Have you ever walked through the filter aisle at your favorite hardware store and been amazed at all the options there are for such a simple part of your homes HVAC system? There's different sizes, thicknesses, numbers, and even scents and colors. In this article we will explain what those different things mean, and which is best for your system.

The first thing you will probably look for is filter size. When you go to your furnace and find the filter it should have some numbers on the side, something like 16x20x1 or 20x25x4. You may also find those numbers written somewhere on the ductwork near your filter so you don't even have to take the filter out to figure out the size. The first two numbers are the outside dimensions of your filter, and the third number is the thickness.

The majority of furnaces are set up to use a 1" thick filter, as this is the cheapest option when installing a furnace and ductwork. If you've had the unit upgraded, or the builders of the home put extra thought into the HVAC system you may have a 4" or thicker filter. If you have the thicker filter, you can often put a thinner one in its place. Unless you are using a smaller filter temporarily until you can get the proper one, I wouldn't recommend using a thinner one. If you have the thinner 1" filter, your options are limited to just to that size, as they don't get any smaller.

When it comes to thickness, the bigger the better. If you were to pull the filter material out and stretch it flat, the thicker filter would have much more surface area that the thinner one. With more surface area, you get more space to catch particles in the air. This makes your filter last longer before plugging up, and helps the airflow through your furnace.

When you go to buy a new filter, its best to stick with whatever size you currently have in your furnace. If you have questions, or want to upgrade to a larger better filter you can always give us here at Air Central a call to help you out.

When it comes to filters, there are also numbers that indicate the MERV rating, or efficiency of the filter. MERV stands for minimum efficiency reporting value. The higher that number, the more efficient the filter is at catching airborne particles. Below I will list some of the common MERV ratings and what they are good for.

MERV 2: This would be basic thin fiberglass filters. They are the really cheap ones that look like horse hair crisscrossed back and forth in a few layers. You find them often in residential systems that have a standard 1" filter compartment. They will catch most hair, large dust particles and stop any larger objects that may find their way into your ductwork, but most smaller particles go right through.

MERV 7-8: This is a pretty standard rating for most residential filters. They are commonly found in the 1" thicknesses, and are usually inexpensive. You also see 4" and thicker filters starting at this rating. These filters work great for most situations where a 1" filter is the only option. They catch everything the lower levels will, as well as pet dander, mold spores and fine dust. If your system was designed properly these filters provide plenty of airflow when changed regularly, usually about 1-2 months for 1" and 6-12 months for 4" and thicker depending on how much dust is generated in your home.

MERV 10-11: You find most 4" filters in this range. You can get 1" filters at this level, but we normally don't recommend it, as the thinner filters at this level become very restrictive to airflow in your duct system. The 4" and thicker filters in this range have plenty of surface area to provide good airflow. These filters catch all the stuff lower levels do as well as finer pollen, fumes from vehicle exhaust, and even some smoke and bacteria. In 1" filters you will have to change them at least every month, possibly more often, but with the 4" and thicker you can safely get 6 months, even up to a year depending on dust load.

MERV 13: This is about the highest rating you find in residential applications. This level is almost exclusively 4" and thicker filters, due to how restrictive they are to airflow. You can find 1" filters at this level, but they tend to be so restrictive that they cause problems with your system after just collecting a small amount of dust. These filters are more expensive, but if you are looking for something to help with smoke, smog, and odors, these may be the choice for you. If your duct system wasn't designed properly these can start to restrict airflow to the point of causing problems even with the 4" and thicker sizes. These filters also plug up much faster because very little will pass through them. Typical life for 4"and thicker is 3-6 months.

Also worth noting, just because the lower rated filter says it will catch dust/lint, doesn't mean it will remove as much of that dust from the airstream as the higher rated filter. A MERV 2 catches less that 20% of particles under 10 microns. Whereas a MERV 13 will catch over 90% of particles down to 1 micron. There are higher ratings available, but they are generally for commercial and industrial applications where there is a need to filter out incredibly fine particles and vapors to protect sensitive equipment or catch bacteria and viruses for medical facilities.

When selecting a filter, try to stay within the guidelines above to prevent problems with your HVAC system. If you get the one that looks really good because it is supposed to trap more stuff, it may be to restrictive for your system and cause your furnace to overheat in winter, and your AC to freeze up during summer. It also puts extra strain on your furnace and AC which may lead to early failure.

In most cases, stick with the same filter that you find in your unit, unless a professional recommends something different. If you are interested in getting a better filtration system, or are just seeking some advice on what filter you should be using, give Air Central Inc. a call and we will be happy to help. We also sell and install other air quality devices like UV lights, Ionizers, and humidifiers to help keep you home clean and comfortable.