The general consensus is that the thicker the air filter, the better the efficiency. However, it is important to never try to force an air cleaner to fit in a space it's not designed for. If you try to use a 4-inch thick air filter for a system that is made for a 1-inch thick filter, the efficiency will actually be worse. Most air filters have their size printed on the side of the frame.
The length, width and depth of the filter are measured in inches. For example, you'll say something like “16x25x1”, where 16 is the length, 25 is the width, and 1 is the depth. Most HVAC systems are designed to accommodate 1-inch thick filters. However, you can choose not to use factory-installed filter grids and have an HVAC professional install an external filter grid for a thicker filter.
The easiest way to identify the size of the AC filter is to remove the existing filter from its slot and examine the filter frame. If your old air filter doesn't have the measurements printed on its side, you can measure the filter yourself with a ruler or any other measuring device. Most pleated and white air filters are inherently the same within each efficiency level and vary slightly, if at all, between brands. A filter that is ready to be changed will be gray and covered with build-up, preventing air from easily passing through it. You also don't want to place a 1-inch air filter in a 2-slot slot because air takes the path of least resistance and will flow around the filter rather than through it if it's not the right size. When in doubt, you can always measure your air filter yourself with a ruler or any other measuring device.
First, 4-inch filters cover a higher range of MERV ratings, with a slight overlap with 1-inch filters. For example, a filter labeled 10 X 12 X 1 is 10 inches long, 12 inches wide, and one inch thick. Having to bend and force the filter into the boiler is a clear indicator that the filter is too large. In this case, you'll need to round down to the nearest whole number to determine the nominal size of the air filter. A restriction of air flow can also be observed through the increase in the heating time of the boiler.
A filter that is too small or two large filters will create holes around the filter, which in turn will allow air to flow on the sides. Once you've determined the exact dimensions that best suit your needs, order the filter size with the exact fraction or decimal of your preferred measurements. It is important to remember that using an incorrect size of furnace filter can lead to decreased efficiency and increased energy costs. Therefore, it is essential to make sure that you are using an appropriately sized furnace filter for your system in order to maximize efficiency and reduce energy costs.