Nominal Size vs. Actual Size
Sizes of air filters can cause some confusion if you don’t understand what they mean.
Whether you are measuring lumber or air filters, it is important to note that the nominal size is not the same as the actual size. But what does nominal size mean anyway? Learn the difference between the two dimensions and make your air filter purchase with confidence.
Let’s examine the details about air filter sizes.
What is Nominal Size?
Nominal dimensions refer to the rounded number used on the label packaging of both air filters and lumber.
Nominal Size vs. Actual Size: How are They Different?
The nominal size of an air filter is the standardized size used for labeling and marketing, which makes it easy to identify compatible filters. The nominal size isn’t an exact measurement but it's the size that is important when you order an air filter.
The actual size is the filter's precise dimensions, which are typically smaller than the nominal size by about 0.25–0.5 inches. The actual size usually includes a fraction in its measurement.
Once you understand that nominal size is not an exact measurement, but rather a rounded number that makes searching for a filter and packaging lumber more efficient, you can infer how that differs from the actual size. The actual size refers to the exact measurements of your air filter, and the true dimensions when you measure the length, width, and depth (thickness). Actual size typically includes a fraction number within the dimensions – for example: 19 1/2" x 19 1/2" x 3/4". Since air filters are most often sold by their nominal dimensions, it is crucial to learn how to properly measure your air filter. When making your purchase, you should order your filter by its nominal size, rather than its actual size.
Do Manufacturers Use Nominal Sizing Instead Of Actual Measurements?
Manufacturers try to simplify the labeling and purchasing process of air filters by using nominal sizing so customers don’t have to know the air filters' precise dimensions. They created a convenient way to identify filters that fit within a size category to make it easier to shop for an air filter.
What Factors Determine The Actual Size Of A Filter?
To maintain industry standards for appropriate fit and function, air filters are manufactured with tolerances that allow for slight variations in size. Filters are designed to be slightly smaller than their nominal size to ensure they don’t get damaged when they fit into the HVAC system’s filter slot. Whether it's a pleated or specialty design, the type of air filter can affect its thickness and size. Ensuring the filter performs efficiently, fits well, and is easy to install, is the manufacturer’s primary concern, as well as following industry standards.
How Can One Accurately Measure The Filter Size Needed?
Check the size printed on the side of your existing filter, which is the nominal size used for replacements. Use a tape measure to determine the exact width, height, and depth of the filter or the filter slot, measuring to the nearest 1/8th of an inch. Compare the actual dimensions to the nominal size, which is slightly larger. Some nominal sizes made by manufacturers include:
The difference in these sizes is mainly due to their thickness which impacts how they capture particles, and how frequently they need replacement. Ensure the measurements allow a snug fit without forcing the filter into the slot because a tight fit can restrict airflow.
Are There Any Potential Issues With Using A Filter That Has Different Nominal And Actual Sizes?
If you use an air filter with mismatched nominal and actual sizes, it might lead to potential issues. If the filter is too small, it could leave gaps surrounding it, allowing unfiltered air to bypass the filter and circulate through the HVAC system. This can cause debris to accumulate on system parts, and cause damage. A filter that is too large or thick may not fit properly. Forcing it could restrict airflow, strain your blower motor, and decrease system efficiency. Air filters that don’t fit might rattle or shift and compromise filtration. Check both the nominal and actual dimensions and ensure they match your HVAC system requirements.
How Does Nominal Versus Actual Filter Size Affect Filter Compatibility?
The nominal size vs. actual size of an air filter can affect filter compatibility if it doesn’t fit properly into the HVAC system filter slot. Ensuring the actual size aligns with your HVAC system’s requirements, even when the nominal size matches, is important if you want system efficiency, proper filtration, and the best performance.
Do Nominal Sizes Differ For Custom-Made Filters?
Nominal air sizes differ for custom air filters. Standard air filters have pre-defined nominal sizes for compatibility with most HVAC systems. Custom-made filters fit non-standard dimensions. The Nominal filter size for a custom filter may still round to convenient measurements, but its actual size matches the dimensions of the HVAC system or filter slot. When you order a custom air filter, make sure you give accurate measurements to avoid size discrepancies.
What Is The Impact Of Nominal And Actual Sizes On Airflow Efficiency?
The relationship between nominal and actual sizes impacts airflow efficiency. Ensuring the actual size of the filter corresponds appropriately to the system's requirements, based on the nominal size, is an important factor in maintaining efficient airflow and system performance. If you select an improper-sized air filter for your HVAC system, it can cause more wear and tear on your system as well as restricted airflow.
Can Different Filter Materials Change The Relationship Between Nominal And Actual Size?
Filters made with flexible materials, like fiberglass, may allow for easier installation. Filters with rigid frames, like metal or sturdy plastic, require precise sizing to fit properly. In pleated filters, which are made with thicker material, size may vary due to their nominal size. The difference in materials air filters are made with, proves the importance of size, if you want maximum performance from your HVAC system and to avoid issues with air filter fit.
Conclusion
Choosing the appropriate filter for your HVAC system is easier if you are aware of the differences between nominal and actual air filter sizes. Verify both measurements to prevent incompatibilities and preserve the longevity and air quality of your system.
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