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.
If the filter is missing or unmarked, measure the filter slot in your HVAC system for these dimensions. Always ensure the size matches exactly, as even slight differences can affect filtration efficiency and system performance.
Going down in thickness by an inch shouldn't be a big deal - for example, you should be able to use a 4-inch filter instead of a 5-inch filter. If you are considering a thicker air filter in order to improve air quality it may be better to focus on the MERV rating.
A standard air filter size for home HVAC systems is often 20" x 20" x 1". However, the size can vary greatly depending on the specific system. Always measure your existing filter or check your system manual to ensure you get the right size.
Selecting an incorrect size might appear insignificant, but it seriously hampers your system's performance and lifespan. Such a minor mistake leads to compromised efficiency and airflow restriction, which result in higher energy use, more maintenance work, and the need for frequent replacements.
UNDERSTANDING FILTER SIZES
Therefore, for example, a 16x25x4 filter is 16 inches long, 25 inches wide, and 4 inches thick. It's important to note that filters with different dimensions are not interchangeable due to variations in frame size and media area.
The easiest way to find out what size filter to get for your furnace is to simply remove your current filter and look at its dimensions. These are typically printed along with one of the sides of your filter.
If the filter is too small, it won't cover the entire space and may allow dust and dirt to flow past. If you're not sure what size filter you need, check the manufacturer's specifications or ask your HVAC technician for help choosing the right product.
Then, there is the actual size, which typically runs half an inch shorter than the nominal size. For example, the nominal size of one of the most popular furnace filter sizes is 20 x 25 x 1 inches. The actual furnace filter size measures to 19.5 x 24.25 x 0.75 inches.
A filter that's too small will leave space where debris and particles can enter; a filter that's too large won't fit in the slot. You can't trim it down, either, since that will create gaps that allow things like dust, dander, pollen, and indoor air contaminants to enter.
Usually they round up or down to the nearest inch, so it is less likely that these two filters will fit the same furnace because of the rounding. However, if your filter is close to 20x25x4. 5, and the actual dimensions of the 20x25x5 filter are closer to 20x25x4. 5, than they might fit.
Stiffer builds for steadfast seal: 1-inch filters have less material to control, which means the frames are thinner. Thin frames have flimsy seals which let through unfiltered air, defeating the purpose of your filter. 5-inch frames are thicker and fit into the furnace more seamlessly.
Damage to the HVAC System
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.
For 1- to 2-inch filters, replace them every 1 to 3 months. For 3- to 4-inch filters, replace them every 6 to 9 months. For 5- to 6-inch filters, replace them every 9 to 12 months.
In conclusion, while the idea of stacking two filters might seem like a shortcut to better air quality, it can lead to more harm than good.
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.
Nominal means "in name," indicating that the nominal size is used for naming purposes rather than to express an exact measurement. The nominal size is usually the actual size rounded to the nearest whole inch. For example, a filter that measures 19.5" x 19.5" x 0.75" would have a nominal size of 20" x 20" x 1".
The four-inch-thick air filter has an advantage over the one-inch in terms of longevity, airflow and maximum filtering potential. If it fits your furnace, then a four-inch air filter will provide better air quality. The cost difference is often minimal.
You can still use a thinner furnace filter, but you need to make sure that it stays in place. This may mean moving your air duct system over so that the filter is properly braced. If you'll only be using this smaller filter for a very short period of time, you can add a bit of tape to the side to hold it there.
If your filter doesn't fit properly, it can interfere with the flow of air from the system to your home. Your air filter may also get sucked into the system, causing damage. Improper installation of the filter or the absence of a filter allows dust and other debris to build up inside the system.
This might seem like a small detail, but cutting an air filter down to a smaller size can sometimes damage the structural integrity of the filter. This is especially true for higher-end filters. The filter size should be listed in the instructional manual that came with your HVAC unit.
Furnace filters are sized by thickness (depth), height, and length. The most common thickness is 1”, with 4” also being a popular choice for larger systems. Height and length combinations range from 10”x10” all the way to 30”x 30”. The most common sizes are 14”x 25”, 16”x 20”, 16”x 25”, 20”x 25”, and 25”x 25”.
How Often Do I Need to Change Out My Air Filter? Generally, most air filter manufacturers and HVAC companies recommend changing your air filter every 90 days, or 3 months. That can change based on where your home is located (ex. dusty, dry climates), if you have any pets, and the age of your system and equipment.
If you can't ascertain your furnace filter's nominal dimensions, there is no need to panic. You can simply measure the opening for filter and round up to the nearest inch. For example, if the actual opening is 14 by 19 ½, the nominal dimensions you want to specify are 14 by 20.