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Introduction to Air Filters | MERV Ratings, Microns, and More

Introduction to Air Filters | MERV Ratings, Microns, and More

December 22, 2017

bigstock Hvac Technician 6396508

What is the most important thing a homeowner can do to protect their HVAC system? It’s easy – change the air filter every month! There are a few easy steps to changing your air filter here in our introduction to air filters!

Air filters are crucial to the performance of your HVAC system and the quality of your indoor air.

In fact, the EPA estimates that indoor air pollutants are around 2-5 times as high than typical outdoor environments. Indoor air pollution is one of the top five environmental health risks.

In addition to monitoring healthy humidity levels, controlling sources of pollution, and investing in air purification systems, make sure that you regularly change or clean your HVAC filters.

Introduction to Air Filters: Why Is Changing the Air Filter So Important?

Dirty air filters clog up your unit and restrict airflow. Not only is this bad for your indoor air quality, but it will also increase repairs and decrease useful lifespans. Sometimes, a dirty air filter will even cause the unit to shut itself off (as a safety mechanism).

How to Check/Change Your Air Filter

Depending on how often you use your HVAC system, you’ll want to replace the air filter every 30-90 days. In the winter and summer when you are using your heating and cooling system nearly every day, it’s important to check and change your air filter more frequently.

How often you change the air filter depends on several factors:

  • The type of air filter you are using
  • Pets, number, and type of pets
  • Occupancy levels of your home
  • Overall air quality, level of air pollution and construction around the home


Manufacturers generally advise changing your standard 1″-3″ filters every 60 days. If, however, you suffer from light to moderate allergies, you may want to upgrade your air filter or change them more often.

We recommend setting reminders to check the air filter status every 30 days. It’s better to replace your air filter sooner rather than later.

A good way to make sure you always have a clean air filter is to set calendar and phone reminders to inspect your air filter every 30 days. Every month, remove the air filter from its chamber and hold it up to the light to see if any light passes through. If light easily passes through the air filter, you should be good to go.

If, on the other hand, your air filter looks dirty and restricts the light from passing through, it’s time for a filter change. It’s better to replace your air filter sooner rather than later.

Why is the number of pets important? Pets tend to shed large amounts of hair, clogging filters quickly, which in turn can severely reduce the performance of your HVAC unit.

Generally, air filters should be replaced as follows:

  • Vacation home or single occupant no pets or allergies: 6-12 months
  • “Average” suburban home, no pets: 60 days
  • Pets, respiratory problems, allergies, young and elderly: 30-45 days

2. Remembering to Change your Air Filters

Sign up for our Mr. Fix-It Service Email Club and receive exclusive offers, helpful home tips, and filter replacement reminders. Simply enter your email in the form on the sidebar. If you are having any trouble, email

In addition to signing up for our helpful email reminders, here are some other ideas for remembering the important home maintenance task of checking and changing air filters:

  1. Set calendar and phone reminders for the first of the month.
  2. Keep plenty of backups available, so you never have the excuse of going to the store.
  3. Use a Sharpie to write the date of replacement on the air filter whenever you replace one. This will tell you exactly how long it has been since you last replaced the air filter.

Make sure you have plenty of backup air filters on hand so you can quickly replace them. It’s also a good idea to write the date of replacement on the air filter itself.

3. Installing the Air Filter Properly

Make sure you follow all the manufacturer’s instructions. When installing the air filter, the most important thing is making sure that it fits properly and is pointed in the right direction.

Always point the arrow (printed on the air filter) in the direction of airflow. The air will flow in the direction of your supply vents. It’s simple: install the filter so the arrow is pointing toward the furnace and away from the return duct that carries air into the system for conditioning.

Shopping for an Air Filter: Introduction to MERV Ratings

When shopping for a new air filter, pay attention to the MERV (minimum efficiency reporting value) rating. The higher the MERV number, the more effective your air filter will be at trapping smaller airborne contaminants.

You have to be careful though. When you start looking at high-efficiency air filters (MERV rating 14-20), the filters will be a lot thicker and may require professional modification of your HVAC system to accommodate the thicker filters.

For instance, HEPA (high-efficiency particle air) filters can capture extremely small particles (smaller than .3 microns), but they are a lot thicker and can block airflow, causing problems for most residential HVAC systems.

Speak with a professional HVAC technician if you want better air filtration ratings (anything above 13).

  • Low-efficiency filters (MERV 1-6)
  • Medium-efficiency filters (MERV 7-13)
  • High-efficiency & HEPA filters (MERV 14-20)

Unless you are thinking about professionally modifying your existing system, we highly recommend going with the medium-efficiency air filters, which provide much better air filtration than low-efficiency filters without having to modify your system in any way.

When shopping for a new air filter, bring your old air filter into the store and search for a similarly sized one with a higher MERV rating (7-13).

Microns and MERV Ratings

Microns are a unit of measurement equal to one-millionth of a meter (Symbol: μ, mu). The higher the MERV rating, the more effective the air filter is at capturing smaller particles (smaller micron sizes).

Most airborne particles in your home are one of three ranges:

  • E1 (0.3 to 1 micron)
  • E2 (1 to 3 microns)
  • E3 (3 to 10 microns)

Higher efficiency filters, such as HEPA filters, which are found in hospitals, require special equipment to allow for the thicker filter. For residential homes, we recommend medium-efficiency filters (MERV 7 and higher). If you have pets, we recommend a MERV 8-12.

Once you hit MERV 13, you will want to speak with a professional HVAC technician to see if it will interfere with your airflow and HVAC system.

View the chart below to see which MERV ratings are able to capture which size particles:

MERV Micron Air Filter Chart

If your HVAC system starts acting up this winter, learn some furnace troubleshooting tips before you call the professionals at Fix-It 24/7.

Fix-It 24/7 Plumbing, Heating, Air and Electric provides reliable, on-time plumbing, electric, HVAC, and indoor air quality services 24 hours a day, 7 days a week (holidays included!).

Call us now at (303) 214-0277 or schedule service online.

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