Ensure Proper Airflow when Using Higher MERV filters

Indoor air quality is at the forefront of everyone’s mind these days, and it’s crucial to use the correct air filters and install them correctly, to create the proper environment for your customer.

How much air resistance the filter provides depends on the type of filter material and the amount of area. The standard fiberglass filter doesn’t resist the air flow much, at least not when the filter is clean. As the filter loads up with the stuff it catches, the resistance increases.

If you want to remove more stuff from the air, you can use a filter with a different material. The pores in these filters are much smaller, allowing the filter to catch more pollutants from the airstream but also adding more resistance. The higher MERV filters have higher resistance.

Pressure Drop

When you add more resistance, you can get a bigger pressure drop across the filter, and that’s where the problems begin. The blower in the furnace or air handler is rated for a certain amount of pressure drop. If you use a lot of it up just to get the air through the filter, there’s not much left to move the air through the rest of the system.

When the pressure in the duct system goes up, it's possible that the blower will use more energy. Paradoxically, that happens with the more efficient blower motors (ECMs), and not so much with the less efficient PSC motors. 

Reduced Airflow

And that brings up the next problem: reduced airflow. If your system is rated for a total pressure drop of 0.5 inches of water column (i.w.c.) and you use up half of it just at the filter, you’re not likely to get the amount of air flow the system was designed to deliver.

One consequence of low air flow is reduced comfort. In the hottest and coldest weather, your system may not be able to deliver enough heating or cooling to the house. Or it may just be one or two rooms that get uncomfortable.

Another consequence of low air flow is that the air conditioner coil may get too cold and even freeze up.  That starts a vicious cycle of lower and lower air flow and a colder and colder coil until the coil is just a block of ice.  Then the refrigerant doesn’t evaporate and goes back to the compressor in the liquid state, which can damage the compressor. The same thing can happen with a furnace. Low air flow means the furnace heat exchanger gets hotter.  This could cause limit switch trips and excess wear on the furnace.

There are positive consequences that comes with low air flow. In a humid climate, the air conditioner will dehumidify better. That’s only a benefit as long as the air flow isn’t so low that it turns the condensate to ice. Bryant Evolution furnaces have perfect humidity built in to lower the CFM and help with dehumidification when needed.

To summarize, a high-MERV filter can:

  • Add resistance to the air flow
  • Increase the pressure in the duct system
  • Increase the energy use of the system
  • Reduce the air flow
  • Cause comfort problems
  • Freeze the air conditioner coil
  • Damage the compressor
  • Crack the heat exchanger
  • Put carbon monoxide in your home
  • Dehumidify better

Watch these videos on HVAC Partners to see how to properly check the airflow of a ducted heating system. When using higher MERV filters, please make sure you have the proper airflow through the system to give you and your customers piece of mind.  We are not discouraging the use of higher MERV filters, just want them to be used correctly and efficiently!

Contact a B-Y expert if you have any questions.

Are you a homeowner or own a commercial property? Check out mybryantdealer.com to find a Bryant dealer near you!