What is the best air filter?
What is the best air filter for my furnace?
This question is asked a lot. The filter has a couple of functions in the HVAC system. First it keeps the heating and air conditioning system clean. If the coils and heat exchangers get dirty or plugged the unit will not cool or heat. It is like putting a hot pad on your hand when you grab a pot off the stove, no heat is transferred; or grabbing something cold with gloves on, no change in temperature. The second benefit to a filter is it cleans the air. Although rather than clean the air it would be a lot easier to get rid of the source of dirt but this is not always practical. So with that said the best filter for your house is one that keeps the furnace clean and cleans the air.
The question is complicated by the many choices that the consumer has to pick from. When picking a filter the best advise is do no harm. Many of the so called high efficiency filters are very restrictive in the air that will flow through them. So some caution must be taken when replacing a standard type fiberglass filter with a media filter. It is not possible to check a filter’s air flow properties without contacting your HVAC professional. So just like the commercial ”Before participating in any physical activities consult a health professional to make sure your heart can handle it” or “Before installing a high efficiency media filter contact your HVAC professional to make sure your furnace can handle the restricted air flow from the filter. Restricted air flow can cause premature heat exchanger failure, over heating, coils icing and heating failure, short cycling on limits, compressor failure. Yes your furnace will die. Maybe not soon but later.
Your hvac professional will use a temperature rise method and compare it to the name tag specification or they will use a static pressure reading. This number is not normally found on the name plate but is always in the install guide or manufacturer’s specification.
What do I do if I want one of the high efficiency filters but my furnace is not set up for it or when I do install one the furnace overheats due to restricted air flow. The solution to this is not simple but it can be done. Sometimes a simple fan speed change will make the system work within the name plate specs.
Other times, modification to the duct system and filter rack need to be done. Installing a larger filter or two filters will almost all the time solve the problem. Duct sizing and filter location and configuration is critical for this to work. This type of change can be costly and is normally not a do-it-yourself project. Even the seasoned professional can struggle with retrofitting. It’s just because sometimes bumblebees just won’t fly.
The last type of filter is the electronic or large media filters. They are normally installed in the duct system. Just like the media filters if installed wrong or not sized correctly they will not work or cause harm to the furnace.
Filters are labeled with size and ratings that tell the ability to remove dirt MERV. Nowhere on the filter does it show how restrictive the filter is or its air flow. The lack of air flow part is what does damage to the furnace. So it should be the most important factor in picking a filter. Yet this is the hardest factor for the homeowner to determine.