Filtering Fiction of Filters

I've written numerous articles about filters and I'd like to revisit the subject as it seems that the most common question I get from homeowners is "what filter should I be using?". Clients often get told that the cheaper filters are best and that couldn't be further from the truth, so lets dive in.

It's true that better filters CAN reduce airflow and add stress to an HVAC system, but that's not because a filter is a bad thing. The primary purpose of a filter is keep particulate matter from getting into the duct system, clogging up the evaporator coil, and soiling the duct system. If you don't mind paying for coil cleaning and duct cleanings then you stop here cause there will be no benefit in continuing to read this post. If you don't like paying for those services, this article may be for you.

Filters are rated by the MERV system and in basic terms, the higher the MERV rating, the better job the filter can do at capturing smaller and smaller particles in the air. To help understand the MERV rating system think of it like this, those extremely cheap see through filters at the store might (probably won't even be rated) be rated at a MERV 3. At the higher end of the spectrum for common household filters we will see MERV 13 (it goes higher fyi). A MERV 13 filter can capture particulate matter in the 2.5 range which has a diameter equivalent to taking a human hair and cutting in half, then taking one of those halves and cutting in half again. It's really tiny, matter of fact a MERV 13 filter can capture some viruses if installed correctly.

Now that we understand the purpose and some design parameters of filters, lets talk about why some people are told to buy the cheapest ones. The reason this misconception exists is primarily because our industry is so good at doing a bad job. Go ahead and read that again it wasn't a typo. 90% of the homes I go to have undersized duct systems and oversized equipment. The blower motor in HVAC equipment has a limited amount of power (regulated by government design standards and efficiencies) and only so much of that power can be used to overcome resistance in the duct system. In our industry we use the term static pressure to measure this resistance, but for educational purposes we are just going to call it resistance. Since we do such a good job at bad work, the average system I see installed in my area is already stressed to the max and adding a good filter system and cause undesirable performance issues, basically the straw that broke the camels back. So untrained technicians just tell consumers to use a cheap filter.

With a trained and competent technician who understands the testing process for airflow restrictions, they can tell you what filter the system can accommodate. I'm going to tell you right now, you want the best filter you can have as this helps with indoor air pollution control, dust control, and it protects the very expensive HVAC system that you don't want to pay to replace. With the right tools and knowledge, a good technician can accurately calculate what the resistance will be with a different filter, compare that to the performance data of your equipment, and make recommendations. It's actually rather easy if you know the math. With the right readings, fan law number 2, pressure drop data of different filters, we can tell you exactly how different filters will work with your system.

I don't recommend just installing the filter you want and seeing what happens as this can cause some other issues such as premature blower motor failures, compressor failures, increased energy consumption, and even biological growth issues. It's absolutely recommended to have your system tested and analyzed by a trained technician as well as have follow up testing to ensure the system is optimized for performance.

That's it! I know this was a shorter article, but I didn't want anyone to get lost in the weeds of filter performance data although I do recommend if you are interested in that to go to different manufacturer's websites and look into it. Thank you for reading.