A Guide to Changing Furnace Filters


Any modern forced-air furnace system will almost certainly have an air filter, and changing it regularly is an easy way to reduce heating and cooling costs, improve your home’s air quality, and avoid unnecessary and costly repairs over time. This is especially important for those with respiratory problems or sensitivities to allergens.

A dirty furnace filter can more than double your heating costs, so it’s most important to change it in winter, during which a monthly change is recommended. But if your furnace also includes an air-conditioning system then it should be replaced monthly throughout the year. Neglecting to change your filter when needed can also shorten your furnace’s life span, leading to more serious repercussions. However, many people fail to change their filter often enough.

Getting Started

Before proceeding, it is crucial to be safe and always switch power off to the furnace fan via the main circuit breaker.

The filter is usually found wherever the furnace draws in cold air, at the entrance to the blower compartment, but may be elsewhere depending on your type of furnace, which may be a horizontal, up-flow, or down-flow unit.

Sometimes there will be a second filter in the cold-air return duct, for example, and down-flow furnaces typically use two filters. Most horizontal units will have a slide-in filter that goes directly into the furnace. It’s also possible your filter will be in the central return grill. In any case, your furnace’s user manual should reveal the exact filter location.

Up-flow and down-flow units will require you to remove the blower compartment door to change the filter. It is important that, when reinstalling the door, it properly crosses over the furnace frame to reengage the safety switch, otherwise the furnace will not operate.

Checking And Replacing

Check the current filter by removing it and holding it near a light source. If the light isn’t visible through it, the filter should be changed. Determine the size of the filter, for obtaining a correct replacement, and have a garbage bag handy in which to put it to prevent germs and particles from escaping into the air.

Concerning replacement filter options, price usually corresponds to quality, and inexpensive filters will likely do little to protect your furnace motor and help keep airborne pollutants out of your house. Options range from simple glass-fiber filters to more complex allergen filters, which may either be pleated or use electrostatically charged fibers to trap smaller particles. Also available are more expensive permanent filters, which are cleaned on a regular schedule, saving you money down the road.

Whatever filter you choose, make sure it is reinstalled correctly by noting the arrows that show you which side faces the blower motor. A filter installed backwards will suffer decreased efficiency and lead to the same issues as a dirty one. After sliding in the new filter and properly reinstalling any compartment doors, you can now switch the power back on and run a cleaner furnace.

Have questions about changing your furnace filter? Contact Bell Brothers for more information!