All furnace air filters are the same, right?

A system you’ve been sfiltersservicing since it was new suddenly starts acting up.

If it’s a furnace the high limit is tripping. If it’s an A/C unit the indoor coil is icing up. The strange part is it ran fine for years. What the heck happened? I’ll tell you…

Open the filter access door and there’s a good chance you’ll find the culprit; the ever popular 1” pleated, super-duper, MERV 7 air filter.
Why are we down on 1″ pleated filters? We’re not! We’re down on home owners installing them without checking with a professional first (yes, we mean you)

Let’s start with how well they work; they work GREAT! I mean it; they filter out dust that you would need a microscope to see. But then, so does a piece of cardboard. I’m not kidding; cardboard won’t let dust pass through it. So, why don’t we don’t use cardboard for air filters? Because it doesn’t let air through either.
If you look at a typical fiberglass air filter the beginning pressure drop is .04”wc. The manufacturer recommends the filter be replaced when the pressure drop is .50”wc. Now let’s look at a typical MERV 7, pleated air filter; its beginning pressure drop is .40”wc, and it’s considered dirty at 1”wc.
The pleated air filter’s beginning pressure drop is almost as much as the fiberglass filter’s pressure drop when it’s time to change it. What does that mean to the furnace or air handler? Reduced air flow, high limit trips, and iced up evaporator coils.
Are we saying 1” pleated air filters are no good? Absolutely not! What we are saying is that something as simple as changing the type of filter can cause system problems if it’s not done correctly. First, the filter needs to be rated to handle the CFM that the unit requires. Second, changing from a standard air filter to a pleated air filter will likely require an increase in blower speed to compensate for the static pressure increase, and accompanying decrease in airflow. And third, you may have to change the filter more frequently.

Patrick is Zen HVAC’s diagnostic and training guy. Patrick started in the trade the day he left technical school and never looked back. He's served in various technical and training roles in the HVAC industry but specializes in system troubleshooting and diagnostics, retro commissioning, and technical training. His moto: If I can understand it, anybody can. Patrick uses the Zen common sense approach to teach Patrick’s Likes- His Wife, kids and dog. Old pickup trucks. Hiking. The industrial Revolution. Patrick’s Dislikes- Taking work too seriously. Anything unintuitive. Emoticons :( Patrick’s Favorite famous person- Theodore Roosevelt “I am only an average man, But I work harder at it than the average man" Famous Patrick Quote- “Well, that was stupid of me”

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