What Can Go Wrong

A dirty or clogged filter can restrict airflow and reduce system performance. The Department of Energy explains why filter selection and installation matter. If the filter is wet or you suspect microbial growth, have an HVAC professional investigate the moisture source; replacing the filter alone may not resolve the problem.

Types of Filters

Most HVAC and furnace filters are disposable, made of biodegradable paper or similar media, and shaped in cells, screens, or fins designed to trap as much airborne debris as possible.  To save money, purchase filters in economical multi-packs.  There are many types that fit different models of furnace/HVAC units.  It’s important to use the appropriate filter for your unit.  Using the wrong size filter creates the same types of problems as having a dirty filter.  Your HVAC installer can show you where the filter goes and how to remove the old one and install a new one.  If it comes down to it, your unit may also have an affixed label with directions for easy filter replacement.

MERV?  Merv who?

If you’ve purchased filters, it’s likely you’ve run across the term MERV.  Who…er, what is MERV?  Minimum Efficiency Reporting Values.  In layman’s terms, it’s the filter’s ability to capture larger particles between 0.3 and 10 microns (µm).  Check out the EPA’s page for ratings and efficiencies.  Essentially, the higher the MERV rating the more particles it filters from the air.  Price is usually directly proportionate to the MERV rating.  (Higher MERV = higher price.) 

Which MERV is right for me?

Choose a filter with the correct dimensions and a rating your system can accommodate. Ask your HVAC professional or consult the equipment manufacturer before changing filter type or increasing filtration. A higher rating is not automatically suitable for every system. See EPA’s guide to air cleaners and HVAC filtration.

How Often?

Your HVAC or furnace technician should service your unit once a year.  Because a furnace/HVAC unit contains moving parts, it’s important that belts are not cracked and dry, and ventilation ductwork is not gapped, cracked, or rusted.  Components, such as coils and fans, should be clog-free and adequately lubricated for unimpeded operation.  This sort of evaluation is best left to the professional unless the homeowner has had the appropriate training.

Check the filter monthly and replace or clean it according to its condition and the manufacturer’s instructions. Disposable and washable filters need different care. ENERGY STAR’s maintenance checklist recommends monthly filter attention.

Conditions that may require closer monitoring include:

• 

  If your filter looks like this, change it.

  You run your unit six months a year to year-round.

• You have pets.  Pet dander can become airborne and circulate through the home’s ventilation system just as typical household dust does.
• You have a large family.  More activity means more household dust, dirt, and debris.
• You smoke indoors.
• You or someone in your household suffers from allergies or a respiratory condition.
• You live in a particularly windy area or experience high winds for extended periods.  More so if there are no nearby shrubs or trees to provide a natural windbreak.
• You live in an area prone to or having recently experienced any wildfires.  Airborne ash outdoors will eventually find its way indoors.
• You have a fireplace that you occasionally use.
• You live on a working farm or ranch.  Activity from field equipment and animals kicks up dust.  Dust and dirt then pull into the home’s ventilation system.
• You have a large garden.  Depending on its size and how often you work it, gardening activity can contribute.  This includes tilling the soil, planting, pulling weeds, using herbicides and pesticides, and even watering.
• There is construction taking place around or near the home.  You may be installing a new roof or a pool, or perhaps a neighbor is building a home or addition.  Even if the activity is only temporary, dust and debris from worksites adjacent to or near the house can be sucked into the home’s ventilation system, and this increased activity can tax your HVAC system.

Change the filter immediately if:

• The filter appears damaged.  Regardless of how the damage occurs, a damaged filter that has bent fins, collapsed cells or holes will not work as well as an undamaged filter, especially if it allows system air to bypass the filter at any point.
• The filter is damp.  A filter affected by moisture intrusion, system condensation, or even high indoor humidity can quickly become moldy and spread airborne mold spores throughout the home via the ventilation system.
• There is evidence of microbial growth or mold on the filter.  Mold spores already infiltrating the home via the HVAC system are not only bad for the unit itself, but they can pose a health hazard for the family, ranging from an irritated respiratory system to a serious allergic reaction.  The musty smell produced by a moldy HVAC filter is also unpleasant and may take a while to completely eradicate from inside the home.  If you discover that you have a moldy air filter, it’s important to have the cause investigated further.  An InterNACHI inspector or HVAC technician can help determine the problem so that it doesn’t recur.

Tips on Changing the HVAC Filter

• Turn off the unit before replacing the filter.
• Use the right filter for your unit
• Follow the directions for your unit to make sure you’re installing the filter properly.  For example, many filters use different colors for the front and back (or upstream and downstream flow).  This prevents improper installation.
• Make sure there aren’t any gaps around the filter frame.  If this is the case, you may have the wrong size filter, or the filter itself may be defective or damaged.
• Use a rag to clean up any residual dust before and after you replace the filter.
• Securely replace any levers, gaskets, and/or seals.
• Turn the unit on and observe it while it’s operating to make sure the filter stays in place.
• Note the date of filter replacement in a convenient location for the next time you inspect it.  A filter that becomes dirty enough to change within a short period of time may indicate a problem with the unit or ventilation system, so monitoring how often the filter requires changing is important information for your technician to have.

Call a technician for servicing if:

• Your unit fails to turn back on.
• The fan slows or makes excessive noise, or the fins appear bent.
• The coils are excessively dusty or clogged.
• You notice moisture intrusion from an unknown source anywhere in the system.

Homeowners who take care of the easy task of changing the HVAC filter can help prevent system downtime and avoidable expenses.  Don’t forget the added benefit of keeping your family living and breathing comfortably.  Your InterNACHI inspector can provide more useful tips and reminders during your annual home maintenance inspection.  For other useful homeowner’s maintenance tips, check out our other blogs here.