High MERV filters, specifically MERV 12 to MERV 16 filters, stop more and smaller particles of pollutants, allergens and other junk that leads to bad indoor air quality (IAQ). That’s a fact. The terms “more” and “smaller” are both important when discussing healthy IAQ in your home.
That seems like a good thing – and it is.
So, what could be wrong with installing a high MERV filter that removes more and smaller particles? Quite a bit – poor airflow, higher energy costs, an uncomfortable home and HVAC equipment failure…Those are facts too.
It turns out that sometimes due to unintended consequences of high MERV filters, the cure is worse than the problem.
Why a high MERV filter might be wrong for your furnace or AC system/air handler/heat pump…
The important relationship between MERV and airflow
How to choose the right MERV rating filter for your furnace or AC/heat pump air handler – your HVAC system.
MERV Rating Overview
If you’re up on MERV ratings, this section might not tell you anything you don’t know.
However, if not, this information will answer “what is MERV rating? What is the best MERV rating for my furnace or air handler?” and similar introductory questions.
MERV Rating: MERV is the Minimum Efficiency Reporting Value – The gist of MERV is a measurement of the smallest (minimum) particles a filter will effectively (efficiency) remove when tested – with test results that are verifiable and reportable (reporting value).
The higher the MERV, the smaller the particles removed, and the more of them removed.
- MERV 8 filters are effective at removing most 3+ micron particles like lint, dust, most pollen and mold spores, dust mites and their debris.
- MERV 10 – MERV 12 filters remove most 1.0 to 3.0 micron particles plus bigger ones. These include all those listed under MERV 8 filters plus pet dander, bacteria, most smoke/smog particles and most auto emission particles.
- MERV 13 – MERV 16 filters remove most .3 – 1.0 micron particles plus bigger ones. These remove all the above and reduce most viruses and odors.
What is the best MERV filter for your furnace or air handler? The short answer is this: The best MERV rating for your heating and air conditioning equipment is the MERV recommended by the manufacturer. It’s usually in the range of 8-12 MERV.
It might be listed in the furnace or air handler owner’s manual, and if you don’t have the manual, a PDF of it can be found online by searching the brand and model furnace.
When in doubt, choose a MERV 8 to MERV 10 filter rated for high airflow, so it won’t damage your furnace or air handler.
Or you can ask a certified HVAC technician what filter is best for your furnace or air handler.
OK, let’s get into the details of why a high MERV furnace filter might not be good for your furnace.
The High MERV Problem with MERV 12 to MERV 16
The highest MERV rating for 1” filters is 14, though those are hard to find. 3M Filtrete filters are available in a 1” MERV 13 as the highest rating. Filters 4” and 5” thick can have MERV ratings up to 16.
Why are high MERV air filters a problem, at least for some furnaces? Because air filters restrict airflow, and your HVAC equipment needs sufficient airflow to work efficiently and durably.
Think about wearing a mask during the COVID pandemic. Yes, I’m sure you’d rather not think about it – but it serves as a good illustration. The point of the mask is to filter virus germs out of the air passing through them. And as you know, some masks are very restrictive and make breathing difficult.
A furnace or air handler filter serves the same purpose.
1. The higher the MERV rating, the more restrictive the filter might be. Plus, a MERV 12 filter that is just 1” (one inch) thick will likely be more restrictive than a MERV 12 filter that is 4” (four inches) thick.
2. If the MERV rating is too high for the furnace or air handler blower motor to handle, low airflow will be the result. A clogged air filter of the “right” MERV rating will do this too.
Low airflow. That is the important phrase. Low airflow is the source of many HVAC equipment problems.
HVAC systems have a blower motor and fan. The fan pulls air through the air filter and into the furnace or air handler where it is heated or cooled before pushing it through ductwork to the rooms in your house.
Low airflow causes lower pressure on the other side of the filter: The systems are designed to handle a small air pressure drop caused by the air filter.
The more restrictive the filter, or the more clogged it is, the bigger the pressure drop. Imagine blowing through the screen in your window. There’s very little pressure change from inside to outside. But then imagine blowing through a thick curtain covering the window – there would be a large pressure drop. You might barely feel the air if you put your hand on the other side of the curtain.
Warning Signs: When You Shouldn’t Use High MERV Filters
The easy answer is that you shouldn’t use a filter with a MERV rating above what your furnace or air handler is rated for.
The more complicated answer is that you shouldn’t use high MERV filters when you notice warning signs like these:
Low airflow through air vents: If you notice that the air isn’t flowing out of the grates as robustly after you install a MERV 12 or higher filter, that might be a problem. Air filter issues are one of the biggest causes of low airflow.
Indoor discomfort: Are rooms furthest from the air handler or furnace not getting enough of the treated air to make them comfortable? In winter, does a distant room feel 5 degrees cooler than others? Or warmer in summer when the AC is running? Low airflow!!!
Higher energy bills: Are your heating and AC bills higher after you put in the MERV 12 or MERV 13 filter? In fact, it could happen with a MERV 10 pleated filter if you’ve been using the cheap spun fiberglass filters that have about MERV 6. The higher costs mean that your system is working harder than it should to push air through a dense filter, and that overtime effort is showing up in higher electricity and/or gas bills. Another result of low airflow!
Danger – Mechanical Problems Related to Low Air Flow
Some homeowners “live with” comfort issues and higher energy bills if they think their indoor air is cleaner.
But when mechanical issues start happening, they usually take notice. Here are potential problems that might occur when you shouldn’t use high MERV (12+) filter for the furnace or ac/heat pump air handler.
Did you ever feel your lungs working harder when wearing a mask? Well, a filter with a MERV rating too high to handle will cause the furnace or air handler “lungs” to work too hard. The motor wearing out prematurely is a common result.
This is a worst-case scenario, but it happens. Our culprit, low airflow, often means the thermostat is hard to satisfy. During AC, let’s say you set it to 75, but the low airflow means everything is working harder, and it can’t get the temperature below 77. So, the thermostat keeps the compressor going well beyond what’s good for it, and it fails. This can happen to a heat pump too during a winter heating cycle.
Coil too Cold
The AC coil in or near your furnace or air handler gets really cold as it takes heat out of the space and pumps it outside. A normal flow of warm air over the coil will prevent its surface from dropping below 32F. Low airflow allows the coil to become freezing cold. Then moisture from the air freezes onto it, and the coil becomes encased in ice. At that point, your AC system can’t do its job.
Unthaw Tip– Turn the thermostat from AC mode to Fan mode, and let warm air flow over the frozen coil. It will soon thaw. Or you can just turn off the system and let it thaw more slowly. In either case, have towels handy to collect any moisture that overwhelms/overflows the pan and drain in the furnace or air handler. And then revert to a lower-MERV filter to prevent low airflow.
AC Troubleshooting– If this problem happens after installing a high MERV filter, the filter is likely the issue. If it happens at other times, it could be caused by a clogged filter or a clogged drain pan or line. See our AC Drain Line Clog & Cleaning Guide for more helpful information on this topic.
Alternatives to High MERV Filters
The point of using a MERV 12 or higher filter is better IAQ – cleaner, more breathable air for all, and especially those with allergies, asthma, COPD, etc.
Have you considered a room air purifier? It’s a piece of equipment with the sole purpose of cleaning the air in the room.
If you’re in a situation when you shouldn’t use high MERV (12+) filter for furnace or ac use and would like to research air purifiers, here are two suggestions.
> Start with our Best HEPA Air Purifiers Guide. A HEPA filter is the equivalent of a MERV 17 or higher filter, so you’ll get very clean air.
> The other option is to browse our Pick HVAC Air Purifier pages for a wealth of information such as separate pages on the best air purifiers for allergies, dust, odors and mold.