The air filter is a component of the heating, ventilation, and air conditioning (HVAC) system that influences both the unit’s operational efficiency and indoor air quality. Using the correct filter size is necessary for the system to function as designed. An improperly sized filter can lead to air bypass, where unfiltered air slips around the edges, or it can restrict airflow, forcing the blower motor to work harder. This increased strain leads to higher energy consumption and can shorten the lifespan of system components. Ensuring the replacement filter matches the system’s specifications protects the equipment and maintains a clean indoor environment.
The First Step: Where to Look
The most direct way to determine the required filter size is to examine the existing filter currently installed in the HVAC unit. Manufacturers typically print the dimensions clearly on the cardboard frame of the filter itself. This printed size is the nominal size, which is the standard measurement used for purchasing replacements.
If the existing filter is missing, illegible, or damaged, the next place to check is the HVAC unit itself. Many manufacturers place a sticker or stamp with the required filter size directly on the access panel or door that covers the filter slot. This information is often located near the filter compartment or on the main housing of the furnace or air handler.
Locating the size information on the unit’s housing is a reliable method because it reflects the system’s design specifications. If the size is not visible on the exterior panel, it may be printed on a label inside the compartment once the access door is opened. Consulting the owner’s manual for the specific HVAC model is another reliable option if physical labels are absent or worn away.
Decoding the Dimensions
Air filter sizes are universally listed using three numbers, representing the filter’s dimensions in inches: Length x Width x Depth (or Thickness). A common residential size, for example, might be listed as 16x25x1, indicating a filter that is 16 inches long, 25 inches wide, and 1 inch thick. Understanding these three measurements is necessary for selecting the correct replacement.
The size printed prominently on the filter frame is known as the nominal size, which is a rounded, standardized figure used for easy identification. The actual size is the precise measurement of the filter, which is always slightly smaller than the nominal size. This nominal size is what consumers use when searching for a replacement filter.
For instance, a filter with a nominal size of 16x25x1 might have an actual size closer to 15.5 x 24.5 x 0.75 inches. This slight reduction in dimensions is intentional, allowing the filter to slide easily into the designated slot without jamming. The filter’s frame covers the small gap created by this difference, ensuring a snug fit that prevents air from bypassing the filtration media.
What to Do If the Size is Missing
If the size information cannot be found on the old filter or the HVAC unit’s housing, the only remaining option is to manually measure the filter slot opening. Begin by removing any existing filter and using a tape measure to find the inside dimensions of the opening where the filter sits. Measure the length and width of the opening from one interior lip to the opposite interior lip.
Measure the length and width in inches, recording the precise measurements to the nearest fraction of an inch. To determine the nominal size for purchasing, round each measurement up to the nearest whole or half-inch. For example, if the slot measures 15.5 inches by 24.5 inches, the nominal size to purchase is 16 inches by 25 inches.
The final measurement to take is the depth, or thickness, of the filter slot, measured from the front of the slot to the back. While 1-inch thick filters are common, many systems use 2-inch, 4-inch, or 5-inch thick filters. Using a filter that is too thick can prevent the access door from closing properly or damage the unit. If there is uncertainty about the required depth, consulting the unit’s specifications or contacting the manufacturer is recommended.