Furnace Filters

By Tom Wurzer Warren Engineering

Homebuyers frequently ask us questions about furnace filters.

• How often should I change my filter?

• What type of filter is the best one to use?

• My kids have allergies. What filter should I use?

• Are electronic air cleaners superior to standard filters?

The Purpose of the Filter

The furnace filter is a critical component of a forced air heating and/or air conditioning system. The primary purpose of the filter is to protect the mechanical equipment (i.e. the furnace fan, the heat exchanger, the air conditioning evaporator coil, etc.). The filter prevents dust and debris from entering the equipment and it is located on the return air side of the system. It is the first component of the system that the air from the house goes through.

Diagram shows the position of the filter to the left of the fan

Many allergists will recommend better, high-efficiency filters for your furnace to try and reduce dust, pollen and other allergens in the home. There is very little evidence that a better filter will reduce allergens in the home. It may slightly reduce the number of smaller sized particles, but most sources do not report that it will reduce dust in the home. Regular vacuuming with a high-quality vacuum and general good housekeeping provide greater benefits in this regard.

Frozen air conditioning coil

If the filter is not maintained and changed or cleaned at proper intervals, the heating and cooling equipment can be damaged. A dirty filter will reduce the amount of air that flows through the system. This will reduce the efficiency of the system and increase utility costs. If the air flow is blocked too much, the filter can collapse. Low air flow can lead to excessive heat and premature cracks in the heat exchanger, frozen air conditioning coils, and other equipment problems.

Extremely dirty furnace filter

Types of Filters

When furnaces come from the factory, many come with a filter. These are typically low-efficiency washable filters that are meant to keep large construction dust and debris out of the unit. These are often referred to as “construction filters”. Although you can continue to wash and use these, we typically recommend removing them after construction or renovation is complete. Replacement with a higher efficiency filter that removes smaller dust particles is typically recommended.

Examples of washable filters that come with some furnaces

Standard residential furnace and air conditioner filters are typically one-inch thick and with varying dimensions for width and height. Typical sizes include 16”x24”x1” or 16”x20”x1”, but there are many, many sizes. With the ease of today’s internet shopping, one can quickly search on-line for retailers that carry the specific filter size that you need.

Standard filters typically range from what is known as a 30-day filter, to a 90-day filter. Flat fiberglass filters are usually the 30-day type. If homeowners are religious about changing their filters, these can work adequately. However, the pleated 90-day filters allow more room for error, in terms of forgetting to change the filter at regular intervals. The greater surface area created by a pleated filter actually reduces the pressure drop through the filter, as opposed to a flat filter.  The pleated filter can hold more dust and dirt prior to needing a change.

For even more surface area, thick pleated filters are used. Typical brands include Air Bear®, Space Guard®, Aprilaire®, Honeywell®, Skuttle®, and more. With the increased surface area of a 4 or 5-inch thick pleated filter, the filter material can be a tighter weave that removes smaller particles. This greater surface area allows the filter to only require a change every 12 months under typical operation and every 6 months if running the fan continuously.

Examples of clean and dirty thick pleated filters

During the 1980’s and 1990’s, electronic air cleaners (also called electrostatic precipitators) gained popularity.  Relatively few of these filters exist today. They are considered relatively ineffective as filtration devices. Once the metal plates in these units are dirty, most dust and dirt passes right through them, into the furnace. Further, these devices generate non-adjustable levels of ozone. This could be an irritant to people with asthma, and there is debate regarding whether or not ozone could be a carcinogen. We typically recommend replacing the pre-filters and filter cells in these older electronic air cleaners with a thick pleated filter designed to fit on the same housing.

There are some newer electronic air filters on the market with more measurable ozone generation levels that use a combination of washable filters and electronic grids to filter the air. These are still relatively rare in the marketplace and the jury is still out in terms of the effectiveness and reliability of these filters.  They do require periodic cleaning maintenance.

Old and new style electronic filters

Filter Ratings

High efficiency thick pleated filters, and some 1-inch filters, carry an efficiency rating known as the “MERV” rating (Minimum Efficiency Reporting Value). The MERV ratings for home furnace filters vary from 1 to 16.  Higher MERV ratings correspond to a greater percentage of particles captured on each pass of air through the filter. Unless engineered properly, with a greater efficiency (i.e. higher MERV rating) comes a greater pressure drop through the filter.

Low-efficiency fiberglass filters typically have MERV ratings of 4 or less. Pleated furnace filters will have MERV ratings ranging from 5 to 16. Typically, a MERV rating of 5 to 8 is adequate for most homes and brings less risk of excessive air pressure drop through the system.  Higher MERV ratings of 11 to 16 can be used, but one must be sure that the filter rack or housing, and the furnace, are designed to use such filters properly. One must also be more careful to change the filter at regular intervals.

Duct build-up on furnace fan from lack of a proper filter

The Basics

In general, furnace filters exist to protect the furnace and the air conditioning coil.  Minor benefits from a good filter might include a slight reduction of allergens in the home, but this is debatable. Main points to remember include:

• Make sure the furnace has a properly installed filter. Many furnaces we inspect have no filter at all, or they do not have a rack or clip to hold a filter. Many times the filter is sitting loosely in the fan cabinet and almost all of the air flowing through the furnace is bypassing the filter. Seal off gaps around filters to prevent air from bypassing the filter.

• Change the filter as frequently as recommended by the filter manufacturer. This is typically a 30-day, 90-day, 6-month, or 1-year interval. If the filter is washable, such as original furnace filters or electronic air cleaners, make sure cleaning is done at recommended intervals.

• Consider replacing older electronic air cleaning pre-filters and filter cells with a thick pleated filter. These pleated filters are actually more effective in most cases and have less maintenance associated with them.

Please do not hesitate to contact our office if we can be of assistance in this regard, or in regards to other issues related to home inspections.

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