"Strain on the system" is one of these phrases that the field techs use to excuse just about anything. They want to sell you something new and they want a reaso

"Strain on the system" is one of these phrases that the field techs use to excuse just about anything. They want to sell you something new and they want a reason that purchase is your fault. If you have an excessive pressure drop across the filter you'll have slightly lower airflow across the D/X coil which results in less energy extracted per second the compressor's running. That doesn't "strain the system" in any straightforward way. It's going to increase your compressor runtime, yes, and the delta between your return and discharge temperatures will be less. (The discharge won't feel as cold) Mostly excess pressure drop is caused by dirty filters. A clean MERV won't drop the pressure low enough to result in the D/X icing up so you're still cooling. Depending on your local thermal load, especially in shoulder season (spring, fall) slightly lower airflow might even improve your system's lifetime because it might reduce short-cycling. Every cooling cycle counts towards eventual failure. That's going to depend on your controller, though. Working out the details for your system is an involved engineering exercise. A field tech telling you "MERVs are no good" isn't doing that mental work. A MERV will get dirty and saturate faster so if you're using MERVs you need to change them more often. A saturated filter will slow air enough to ice the coil and then you have a real problem. All that said... if you want very clean air, buy a box fan or squirrel fan and tape a MERV filter to it. Squirrel fans deal with this better but the shape's less convenient. Stick it in a forgotten corner. Don't buy a stand-alone commercial air filtration device. They're a total rip-off.