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Air Filters 101: What You Should Know for Comfort and Savings

The job of HVAC air filters may not seem complex, but stop by any major home improvement store and the variety of filters you'll see suggests otherwise. Despite their small size, furnace filters have a big impact: Your heating and cooling system's energy efficiency, the lifespan of your system's components, and the air quality throughout your home all depend in part on the right air filter.

Getting the most from your air filters depends on two main factors:

  1. Choosing the right filter
  2. Keeping the filter clean

Selecting and maintaining air filters may not be intuitive, but it's not difficult once you understand the basics.

Why Your System Depends on Clean Air Filters

It takes just minutes, but changing your HVAC air filter as needed is one of the most critical home maintenance jobs to keep your furnace, heat pump or air conditioner in top form. A clean filter lets you enjoy better system performance while spending less money on energy.

Save on energy bills – Your system is designed to operate with a specific amount of air flowing into it. A dirty filter makes it harder for air to enter. When the system "senses" this restricted airflow, the blower fan motor works harder in an attempt to draw in the required amount of air. To do this, it expends more energy, which in turn, increases your electric bill.

The problem gets even worse when you're running the A/C. When less air flows over the evaporator coil, the system extracts heat less efficiently and is forced to run longer to achieve your selected thermostat temperature. Overall, a dirty filter can reduce your system's energy efficiency by up to 10 percent. Keep your filter clean and you'll enjoy lower heating and cooling bills.

Improved performance – Your furnace's burner assembly requires proper airflow for fuel combustion. Without the right amount of air, the system will burn less fuel than it should, and consequently, produce less heat. An A/C evaporator coil that's not receiving sufficient airflow is not only less efficient, but there's also the risk that the evaporator coil may freeze, which can stop your system from running at all. Poor airflow also impairs your A/C's ability to remove humidity. If you expect your system to do its job of keeping you comfortable, remember to change the air filters as frequently as necessary.

Longer component lifespan – A clogged filter is one of the most common causes of system failure. Restricted airflow places strain on the blower motor and causes excess wear. If you consistently leave dirty filters in your system, your blower motor may fail years before its life expectancy runs out. In the worst-case scenario, the motor may burn out completely. With air no longer flowing through it, your HVAC system can overheat and start a fire. Airflow that's restricted enough to freeze the evaporator coil can damage the coil and the compressor. Just by changing your air filters on schedule, you can avoid expensive repairs.

A cleaner system and home – A furnace air filter's primary job is to keep debris out of the system components and ductwork. Very dirty filters can't perform this task. Left in long enough, a filter can become so clogged that it completely blocks airflow. As the fan tries to pull in air, the resulting air pressure buildup can push the filter loose or even blow it out completely. When this happens, your system is left open to dust, lint, pet hair, and every other type of air contaminant. These can build up on your system's components and in the ducts. Get enough of them and they'll start coming out in clumps through your registers, too.

Know When Your Filter's Ready for a Change

To change your filter on schedule, you'll need to recognize the difference between a truly dirty filter and one that's simply picked up a little dust. By replacing your filter when it's still clean, you'll only waste money on new filters you don't need. Wait too long, though, and you'll subject your system to all the problems a grimy filter can cause.

No matter what kind of filter you have, it's wise to inspect it once a month. The easiest way is to remove the filter and hold it up to a light source, such as a ceiling light. If you can't see light through the filter, it's time to replace it. 

Filter monitoring devices can help you keep track of when to put in a clean filter. These devices measure air resistance across the filter and alert you when the resistance has become abnormally high, indicating a dirty filter.

Your system user guide may also offer recommendations on when to change the filter based on the type of filter, your indoor air quality, and how often you run your system. For instance, a low-efficiency filter will need to be replaced once a month during the heating and cooling seasons. A medium-efficiency filter, however, may last the entire spring or fail when you're not heating or cooling very much.

In general, you'll need to change your filter monthly if:

  • You have a large household
  • Someone in your home has respiratory health problems
  • Someone in your home smokes
  • You have pets
  • Construction or agricultural work is taking place near your home
  • You live in area with high winds that blow contaminants indoors

Signs You Need a New Filter

When your filter reaches the point where it should be changed immediately, you may notice certain symptoms in your system or around your home.

  • Your furniture gets dusty quickly. A clogged filter can release built-up dust into your ducts. This dust eventually will reach your rooms.
  • You notice a dusty or musty smell when the system kicks on.
  • Your system turns on and off more frequently than usual. This problem, known as "short cycling," is caused by impaired airflow.

Three Considerations for Finding the Right Filter

When you're ready to buy a new filter, there are several factors to consider.

Efficiency – A filter's efficiency refers to how well it traps airborne contaminants. The MERV, or minimum efficiency reporting value, rating system makes it easy to compare the efficiency of different air filters. A higher MERV means greater efficiency. Low-efficiency filters are designed only to protect your system, not to clean your air. Lower-end medium-efficiency filters (MERV 5 to 9) clean the air sufficiently for those without respiratory health issues.

For someone with allergies, asthma or a similar concern, MERV 10 to 12 filters are a better choice. Keep in mind that using a higher-efficiency filter can cut your system's operating expenses by up to 10 percent since it reduces your fuel use and maintenance costs. So even if you're in perfect health, you'll still benefit from a higher-efficiency filter.

Filters with efficiencies higher than MERV 12 are so dense they can impair airflow in some HVAC systems, so consult a heating and cooling technician before buying one.

Design – The efficiency of a filter is largely influenced by its design. In the low-efficiency range, you'll find flat disposable fiberglass filters. Among medium-efficiency residential filters ranging from MERV 5 to 13, pleated models are more common. The filter material in these models is bent into a row of V shapes to fit more material in a smaller space.

Size – Air filters come in a variety of heights, widths and depths. To ensure you buy the right size for your system, write down your current filter's measurements before you go shopping. Better yet, take the dirty filter with you.

Changing Your Filter is a Cinch

Before you replace your filter, turn off your HVAC system. In most heating and cooling systems, the air filter is located beside the system's air handler, which may be in a utility closet, crawlspace, attic or basement. The filter is usually secured under a removable or hinged cover.

Pull out the old filter. Check your new filter for damage. If it's in good condition, write down the date of your filter change in a convenient location, such as on the filter frame, so you'll have an easy way to tell how long the filter's been in use.
 
Then look for indications of how the filter should be installed. Most have arrows or a color code indicating the front and the back so you'll know which direction to install it. The filter should fit without gaps around the frame. Noticeable gaps suggest you've bought the wrong filter size. Turn your system on again and pay attention to how it runs. If it doesn't start up, the fan seems slow or you hear unusual noises, turn the system off and contact an HVAC expert.

If you need help making sense of air filter efficiencies, designs and changing schedules, contact us at Comfort24-7.com. We help homeowners around Chicago, northwest Indiana and southwest Michigan, choose and maintain the equipment that keeps them comfortable without breaking the bank.

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