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Nicor Gas

Furnace Filter FAQ

Furnaces are one of the most crucial pieces of a comfortable home. You’ve probably heard that changing your furnace filters is a small step than can create big energy savings, plus increase the efficiency of your equipment and help keep your furnace running longer. This DIY energy-efficiency step is simple once you’re fully in the know about furnace filters.  

Why do furnaces have filters?

Furnace filters are an important part of your overall furnace system. When the blower draws air in, it passes through the filter, helping to keep dust and pollutants like pollen, pet dander, and tobacco smoke out of the air.  According to Consumer Reports, leaks from windows, doors, and other places in a home mean that the air in the typical house is replaced by outside air every 2 hours. Filtering indoor pollutants can make a huge difference in air quality.

Why do they need to be replaced?

Dirty filters increase energy costs and can damage your equipment. Clean filters also ensure better air quality throughout your home.


dirty and clean filters

What do the ratings on filters mean?

All furnace filters have a MERV Rating (Minimum Efficiency Reporting Value). Established by the American Society of Heating, Refrigerating, and Air Conditioning Engineers, MERV rates a filter’s ability to capture and hold particles and pollutants. The rating numbers range from 1-20, with 1 capturing particles about the size of carpet fibers and larger and 20 capturing particles as microscopic as viruses. While a place like a lab or hospitals will need to install higher-rated furnace filters at their facilities, most homes can use a filter rated 3-4, which will trap dust mites, pollen and other common pollutants.

How often do you replace them?

Most guidelines recommend replacing them every one to three months, depending on the type and efficiency of your furnace. Your particular furnace should have specific guidelines. Ask the contractor who installed your HVAC system or find the brand name on your unit and search the manufacturer website to find their specific recommendations.

If a filter has a plastic frame that means it’s reusable and only has to be cleaned periodically with a vacuum and water, drying completely before putting back in the furnace.

How do you replace them? Which way do you put the filter in?

Before changing your filter or performing any maintenance, turn off your HVAC unit. If you don’t know how to turn off your equipment, switch it off at the breaker. Air filters are often located on one of the sides or the bottom of the unit and easily pull out of a slot. Some filters are found in air vents instead. Look for the markings that tell you which side of the filter should face the furnace – air flow arrows should face the furnace. 


Which furnace filter should you use?

There are plenty of resources to help you find the right filter for you. The HVAC department or filter aisle of your local hardware store is a good place to find a quality furnace filter at your budget. You can find the Consumer Reports picks for best filters here.

Which size furnace filter do you need?

The best way to find out what size filter you need is to pull out your current filter and look at the dimensions listed on the side. Dimensions range from 10”x10” to 30”X36” and most, filters are 1” thick, but some homes will have 2”-5” thick filters. The actual size of a furnace filter are the true dimensions, for instance 19 ⅝” x 19 ⅝” x 13/16. The nominal size of a filter is the rounded number used by manufacturers, marked as 20 x 20 x 1.

Does the filter have to be an exact fit?

Filters should fit as closely as possible. Using an incorrectly sized filter is like not using a filter at all because particles will just pass around the filter. This can cause damaging build-up inside your furnace and decreased air quality.

Where can you buy furnace filters?

You can find furnace filters at home improvement stores and hardware stores. Some online stores that exclusively sell furnace filters often have an auto-ship option, making sure you always have the right size filter when it’s time to replace them.

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