI CELL air purifier that can be installed by your HVAC contractor. You will be money ahead.