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Warm weather is just around the corner and before you turn your air conditioner on it’s time to change the filter. While your air conditioner is off take out the filter, write down the size, and head to the hardware store.
If you walk down the filter aisle at Lowes, Home Depot, or Menards the first filters you will see are high end ones that promise to filter just about everything. Most people have a 1” thick filter so most filters on the shelves will be 1”. All these wonderful filters use a rating system called MERV. MERV stands for Minimum Efficiency Reporting Value and it’s a standardized value so you can compare filters. The higher the MERV, the better the filtration. The highest MERV you’re likely to find on the shelf is 13. At MERV 17 you’re getting into HEPA filter range. HEPA filters were originally developed by the military to remove super fine particles that had become contaminated by radioactive material. I guess that means they're good enough for your house, right?
Like many people I wandered down the filter aisle wanting to get something nice. So, I came home with this filter.
This was the low end of the nice filters with a MERV 8 rating. I also picked up the 8500 filter, which is MERV 11. Below the Basic 7500 filter were the cheap disposable filters with no MERV rating and no marketing materials for me to read. I figured those unnamed filters were junk.
After looking at the high MERV filters, I want you to put all of them back on the shelf and walk down to the generic filters with no MERV ratings or advertisements. These filters will probably say disposable or something similar (apparently the high MERV filters that get thrown out just as often aren’t ‘disposable’). You want a cheap filter.
All the advertising about filters seems to have nothing to do with what your air conditioner needs. The advertising has everything to do with what you’re afraid of. Mold. Bacteria. Smoke. Oh My!
Your central air conditioner needs two things. First, it wants a good supply of air. All of the high MERV filters restrict air flow beyond what your air conditioner wants. After a little dust builds up on the filter it restricts air flow even more. This summer if your air conditioner stops working the first thing you should do is check to see if the air conditioner has iced up. Without a sufficient flow of warm air blowing over the evaporator coil in your air conditioner ice can start to build up. As more and more ice builds up the air flow is reduced even more in a kind of snowball effect, literally.
Icing up can also lead to excessive moisture in your air conditioner. When the thermostat turns off the air conditioner any ice that is on the evaporator coils will melt. At this point it is possible that mold or bacteria can start to grow, especially if this freeze-thaw cycle is happening consistently. The mold or bacteria can start to make your house smell in what is actually dubbed “Dirty Sock Syndrome.” No joke. Google it.
The second thing your filter needs to do is remove the large particles and contaminates from entering the air conditioner. Pet hair and large particles can attach themselves to the inside of the air conditioner clogging up and reduce the unit’s efficiency. All those particles in the air conditioner are food for bacteria and mold if they are not filtered out. The fine particles don’t really affect the air conditioner because they stay airborne and pass through the air conditioner. So a high MERV filter does not help the air conditioner function better.
What we need is to find the right balance between airflow and filtering. Many manufacturers recommend using the cheap blue fiberglass filters. These filters meet the filtering and airflow requirements for central a/c units.
Despite the airflow problems that high MERV filters can cause some people need their air filtered for health reasons. Allergies are possibly the most common reason. I sympathize. I have allergies and admit to coming home, closing the windows, and turning on the air conditioner to help get rid of the pollen in the house. I don’t care what the scientists say, it helps.
Even worse than seasonal allergies is asthma. Asthma can be brought on by pollen, mold, dust mites, or smog. High MERV filters can filter particles this small so putting a high MERV filter in your central air conditioner makes sense. But it’s wrong.
Here’s why. Your central air conditioner’s blower fan moves enough air to completely circulate the air in your house around 7 times when it’s running. If your air conditioner blower fan only runs 15 minutes every hour then you’re only completely circulating the air in your house a little under 2 times. In order to really control the air quality of your house you need to completely circulate the air around 15 times an hour. High MERV filters are not significantly effective because the air conditioner does not move enough air.
I ran my MERV 8 filter for about 6 months during last winter and here is the result.
On the left you can see the overall filter with no large particle debris. On the right is a closeup of the filter and you can see a little coating of dust. This dust was most certainly restricting the air flow of the filter. I also want to point out that the amount of dust on this filter is about the same amount of dust that settled on one of my ceiling fans! So, it did not significantly remove more dust from the air than what settles out naturally in my house.
I should confess though, that I have seen filters that were horrible covered in debris after 6 month use. It depends on your house, if you have pets, how you live, etc.
If you have health issues and really want to affect the quality of your indoor air for the positive you can do two things. The easiest is to use a spot HEPA air purifier. These little machines are not big enough to handle your house, but studies indicate that when they are directed over your bed while you sleep they can relieve allergy and asthma symptoms. Air purifiers start at about $100 for something large enough to handle a small bedroom. Here are some examples available from Menards.
The second option is to install a Heat Recovery Ventilator (HRV) with a HEPA filter. HRVs work by diluting contaminates in the air. The HRV pulls air out of your house while bringing fresh air in from outside. HRV units cost hundreds before installation labor for a small unit. HRVs designed for entire houses run over $1000. Here is an example of one made Broan. I don't endorse the model because I've never used it, but the information will give you some ideas.
Back to filters.
Many furnaces have 1” thick filters. However, the recent push has been to start using thicker filters. The benefit of a thicker filter is that they have more surface area for air to be sucked through. This causes air speeds through the filter to be reduced which in turn lowers the air resistance.
The common 1” pleated filter also comes in a 2”, 4”, and 5” version. The 2” filter is not much of an upgrade to be honest. The 4” and 5” filters do start to see better air flow at higher MERVs.
You can go even bigger with a whole house purification system that looks like a super wide electronic version of the 4” filter. Here is an example made by Honeywell. These systems can work quite well if the blower fan is run more often. There is a setting on thermostats that allows you to turn just the fan on. Instead of circulating the air in your house only 2 times, leaving the fan on can get you closer to circulating the air in your house 8 times an and hour. Now you can start to make a difference, but it comes at a high price.
So what’s the best 1” filter for your central air conditioner if you don’t want to size up? That’s a tough question but we can narrow the selection down to two options. First, the cheap blue fiberglass filters are a decent choice if you’re primarily interested in the efficiency of your air conditioner and you don’t have health issues. If you want a little more filtration a pleated filter up to MERV 8 is an ok choice. At MERV 8 many, but not all, pleated filters really start to restrict air flow like a high MERV filter. If purifying the air in your house is a major goal you’re going to need to spend some money and look at options other than a 1” filter.
Before you turn your air conditioner on this year, make sure you change the filter. Realize that air conditioners are more sensitive to reduced air flow than furnaces. All the hype about 1” filters being able to purify the air in your house is overblown. Filters are meant to protect your air conditioner, not purify the air in your house. If you want to purify the air in your house do it right. As for your central air conditioner, don't feel guilty for buying a cheap filter. A cheaper filter changed regularly is the best decision for your air conditioner.