Rumer-Loudin, Inc.

What you didn’t know about filters!

An important but often overlooked part of home maintenance is checking and changing your furnace filters.  In fact, get up from the computer right now and go check them.  I am serious!  Many furnace, heat pump, and air conditioning repairs could have been prevented by simply keeping up with filter cleaning and/or changing.  Often a homeowner will clean/change it right before we get there, but we can tell from the condition of the equipment that the filter has been neglected. 

The return (often called cold) air duct provides an airway by which the central forced air heating and air conditioning system draws in air to be conditioned (heated, cooled, humidified, cleaned, filtered).  The air is then distributed throughout the building via the supply air ducts.  The air being drawn in is full of airborne pollutants such as dust, pollen, pet hair and dander, smoke, odors, etc.  It is the filter’s job, which is on the return air side of the system, to stop the debris.  How much is stopped depends on the type of filter.  What is not stopped accumulates in the furnace and supply air ducts.

 When a system operates with a clogged filter, it is working harder than necessary, which decreases efficiency.  In addition, it can stress the parts on your furnace including the heat exchanger because the hot air cannot be carried away. Insufficient air flow can cause an air conditioning evaporator coil to freeze up which is very damaging to the system.  Newer high tech motors are more sensitive to air restriction and can become noisy as well as damaged.

Filter types (except electrostatic) are assigned a MERV (Minimum Efficiency Reporting Value) rating between 1 and 16 which indicates the ability of a filter to remove dust from the air as it passes through the filter.  This is also called dust loading capacity.  The higher the rating, the fewer pollutants pass through the filter.  The size of airborne contaminants is measured in microns. For instance, a human hair is about 100 microns, pollen and spores 10 to 100 microns, dust about 8 microns, tobacco smoke is .01-4 microns.

Deciding on the type of air filter depends on whether you want to remove as many contaminants as possible due to allergies or whether you just want to keep larger particles out of your system for good maintenance.  The most desirable filter is one that has the least restriction while catching the most pollutants.

Regardless, fiberglass throw away filters are not the best option.  They were originally designed to keep large items, such as a piece of plastic or paper, from being sucked into the system and causing major damage.  They were never designed to provide small particle filtration.  Their MERV rating is typically between 1 and 4 and will not stop anything smaller than 10 microns.

Here is a summary of additional filter types plus some pros and cons.  Media air filter pads are used with or without a frame, is typically purchased precut or in a role and cut to fit, is more efficient than fiberglass, and do not need changed/cleaned as often as the more efficient filters but they can be messy to change and will not be much help for allergy sufferers with a MERV rating around 3 or 4. 

Electrostatic air filters use static electricity caused from the friction of air flow to attract debris.  They are a framed, permanent, washable filter available in the most common filter sizes.  The best quality electrostatic filters are about 90% efficient.  If not cleaned monthly, they cause restriction and eventually service problems so maintenance diligence is important. 

Pleated air filters use the increased surface area from the pleats to increase dust loading capacity with MERV ratings between 5 and 12, which makes it a good choice for allergy sufferers.  They are cardboard framed, disposable and available in many sizes.  They must be checked frequently so restriction does not cause air flow problems.

HEPA (High Efficiency Particulate Arrestance) filters are very efficient with MERV ratings of 14-15.  Their pleated design increases dust loading capacity and are a good choice for those with allergies.  Most are disposable, but some can be cleaned and reused.  These filters, when clean let alone dirty, can cause restriction so you may need to speak with a heating and air conditioning contractor before installing them.  Some brands require fabricated sheet metal return air adapters and professional installation, yet they are the most efficient filters available for residential use.

This has been a review of filters not air cleaners, which is a subject for a different article.  Remember, your furnace and ductwork is the lungs of your home.  If you don’t keep up with filter maintenance, it’s like trying to breathe with a hand over your nose and mouth. For more information, go to

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