Updated: Aug 13, 2019
Recently, while doing my job, I ran across a customer objecting to my filter advice. "My husband's maintenance helper recommended that filter." It is a common, complex problem that affects every air conditioning system--AIR FLOW.
It is all about the AIR FLOW. Air Conditioning requires proper air flow both inside and outside your home to function effectively and efficiently. Think of your car. Think of yourself. If something can not breathe, it will not function right.
When it comes to cars, I go to my Dad for advice. He is right every time. When I first purchased an older pickup I have, albeit with very very low miles, it stopped running properly several months later. I went through everything I did to fix it with my Dad, and answered all his questions--the problem was not being solved by typical solutions. He said, "Let me sleep on it, and I will call you in the morning." Next morning, he called me and asked, "Is your exhaust muffler stopped up or collapsed?" It was the one thing I hadn't checked. Dad was right. I had the muffler checked, and that was precisely the problem. Power and gas mileage efficiency returned immediately once the truck engine could breathe properly.
Like your lungs, like an engine, your Air Conditioning system must circulate air properly to function correctly. Air Conditioning systems are designed by engineers to operate within a certain total static pressure. Because the squirrel cage fans suck in air, pressurizes it and blow its out, your entire duct system has a total static pressure if you add up the numbers on both sides of the furnace/air handler. AC guys add the negative pressure number coming in to the positive pressure number of the exiting air to determine if your system is operating within its designed parameters. Anything over .5 inches of water column is unacceptable.
If you have a restrictive air filter, you can eat up all your available static pressure the minute you put it in the filter grill. If your return has a -.35 static pressure, that doesn't leave much available pressure on the positive end before you start to see your overall efficiency, home comfort, and Seer rating suffer.
Going back to the maintenance man. I do not know of many maintenance people who carry around manometers, and test different air filters on how they perform under operating conditions in regards to static pressure. When you are in my profession and are a mechanical contractor, you just might actually do that after you run into the same problem over and over. And I can say I have tested numerous static pressures on typical-designed duct systems.
Unless you have an uncommon, highly ducted return system with multiples return grills in your house, here is what I have found, and is my advice regarding air filters: Almost ALL air filters are unacceptably too restrictive for the normally designed AC duct system. Nearly ALL of them. There just went any possible air filter endorsement deal for me. All those fancy allergen filters that look like your system has to breathe through a piece of paper...they are a no go. Most of the average pleated filters are a no no also. If you need allergen filtering, you should have those 4 inch thick merv filters that compensate for their microfilters by increasing the overall surface area of the filter. Otherwise, you are choking your system, putting stress on the condenser, and increasing your electricity bill.
"Why use a filter at all?," you might ask. If you are not filtering your air, start to do so immediately, here is why. Your squirrel cage blower fan has those curved, cupped long blades to capture the air and throw it forward with centrifugal force into your duct system. You need those cups. Without the curve, it greatly reduces air flow and efficiency. Those cups will fill up with baked on, compacted dirt when you don't use a filter. It will happen, you will ruin your system if you don't use a filter. Dirt will also bake onto the evaporator coil. I despise those cleaning service calls, and it can be difficult and near impossible to clean equipment sometimes in those situations--and I charge considerably more, accordingly, for those customers.
"So what filter should I use?" Glad you asked. In my home, I spend top dollar for the best hvac equipment, but spend the cheapest on the air filter. I recommend a non-pleated, fiberglass cheap cheap filter. You change them a little more often, but they let the most air through. Be careful when you dispose them, that dust collected is on average 40% human skin. Haha. If you have common allergy problems, I recommend a separate filtration system. Cheers.