by Garet Dennis
The single most important thing you can do to prolong the life of your furnace is to change the filter regularly (regular professional service is a close second). A dirty filter will cause the furnace to overheat and wear out sooner. An incorrectly installed filter will let air go around the filter instead of through it. This will clog up an air conditioning system. Air filters should be changed monthly during the heating season. If you have air conditioning then change it monthly during the summer as well.
The first step to changing the filter is to turn off the electrical power to the furnace (remember to turn it back on afterwards). Look for a switch near the furnace - it might look just like a light switch, or it may have a small silver fuse box with the switch.
Next find the filter. There are 3 typical locations:
On many older furnaces (and some newer furnaces) the filter is inside the blower compartment. You will need to open the furnace cabinet to change the filter (that's why you want to turn off the power first).
On many newer furnaces the filter is inside a door cut into the return duct next to the furnace. In this case it's important to set the filter on an angle (see the diagram). If you try to set the filter upright it will fall over and be useless.
Sometimes the filter will be located behind the return air grill in the living space. This is rare, but is becoming more common in houses with an attic-mounted furnace - it saves you from going into the attic to change the filter.