Using the Wrong Size HVAC Air Filter

Hvac technician performing air conditioner maintenance inspection

Updated: 10 October 2024

Your HVAC (Heating, Ventilation, and Air Conditioning) system is critical in keeping your home comfortable throughout the year. While an HVAC’s air filter is often the focus of cleaning lists, one often overlooked aspect of this component is making sure it’s the right size for your system.

In this blog, we’ll explore what happens if you use the wrong size air filter for your HVAC system, the potential risks, and how to ensure you’re using the right filter to keep your system running smoothly.

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Why Air Filters Are Important in HVAC Systems

An air filter’s primary job is to trap dust, dirt, pollen, pet dander, and other airborne contaminants before they enter the HVAC system. By doing so, air filters:

Improve indoor air quality
Protect the HVAC components from dust build-up
Promote energy efficiency
Prolong the system’s lifespan.

However, to achieve these benefits, you must use the correct size filter for your system.

What Happens When You Use the Wrong Size Air Filter?

While it may be tempting to use a filter that is readily available, cheaper, or a size that seems “close enough,” doing so can lead to myriad issues that can affect both air quality and system efficiency. Here’s what can happen:

Damage to the HVAC System

Overheating Components: When the airflow is restricted due to a clogged or oversized filter, the HVAC system must work harder to pull air through the filter. This increased workload can lead to overheating, particularly in components such as the compressor and the fan motor. This can cause these parts to fail, leading to costly repairs or even the need for a full system replacement.
Reduced Efficiency of the System: An improperly sized air filter can create an imbalance in the airflow, which means the system may not operate at its designed efficiency. When the HVAC unit is forced to work harder, it consumes more energy, leading to increased utility bills. According to the U.S. Department of Energy, addressing airflow issues caused by improper filtration can improve HVAC efficiency by up to 15%.
Increased Wear and Tear: HVAC systems have many moving parts that are subject to wear and tear over time. When the system is forced to operate under less-than-ideal conditions due to an improper filter size, it can lead to accelerated wear on components such as belts, bearings, and motors. This can result in frequent repairs and a shorter lifespan for the system.

Reduced Air Quality

Increased Allergen Levels: A filter that is too small may not effectively capture dust, pollen, pet dander, and other allergens. This can lead to higher concentrations of airborne irritants, exacerbating allergies and respiratory issues for occupants, particularly those with existing conditions like asthma or COPD (Chronic Obstructive Pulmonary Disease).
Dust Accumulation in Ducts: An improperly sized filter can allow dust and debris to bypass the filter and enter the ductwork. This accumulation can lead to dirty ducts, which can not only reduce air quality but also promote the growth of mold and bacteria, causing further health concerns.
Unpleasant Odors: Filters that are too small can become saturated quickly, leading to musty or stale odors circulating throughout your home. If pollutants are not adequately captured, they can contribute to unpleasant odors that linger in the indoor environment.

Increased Energy Consumption

Higher Utility Bills: As discussed, an incorrectly sized air filter can lead to increased energy consumption. The HVAC system works harder to maintain the desired temperature, resulting in higher electricity bills.
Environmental Impact: Increased energy consumption not only affects your wallet but also contributes to a larger environmental footprint. Higher energy use can lead to increased greenhouse gas emissions, particularly if your energy source is fossil fuels.

Shortened System Lifespan

Frequent Repairs: As the HVAC system experiences more strain due to inefficient airflow and increased wear, it may require more frequent repairs. This can lead to frustration and unexpected expenses for homeowners who may not have budgeted for such costs.
Premature Replacement: Ultimately, consistent use of the wrong size air filter can lead to a shortened lifespan for the HVAC system. Homeowners may find themselves needing to replace their entire system years earlier than expected, which can be a significant financial burden.

Need your HVAC serviced, repaired or replaced? Request an appointment today.

How to Choose the Right Size Air Filter

Woman is replacing furnace filter at home

Now that we’ve discussed the risks of using the wrong size air filter, here’s a quick guide to help you purchase the correct size for your HVAC system.

Follow these simple steps to ensure you’re using the right filter:

1. Check the Current Filter Size

The easiest way to determine the correct size is by checking the size of your existing air filter. Most filters have the dimensions printed on the side in the format of length x width x depth (e.g., 16 x 25 x 1). If the filter is a perfect fit for your system, use these dimensions to purchase a replacement.

2. Consult the HVAC Manual

If you’re unsure about the correct filter size or can’t find the dimensions on your current filter, consult your HVAC system’s manual. The manufacturer will typically provide recommendations on the appropriate filter size for your unit.

3. Measure the Filter Slot

If you don’t have access to the manual and can’t determine the size from the current filter, you can measure the filter slot in your HVAC unit. Measure the length, width, and depth of the slot to get the exact dimensions of the air filter you need.

4. Consider the Efficiency Rating

In addition to the size, you should also consider the Minimum Efficiency Reporting Value (MERV) rating of the air filter. The MERV rating indicates the filter’s ability to capture airborne particles. Most residential HVAC systems use filters with a MERV rating between eight and 13, which is sufficient to trap common household contaminants like dust and pollen without restricting airflow.

If you’re still unsure about what size air filter you need to make sure your system runs as smoothly, get in touch with us today — we’d be happy to help!

What to Do If You’ve Been Using the Wrong Size Filter

If you suspect you’ve been using the wrong size air filter, take the following steps to minimize any potential damage:

Replace the filter immediately: Swap out the incorrect filter with the right size to ensure optimal airflow and filtration.
Schedule a professional inspection: Have an HVAC technician inspect your system for any signs of damage caused by using the wrong filter. It’s best to catch any issues as soon as possible to avoid more costly repair charges in the future.
Regularly check and change your filters: Make it a habit to check your air filters monthly and replace them as needed. Most filters should be changed every one to three months, depending on factors like usage and indoor air quality.

If you’re ever unsure about the correct filter size, consult your HVAC manual or contact us — one of our professional HVAC technicians will be able to guide you in making the right choice.

HVAC Filter FAQs

Can using the wrong air filter size cause my HVAC system to leak?

Yes, using the wrong size filter can lead to improper airflow, causing condensation to form in unwanted areas. This excess moisture can lead to leaks, particularly around the evaporator coils.

How often should I replace my HVAC air filter, regardless of its size?

In general, air filters should be replaced every one to three months, but the frequency can vary depending on factors like the filter type, air quality, and how often the HVAC system runs.

What is the difference between nominal and actual air filter sizes?

Nominal size is a rounded number used for marketing and labeling, while the actual size refers to the filter’s precise measurements. It’s important to check the actual size to ensure a proper fit.

Does using a thicker air filter provide better protection for my HVAC system?

Not necessarily. A thicker air filter with a high MERV rating might trap more particles, but it could also restrict airflow if your system isn’t designed to handle it. Always consult your HVAC manual before switching to a thicker filter.

Can I trim an air filter to fit if it’s too large?

No, trimming an air filter can damage its structure, reduce its effectiveness, and create gaps that allow dust and debris into your HVAC system. Always buy the correct size for your unit.

Are reusable air filters more prone to size-related issues?

Reusable filters must be carefully sized to your system, just like disposable ones. Improperly sized reusable filters may not form a proper seal, leading to issues similar to those of incorrect disposable filters.

Can using the wrong air filter size affect the humidity levels in my home?

Yes, improper air flow caused by a wrong-sized filter can impact your HVAC’s ability to manage humidity, potentially leading to excessively dry or humid indoor conditions.

What should I do if my air filter keeps getting dirty too quickly?

If your air filter gets dirty faster than usual, it could be a sign that the wrong filter size is being used, allowing more dust and debris into the system. Alternatively, it could indicate poor indoor air quality or a high-traffic environment.