clean furnace filter

There seems to be an air of mystery surrounding the air filter in your furnace and we often get the same questions about them from homeowners on a daily basis: Why do I need one? How often should I change it? What kind of filter should I get? And what does that little arrow mean?


First, a basic summary of how your heating and cooling system works to move the air in your home. Throughout your house, there are return vents (they suck air in) and supply vents (they blow air out). The movement of air in these vents occur anytime the blower at your furnace comes on. The blower sucks the air in your home through the return vents and back to the furnace. This air is then conditioned (heated or cooled) at the furnace compartment. Finally, that conditioned air blows through the supply vents to heat or cool your home. This process can happen 20 or more times per day.

Now imagine the things that may be floating around in the air of your house: Dust? Pet Hair? Pollen? Anything and everything, whatever it may be, will be sucked through the return vents and back to the furnace anytime the blower comes on. The air filter is there to catch these things from entering the “inner-workings” of the heating and cooling system. Without a filter in place, these particles would be allowed to enter into the system, and over time, clog it up. This would cause the system to run much less efficiently (higher utility bills), shorten its life, and become the cause of home heating and cooling problems.

Along with protecting your furnace, another advantage to an air filter is they clean the air you breathe in your house. As we like to say, there are basically two air cleaners in your home – your furnace filter and your lungs. With a dirty filter, or none at all, all those airborne particles continue to circulate throughout the home anytime the blower comes on. Yuck!


dirty furnace filter The simple answer – “When they’re dirty!”, but there are no hard and fast rules. Maybe you’ve seen “3 Month Filter” or “Replace every 30 days” printed on the packing of a filter bought at the store. These are generally a guideline and an estimate of how long the filter will last. There are a number of factors that affect how long that actually may be however, and every house is unique. Imagine two examples:

A) A 1200 square foot home in the city with two occupants, no pets, and whose furnace is located in a basement


B) A 3200 square foot home near a dirt road with six people, three dogs, two cats, whose furnace is in a dusty crawl space.

While example “A” may be able to go 3 months between filter changes, example “B” may have to change their filter every 30 days.

Letting a filter “go too long” has several negative consequences Filters are manufactured with tiny pores allow air to pass through while the “pleats” of the filter catch the small particulates in the air and prevent them from entering the furnace compartment. Over time, these holes get clogged up with the trapped dirt and dust, making it difficult for air to pass through. Lack of air flow causes the HVAC system to work much harder to heat and cool your home (higher bills), makes for longer run times (less comfortable) and puts strain on the components system (early breakdowns). It’s like putting a towel over your mouth while trying to breathe.

A good rule of thumb to follow: If there is a light gray coating of dust of the filter, it’s time to replace it. It’s much better to replace more often than trying to squeeze another month out of a dirty filter.


-The correct size

The best way to see what size filter to purchase is to look at the filter that is already installed in the filter rack of your system (assuming your system was installed properly). The size is printed somewhere on the filter and the most common sizes are sold at home improvement stores. A correctly sized filter should fit snug in the filter rack and filter all the air before it enters into the furnace compartment. The wrong size can allow air to pass around the the filter rather than through it, essentially rendering the purpose of the filter obsolete.

-A filter that catches a majority of airborne particles, while still allowing airflow through the filter-

The effectiveness of a filter is determined by a number called the “MERV Rating” and can range anywhere from 1 to 16 in residential settings. Generally, the higher the MERV rating, the more effective the filter is at catching airborne particles, You might think a higher rating MERV rating is better, but that’s not always the case. A filter with a higher MERV value has smaller pores for which the air to pass through, which clog up sooner, creating resistance of airflow into the furnace, causing the system to become inefficient. These more expensive filters, in turn, need to be changed more often. The same applies for “allergenic” filters. The pleats in these are tight and compact and can cause the same issues if not changed on a regular basis. We recommend a MERV 8 pleated filter

An electronic air cleaner (EAC) is the best of both worlds! It simultaneously filters out over 99+% of the tiniest airborne particles, including allergens, smoke, and dust mites while still allowing proper air flow through the filter. The up front cost of an electronic filter can seem significant, but the value over the long term make them a great investment. They clean the air in your home and prevent dirt and dust from entering into the HVAC system better than any pleated filter, potentially saving you money on avoidable furnace and air-conditioning repair calls. I have seen furnaces that are over 10 years with an electronic air filter in place that look nearly brand new. Depending on the type of EAC, it either has cartridges that need only to be taken out and washed a few times a year or a disposal insert that is replaced every few months.


The air goes in the direction of airflow, meaning it points towards the furnace. On the front side are the pleats and on the back is either a metal mesh or sometimes a cardboard frame. The back side is built as a support system so as air is pushed through the pleats, the back side acts as support to keep the filter from collapsing on itself.

Often overlooked and sometimes forgotten, the air filter in your home’s heating and cooling system is one of the most vital pieces of a smooth, efficient and long lasting HVAC system. By simply replacing this every few months, a majority of costly A/C and furnace repair calls could be avoided, provide cleaner air in your home, and keep your system at peak efficiency.