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What You Need to Know About Furnace Filters

furnace filters

When you buy a new furnace, you want to make sure that you take care of your investment. After all, the furnace is used to keep homes warm in the winter by heating the inside air, and during the summer it helps to circulate the air from your cooling system.

Are furnace filters really necessary?

A furnace filter is a very important part of an HVAC system. Replacing the filter regularly will not only improve the quality of air in your home but will also prolong the life of your furnace.

What happens if you don’t change your furnace filter?

Not taking care of the filter will result in greatly increased health risks due to air contamination and a shorter life span of the furnace.

What types of furnace filters are there?

To choose your furnace filters, you must first know about MERV ratings. MERV stands for Minimum Efficiency Reporting Value. The higher the number, the cleaner it will make the air. High MERV ratings will also restrict air flow more.

Furnace filters come in a variety of sizes as well as MERV ratings. The most common type of filter would be the 1” filter, which can be found at any local hardware store. The main benefit of a 1” filter is the small size - there are some situations where there may not be enough space for a larger, more efficient filter. Most of the 1” filters are rated around MERV 8, but some can go all the way up to MERV 13.

HEPA filters are another type that are great for cleaning air, but they normally would restrict airflow too much for the furnace. HEPA filters are normally found equipped to larger-size air cleaners or dehumidifiers. Many commercial air cleaners will incorporate HEPA technology due to the industries that tend to use them, such as medical facilities.

The other common type of filter is the 5” thick filter which will allow for better air cleaning, a higher MERV rating, and has a longer replacement period. These thicker filters are slotted into a rack that supports the weight of the furnace, unlike most 1” filter racks. Most 5” racks have a door you can pull off for easy access to replace filter.

What is a good MERV rating for furnace filters?

A furnace filter should have a MERV rating of at least 8 MERV. Because these filters remove the majority of pollutants from your indoor air, using 8 MERV filters or higher is the general recommendation. If you have serious medical conditions that make it difficult to breathe, a filter with MERV 9-12 rating will likely be your best choice.

There are two types of filters commonly used: fiberglass and synthetic.

  • Fiberglass filters have been around the longest. These filters have a variety of densities, with the highest density filters boasting some the most powerful filter efficiencies available. HEPA filters are one of the more common types of ultra-dense fiberglass filters. They are also a media-type of filter, as opposed to being a pleated filter.
  • Synthetic filters a slightly more low-cost option and are typically more durable than fiberglass. You may commonly find these in the automotive industry, but they still have their place in HVAC. Synthetic filters do not need to be replaced as frequently as fiberglass filters, leading to less being spent on them overall.

Three other less common types are carbon, UV light, and electronic filters.

  • Carbon filters are very efficient and are known for their ability to remove smells like smoke and other odors. The catch? These filters usually have a heftier price tag because of the greater benefits they entail.
  • UV Light filters are used for killing germs, mold and other organisms. This does not stop them from being in the air, but it will stop them from growing and spreading. This dead debris will dissipate over time.
  • Electronic filters, such as the type of filter used in the AE14-2025-52G from Clean Comfort, are a great long-term choice. These have two metal plates which are electrically charged to attract dust particles and also have two mesh filters to catch the bigger chunks of debris. These filters are able to last indefinitely, but you will have to take the plates and mesh filters out and clean them every now and then. This may take some time depending on how dirty your air was, but the cleaning process is usually fairly simple - most times it will involve rinsing them out with running water or wiping them with a damp cloth. They also have a rather large upfront cost, but it can be justified due to not having to replace plates or mesh filters in the future.

Where do the filters go?

There is some variety in the placement of filters. For the most part, a filter would be attached to the furnace right where the return air duct is attached.

For an upflow configuration, you can have the return air flowing through a filter on the sides or up through a filter on the bottom. The upflow configuration is one of the most versatile, allowing you to attach 1, 2, or even 3 filters with return air ducts attached. With the filter attached to the furnace, the return air duct would transition to the size of the filter.

A horizontal configuration is relatively the same as the upflow configuration except that it is laying down on its side. You can still attach return ducts and filters to its “sides” and “bottom”.

Where you may run into trouble is with a downflow system. They are not as versatile in their ability to connect return ducts with it mostly being restricted to coming in straight from the top. Many scenarios would use a plenum box on top with branching ducts.

You can also attach return air duct to a support return air box, which is just a sheet metal box for the furnace to sit on. The main reason for a support box setup is if you want to use a 1" filter and have the return air come in through the bottom of the furnace. Since a 1” filter rack cannot support the furnace's weight, a support box is needed. Another reason might be to make the furnace taller.

On a 2-in-1 air handler (blower and evaporator coil together), there is usually a 1” filter slot already attached. However, it can sometimes be found on the supply air side. This is not the end of the world, but it is recommended to buy a separate rack/filter to put on the return side.

Caring for your filter

Caring for your furnace means caring for your filter and caring for your filter means caring for your respiratory health. Every filter will have a different replacement period. 1” filters usually need to be replaced every 2 months. Depending on the filter used, it may sometimes be every 3 months. 5” filters can last up to a year before needing replaced. Some factors come into play to determine replacement time, such as how dirty the house is due to pets or lots of people living in it as well as how often the blower on the furnace pulls air through the filter.

The best way to determine if it needs replaced is to look at it. Below are a few examples:

  • If the filter is filled with dust, it will not let air pass through. If air cannot pass through then the heat cannot be moved away from the furnace and the furnace will overheat. If there is no filter, then dust will accumulate in the furnace. The dust will trap heat and cause operational problems or it will collect on your evaporator coil. This can result in a costly cleaning or expensive repairs.
  • You may have also heard of vacuuming or washing a filter. This is something commonly seen with electronic filters. It is not recommended on other filter types unless you are in a pinch. The materials do not hold up after being wet, and vacuuming will only temporarily relieve the airflow problem since you will not be able to get most of the small particles out of the filter.

To sum everything up, although a filter seems like a nice little add-on, it is in fact a crucial component in a complex system. Have you checked your furnace's filter recently? If your answer is no, then it might be time to replace it.

December 17, 2021
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