How to Replace a Furnace Filter
Independent professionals recommend changing ventilation filters at least every 3 months.
Energy Star recommends checking filters each month; particularly during the high season (summer and winter are both high seasons if air conditioning and heating are used). If the filter looks dirty after a month of use, it should be replaced.
A dirty filter will slow airflow. This means more energy will be required to heat or cool. A clogged filter can also cause some parts of air conditioning systems to freeze.
A clean filter will prevent the accumulation of dirt in the most vulnerable areas of your ventilation system. This means that in the course of years your system will need less maintenance and costly repairs.
Particular conditions may require more frequent replacement.
Where to locate the filters
Before beginning, switch the breaker of the furnace to OFF in order to cut the current.
The furnace filter is located on the return side of your system, the inlet side. Not the side where there is hot air (or cold air in the case of an air conditioning system).
The filter is usually located inside the furnace just in front of the fan: You get access by removing a back panel or a small side plate in the case of an electronic filter. Usually you have to unscrew a couple of bolts to get access to the filter.
In certain cases the filter can be located directly in the return duct.
How to measure a filter
If you already have a filter, you only have to take its dimensions.
IMPORTANT: Stock filters are usually identified by their nominal dimensions. In fact, the real physical dimensions of filters are about ¼" less than the advertised nominal dimensions. Please take this into account when ordering. In the case of filter thickness, the nominal dimension is the same as the real dimension.
How to install the new filter
Some pleated filters have an arrow indicating the air flow direction. Place the filter in accordance with the arrow.
Attention: Check for air leaks
Pay special attention to the airtight seal around the perimeter of the filter.
An air filter is useful to the same extent as it is airtight around the perimeter. Air always follows the least resistive path. If the filter can move in its casing, air will pass around it. Take care to secure the filter against its frame.