If the filter is too small, it won't cover the entire space and may allow dust and dirt to pass through. 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. The filters only work if air has to pass through them. The filter has air resistance and a gap does not, so if you leave a gap of 1, a disproportionate amount of air will flow through the hole, making the situation even worse.
If the filter is too small, there will be a gap around the air cleaner. This allows dirty air to circulate around the filter. It's almost like not having any filters. 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.
For common brands of HVAC systems such as Goodman, Lennox, Trane, Rheem York, Coleman and Carrier, most standard air filter sizes will work. Be sure to consider overlaps or recessed edges that may obstruct the installation or removal of the filter. When in doubt, you can always measure your air filter yourself with a ruler or any other measuring device. Installing several 1-inch air filters in a 2-slot, for example, can cause costly damage to the air conditioning unit, since residential ovens are not equipped to withstand this level of air flow restriction.
Note that the size that appears in bold on the filter frame is the nominal size of the filter, which is likely to differ from its actual size. It's best to have a little wiggle room in the slot of the HVAC unit, so you can easily slide the filter in and out without it bending or breaking. The MERV air filter rating is a measure of how effective an air filter is at removing particles from the air. This also gives you a pseudo “handle” that sticks outside the opening and allows you to grab it for easy removal when it's time to change the filter.
The first (and easiest) way to find out the size of your oven filter is to check the size of your old air filter. Look for where the return duct connects to the boiler and there should be a plate that covers the slot through which the air cleaner enters. If the filter is smaller than it should be, it won't cover the entire space, allowing dust and other contaminants to pass through the gap. To ensure that you get a filter that fits perfectly in your system, you should measure the interior dimensions of your air filter inlet before ordering a new one.
HVAC service companies often offer air filter replacement services in addition to system repair and unit installation services. If your boiler has an air filter in its return vent, changing it is easy since you can do it from inside your home. If you can't find a standard size filter within that parameter, you'll need to order a custom-sized one.