When Should I Change My Furnace Filter?

Conventional furnace filters capture dirt as it returns from the house before it enters the blower compartment. This means that the side furthest away from the blower, or fan, is the one that’s going to get dirty. As time goes on, the dust and dirt clogs in the pores of the filter and then starts to accumulate in a layer on the filter. If you can easily tell the difference in colour from one side to the other, the filter is ready to be changed. The change is overdue if it looks like there is a dirty blanket of lint built up on the filter.

Filter change past due. Collapsed furnace filter.
Way past due. Sucked into the blower.

So why do some people say to change it every month, while others say three months and some say as long as a year? All these answers are correct for someone, but might not be right for you. The time frame that it takes for a filter to get dirty enough to replace varies by a number of factors. Families with pets, or that have a woodworker creating sawdust in the house are providing more particulate matter for the filter to catch. Some people buy filters that have very fine pores in them to capture pollen and such to help a family member with allergies. These smaller pores clog up faster. In each of these cases, furnace filters are going to need to be changed more frequently.

Conventional filters are one inch thick. Some high end filters have a special housing that hold filters that are as much as five inches thick. Because the pleated filter medium they use has so much surface area, they typically last much longer.

A clogged filter lets less and less air through to the furnace. The blower motor has to strain to provide air to the furnace and may eventually fail; requiring an expensive repair. In other cases, the fan can pull the clogged filter right into the fan compartment. With the filter collapsed and out of the way, dirt will get straight through to the furnace components that the filter was designed to protect.

As a home inspector, I often find filters that are past due for replacement. It seems like a minor oversight but the cost can be significant. You should check your filter every month until you get used to the frequency that suits your family. Set a repeating alert in your smart phone to remind you when you need to have a look.


Author: Rob Cornish is a Home Inspector in Ottawa, Canada. © 2013 HomeXam Inc.
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