If you are trying to replace the air filter in your furnace then you need to look for a furnace supply that can help you. You can find these at most home improvement stores and department stores that sell new and used appliances. These supplies include an air filter and they also include a filter tool that is to be placed inside the filter. If this tool does not fit your filter then it may result in build up in the filters and it will block the air coming out of the unit. It is best to get a good quality filter and to change them often.
There are other ways you can replace the air filter and the most common way is to use a vacuum cleaner. A vacuum cleaner with a filter vacuum can be used to suck the air out of the filters. This method is good because the compressor can suck out the dirt and debris from the filter. This method can also be used with pressure washers, blowers, dryers and other type of furnaces.
You may also want to consider getting a new furnace supply if your existing one is very old. This can be purchased on the internet or from the stores near you. A lot of times if you purchase a new supply you will get a free furnace hose as well. Just remember that when you buy a new supply make sure you have all the tools and ductwork you need. With most furnaces, if you have the wrong tools you can damage your unit. When you purchase a new supply, you should consider how long you plan on keeping your unit and what type of furnaces you own.