Why do Air Conditioners Have Air Filters?

A dirty AC air filter

Have you changed your air filters this month? Yes, you read that correctly - I asked if you've done it this month because it really is a good practice to change your AC systems air filters every month, especially during the peak demand seasons when the AC is running everyday. But why do we change our AC system's air filters? Why do some air filters cost a dollar while other's cost $20? Replacing our air filters is like changing the oil in our vehicles and it turns out air filters play a pretty important role in the health and performance of any air conditioning system by keeping the system clean. This article is going to go over the importance of replacing AC air filters frequently, why we change air filters, what happens when we don't change our HVAC system's air filters, the different types of air filters, and the benefits we gain from changing our air filters.

"Why do we change our AC systems air filters?"

Air conditioners have a big job to do when it comes to cooling your entire living space and they are able to do this by moving all the air in your home through their vital cooling components. The air first gets sucked into a component of your air conditioner called an evaporator coil, where it is cooled, and then once the air is cooled the air then gets blown and distributed into each individual room of your home through vents, and then the cycle repeats over and over until your desired comfort setting on your thermostat is reached. So, in order for your air conditioner to do its job, it's components have to be clean. You can think of the component that cools your home, the evaporator coil, as the lungs of your air conditioning system, and, when your air conditioners lungs get dirty, it begins to experience health issues and cannot do its job. This is where air filters come into play - Air filters keep your AC systems evaporator coil clean, so all the air in your home can easily pass through it, and therefore can easily be cooled. A dirty evaporator coil, or dirty air filter, will ultimately cause the entire system to freeze up and form ice, which can cause further mechanical issues with the entire system, like a freon leak, and even ruin components like the system's compressor and turn into an expensive AC repair job. If the evaporator coil is considered the lungs of your AC system, then the compressor is surely the heart of your AC system. So that inexpensive air filter plays a pretty big role in the entire cooling process and the over all health of your entire AC system, which is why it's best to keep them frequently replaced, they also help keep the air you breathe a little cleaner and your house a little cleaner too.

A dirty cooling coil

"What happens if I don't replace my air filter?"

If you forget to change your air filter (believe it or not, some people do for a year or two), eventually the system is going to freeze up and stop cooling. By then you have most likely forgotten you even have air filters that need to be replaced and end up calling an HVAC contractor because the system isn't working and it's a shame when technicians go out and visit people's homes because their system is experiencing issues because they haven't changed the air filter in a couple of years. What happens with a dirty filter (or evaporator coil) is air cannot get into the evaporator coil, and since air contains heat, it keeps the evaporator warm. When this air cannot get into the evaporator coil and keep it warm, the coil forms ice, and freezes up (shown below).

A frozen evaporator coil.

When the coil is frozen, air cannot pass through it at all and either can freon and this makes the whole system begin to experience issues. The indoor blower motor that moves all the air through your home cannot pass air through this thick wall of ice, so it tends to over-heat and burn up. The compressor cannot pump freon through the frozen evaporator coil, so it tends to over-heat and burn up which can force you into purchasing a new AC system entirely. The ice can also bend and break the evaporator coil itself, causing freon leaks and all this can happen because of a dirty air filter. It really is a lot easier to save yourself the trouble and just make sure you get the filter regularly changed. A clean air filter extends the lifetime of the system's components and keeps the entire system from having to run as often in order to cool down the air in your home, which reduces your electric bills.

"What is the difference between the cheap air filters and the expensive ones?"

Air filters can cost anywhere from 99 cents to over 20 dollars with the difference being the sizes of the particles they can trap and prevent from getting into your AC system and ductwork. The cheaper air filters can only trap larger particles, whereas the more expensive filters can trap much smaller particles. If you look closely at the filters you are picking out at the store you'll notice that the more expensive ones list something called a "MERV" rating (see image below). This rating is an indication of the scale and size of the particles the filter can trap and the larger the rating, the smaller the particles the filter traps. If you're going to buy an air filter with a high MERV rating, you can expect it to get dirty fast, especially when compared to the cheaper air filters. This is because the filter is trapping more particles and removing them from the air you breathe and also preventing them from entering the AC system. So if you notice the filter is getting dirty faster, you need to replace it faster, otherwise it starts putting strain on the entire AC system. The small 1 inch thick air filters we use in our homes are pretty limited on the MERV scale, they may be able to trap small particles but the amount or percentage of those smaller particles they have the ability to trap is very limited. So if you're looking to trap particles such as smoke and aerosols and viruses, you're probably going to have to invest in a whole home air purifier or HEPA filtration system, which are mechanical additions that are made to your current HVAC system that greatly improve indoor air quality. HEPA filters and air purification systems are the kind of set-ups they used to purify the air health care facilities to prevent people from getting sick and spreading illnesses. So don't expect much from a 1 inch filter with a higher MERV rating unless your super sensitive to allergens like pollen and dust, in which case these filters should provide some benefit to your breathing comfort and will definitely help keep your house a little bit cleaner by not spreading around dust and debris for you to sweep up later. If you simply trying to keep your AC system clean, a cheaper filter will do just fine, just remember to change them.

MERV rating scale