If you’ve done any research online about how to lower your heating bills during the wintertime, one of the most frequently talked about topics is your furnace’s filter. If your furnace runs with a dirty filter, its efficiency will be reduced and you’ll have higher energy bills. However, if you’re constantly checking your filter and it seems to be clean all the time, it’s important to understand why.

What Does a Furnace Filter Do?

Your furnace’s filter is in place to protect the internal components of your HVAC equipment from particulate matter like dust, pollen, pet dander, dust mites, mold spores, and other contaminants. This helps to ensure the overall longevity of your furnace and reduces its risk of experiencing a breakdown. A furnace filter is also a great way to help enhance the quality of air inside your home.

This filter is located within your return ductwork just before air enters the furnace. Your furnace’s filter works by physically trapping these harmful airborne particles. Most of these filters are made of a fiberglass mesh-like material and can even be pleated to help increase the surface area of the filter.

How Often Should Your Filter Be Checked and How?

Generally, it’s best to check your furnace’s filter every month to see if it needs to be replaced with a new one. Checking the state of your filter is simple to do. You want to hold it up to a light source. When a filter is new, you’ll be able to see through it. When a filter has reached the end of its lifespan, you’ll no longer be able to see through it. Most homeowners find that they need to replace their air filters every two to three months.

Why Is Your Filter Continuously Clean?

If you notice after a few months that your air filter is consistently clean, it’s a cause for concern. While you’re certainly saving money by not having to replace the filter, the fact that it’s not accumulating airborne particles is a problem. This could mean that the particles are going directly into your furnace and accumulating where they will eventually cause harm.

Wrong Filter Size

One of the most common reasons your filter may never get dirty is that you have the wrong size filter. If the filter you put in your heating system is smaller than the recommended size, all the air traveling through your return ducting could simply be moving around the filter. The air is going to go in whatever place is less restrictive. If there’s open space around your filter in the ductwork, air will choose to go through the open space instead of through your air filter.

There is no one air filter size that every furnace uses. If you look at the filter housing on your ductwork, the filter size will typically be notated somewhere. It comprises three numbers: width, height, and depth. It can be helpful to look at the old filter to see what size it is. However, make sure that you verify the old filter physically fits in the housing correctly.

If you’re not sure what size filter your furnace takes, you can look this information up online or contact your furnace installer. You can also opt to measure the filter’s housing. Just be very mindful that filters are given in nominal sizes. This means their actual depth may only be three-quarters of an inch but the filter size for the depth will be 1 inch.

Backward Installation

Another reason that your filter may appear to be clean is that it was installed incorrectly. All filters are designed with a porous side that allows for efficient airflow through the filter. At the very top of the air filter, you’ll see an arrow that notates the appropriate direction of airflow through the filter.

The arrow should be pointing from your return ducting toward your furnace. There’s even likely an arrow on the top of your filter’s housing showing this direction. If you install your filter with the arrow in the opposite direction, meaning pointing from your furnace to your return ductwork, it’s going to greatly decrease the efficiency of your furnace.

With less air being able to travel through your furnace, there are going to be significantly fewer airborne particles trapped in your filter. Alongside checking the top of the existing filter in your furnace to see if the arrow is in the wrong direction, you’ll typically be able to tell that you have a problem because your heating bills will rise while your heating system won’t be able to adequately heat your home.

Low Air Exchange Rate

Another culprit behind why your furnace filter may remain clean after months of use is the low air exchange rate. This means that your furnace isn’t running long enough to adequately move a large amount of air through the filter. One of the biggest symptoms of this problem is that your furnace only runs in short cycles.

The average furnace should run about 15 minutes for every cycle. It shouldn’t cycle more than three times every hour. If you come to notice that your furnace is constantly kicking on and isn’t running for the full 15 minutes, there’s likely an issue with the internal components of your furnace that should be addressed by a licensed HVAC technician.

The Filter Is Too Powerful

When many homeowners get concerned about the quality of air inside their homes during the winter months, they tend to learn about MERV filter ratings. Because filters can have a MERV rating anywhere between 1 and 20, it’s common to think that purchasing a MERV 20 filter is a good idea for getting maximum air filtration for the air inside your home.

While filters can have a rating as high as 20 MERV, not all these filters are recommended for residential furnaces. These furnaces are only designed to move so much air and only have so much power. When you invest in a filter with a rating that is too high, the overall operating efficiency of your furnace can decrease. In fact, your furnace is going to struggle trying to pull any air through the filter.

The maximum MERV filter that can go in a residential furnace is 13. It’s important to keep in mind that this holds true for newer furnaces. Older furnaces may not handle such a high filter rating. It’s best to check with the furnace manufacturer to determine the maximum MERV rating your air filter can have.

Expert Furnace Maintenance Service

Summit Heating, A/C, Plumbing & Electrical offers expert furnace maintenance service for homeowners throughout the entire Denver, CO area. Our technicians will assist with all your heating, cooling, plumbing, electrical, duct cleaning, duct sealing, home energy audit, commercial, indoor air quality, and thermostat needs. This includes installations, repairs, and routine maintenance. Simply call our friendly staff today to book your next heating service consultation with one of our highly knowledgeable HVAC technicians.

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