Understanding Air Filters – Part One

Posted in Air Filters Mar, 01, 2013

Filters for your HVAC system are essential; yet most people do not fully understand their importance. Prevailing marketing hype only tends to confuse people even more! This article will establish what a filter’s job is, how we select the proper filter and the differences in the filter types.

A Filter’s Job:

Easy right; a filter is designed to “clean the air” of dust particles! Well, yes and no… The primary job of the filter is to protect the equipment from foreign particulate contamination. When an equipment manufacturer (OEM) designs a system, and specifies a certain filter, their goal is to protect the equipment; not protect your health!

Using the incorrect filter can lead to many system failures such as:

  • Allowing too much particulate to pass through will cause the evaporator coil to become dirty. This leads to frozen evaporator coils, which could lead to compressor damage! Yes, that’s right; the filter helps protect the compressor!
  • A filter that is too restrictive will lead to low airflow. This could also lead to a frozen evaporator coil and a dead compressor. At the very least it will cause the blower to operate inefficiently (uses more energy) and reduced airflow reduces the system BTUs! You may have a 5 ton system but a restrictive filter may cause it to operate at closer to 4 tons; you are paying for 5 tons worth of energy but only getting 4 tons of return!

As you can see, there needs to be a nice balance point that allows the correct amount of airflow but still removes the particulate from the airstream. This is the basis for sizing and choosing the correct filter.

A Healthy Home:

People who suffer from allergies may require special filtering needs. This is where we go from protecting the equipment to creating better Indoor Air Quality (IAQ). Again, we need to choose a filter that is more efficient at removing the allergens but still allows for the correct amount of airflow.

Choosing The Correct Filter:

The American Society for Heating, Refrigeration and Air-conditioning Engineers (ASHRAE) has set a test standard for evaluating all filters; it is ASHRAE 52.2-1999.

I’m not going to bore you with the entire test standard; but suffice it to say that it produces a very repeatable evaluation. The result of this evaluation is called the Minimum Efficiency Reporting Value (MERV) and is the baseline by which all filters can be compared.

MERV is not a RATING, it is simply an evaluation of a filters efficiency. There is NO set standard for filter RATINGS; so buyer beware when any company offers a rating for filters. This rating system is subjective and has no basis in science; it is purely subjective to the company’s “rating system” which may change at any time.

The question then becomes this: If a filter has a “good” rating what are the test conditions? Is this rating relevant to your system? Or is it specific to certain “type” of system i.e. heat pump or furnace? Does the “rating” system address this? If not, and they never do, how do you know it is “good” for your system?

The answer is, you do not know; and that is where the danger lies.

We have established what is a filter’s job, how they can be used to improve IAQ and that we have a standard test for evaluating all filters. In the next article we will use this information to compare a few different types of filters.