I love this topic because I hear all the time that homeowners are told to use the cheapest filters possible so it doesn't restrict airflow. I loth blanket statements like this as they are thrown around without a scientific basis to back up the claim that good filters restrict airflow and hurt systems. I was once told that there's no guess work in HVAC and that every issue can be quantified and it's true, so long as you have the right tools and you know how to use them. Before we talk about filter selection, lets discuss why homeowners are told to buy cheap filters.
When contractors do duct designs they are supposed to start with subtracting the pressure loss devices like filters from the blower motors total available static. This means that the blower motor only has so much power to push through resistance and so we need to account for this. Different filters do render different levels of resistance at different airflow rates. This two part equation often gets overlooked as does the proper duct design process. I have found that even poorly designed duct systems typically support acceptable return airflow for at least a moderately rated filter. Some technicians are telling homeowners that cheaper filters are better because they went to maybe 2 or 3 calls in the last year where they blamed a "restrictive" and dirty filter for the cause of the indoor (evaporator) coil freezing up. The same techs are the ones not using a manometer to measure the pressure drop across a filter, or the static pressure of the return, or the total external static pressure of the system. By using a manometer we can actually determine the amount of resistance to the airflow with a certain filter and properly diagnose this assumption. I can tell you that in my entire career I have found only 5 times that a client was using a filter that was too restrictive for the application and all 5 of those were using Merv 13 or better filters. What is a Merv? I'm glad you asked!
Merv is an acronym standing for minimum efficiency rating values. In laments terms that alludes to the filters ability to capture particulates. Filters with a higher Merv rating capture smaller particulates than filters with a lower rating. The smaller the particulate the filter captures, the better the filter is... However there is a tipping point if your system wasn't designed to accommodate a Merv 13 filter, then the blanket forementioned claim has value.
Now that we understand how the Merv rating affects the types of filters available lets discuss the range in detail. The really cheap filters you can find at walmart, lowes, home depot, amazon, and hard ware stores usually don't have a Merv rating because they're not capturing much debris to talk about. In my personal opinion the Merv scale isn't important until we get to the Merv 7 and 8 filters. This is as basic a filter I will ever recommend as I've done many field test and found that even the worst duct systems out there will still accept a Merv 7 or 8 pleated filter without causing issues. I also don't recommend going any higher than that without first having your system tested with a manometer and different filters to ensure your system won't see a negative effect. I've told clients that have my maintenance plan to have a Merv 13 filter handy during a service and we can test the system with that filter to see if it will work. Furthermore I will instruct them that before the next service, let whatever Merv filter we went with to stay in for about 2 months prior to the next service so I can check the resistance to airflow with a dirty filter. This allows me to offer advise for the frequency at which the filter needs to be changed.
Every situation is going to be different and unique. We all use our homes differently and expose them to different environments so there's no such thing as a "one size fits all". Just another reason I don't like baseless generic claims without proper testing and analysis. So what can you do if you have a poorly designed system, but you want better filtration?
There are two easily implemented solutions to making your system accept a great filter. One way is you can have your 1 inch filter grille changed out for a 2 inch (thick) filter grille and use 2 inch deep filters. You might think that a thicker filter will be more restrictive, it's actually the complete opposite. When we use thicker filters we increase the surface area of the pleats for airflow to move through the filter thus reducing the restriction to the airflow. The only down side to this option, if any, is that 2 inch filters are less common so you will likely need to purchase these filters online which may not be an issue for most as we live in the age online shopping anyways. The second option you have is to increase the size of your return duct(s) and maybe a slight modification to the way the return is ran to your equipment. Sometimes this option can be easy and sometimes a little invasive, but the benefit is you can stick with a 1 inch filter that's easily purchased locally. My personal preference is a thicker filter rack because of the increased surface area, this allows for a longer service life of the filter and reduces the frequency at which you will need to change it.
There is another option although it's not always practical is having a 4 inch media filter cabinet installed. 4 inch media cabinets are amazing products with typically very low pressure drops, but the biggest draw back to them is they are designed to be installed in the duct system or at the air handler. If your air handler is in an attic or crawl space, someone needs to access this at least a couple of times a year to change the filter. If you have a service plan with a provider that can do this for you on a routine basis then I see no issues, but that cost money so you'll have to accept that with the 4 inch media cabinet or be willing to do it yourself. In the rare occasion you may have your air handler in the conditioned space in which case a media cabinet is absolutely the best and appropriate solution to air filtration.
I hope this sheds some light on the issue and helps you to understand the differences in filters. I also hope that the next time you get told to buy the cheapest filters possible that you either dismiss it or ask your tech what the pressure drop is of your current filter as supporting evidence for the claim.
Kenneth Casebier
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