If your furnace uses an air return in the ceiling or hallway, you can replace your air return grill with a filter grill assembly. The filter grill is simple to install and is obtainable at your favorite home improvement store. Simply follow the instructions that come with the assembly.
The filter in the filter grill is far easier to change than the one in your furnace. The grill just drops open where you can pull out the dirty filter and put in a clean one. Another advantage is that the filter grill effectively filters air from inside your home which is recycled to your furnace. This kind of filtration truly keeps your ducting cleaner that filtration from the furnace.
Always use a pleated kind filter which are a little more expensive that the standard kind This kind collects about 40% more dirt. They also need to be changed more often. Allergenic and HepA types are also obtainable if you feel you need this level of filtration. Generally the washable kind filers are not effective. If you can see by the filter, it is not filtering any air. The filter material that you can cut to fit is the worst possible filtering medium. In the industry, the cut-to-fit material is said to be effective against “keeping out birds and kids”. The exception is the washable kind static filter. These are very good. There are other filtering systems that use static electricity to attract particulates. These filtering systems need to be installed by professionals.
With the filter in the furnace, air is filtered above the heating mechanism. With the filter in the air return, the air from the house is filtered before the air is recycled back to the furnace.
Filter grills are manufactured to fit into standard air return grills so finding the correct size is not an issue unless your air intake grill is a non-standard size.
Use either the filter grill or the filter inside the furnace. Not both. Using both restricts air into the furnace too much.
When changing the filter, use an anti-bacterial spray (Febreeze is a good choice) on the filter to help kill bacteria going by the filter in addition as distributing a fresh scent throughout the house.