What Happens If You Don’t Change The AC Filter? (How Bad?)

Do you want to know what happens if you neglect to change the AC filter?

If the AC or furnace filter isn't changed frequently, numerous HVAC and even health issues can result from the lack of this simple maintenance item.

In this guide, I will go over...

  • The #1 biggest consequence of not changing your filter
  • The numerous potential hazards of not changing the AC filter
  • And how to know exactly when to change your filter

Let’s get started with this guide!

what happens when you don't change ac filter

What Happens If You Don't Change The AC Filter?

During my home inspections, I frequently come across extremely dirty filters. Sometimes, the owners or tenants didn't even know where the filters are located — it's mind boggling.

Probably the biggest hazard of not changing your furnace of AC filter is that it can harm your HVAC system and reduce its life expectancy. In my opinion as a licensed home inspector, if you don't regularly change your filter, it can reduce the life expectancy of your AC by 6 to 12 years. 

And if you are really bad at not changing the filter (I mean, you really work at it) — then you may experience the result of a system failure such as a broken compressor, leaking refrigerant lines, or a failed blower fan.

Besides harming your AC system, neglecting AC filter changes can also lead to mold growth on your air vent covers, in your ducts, and inside your AC air handler.

Read Also: What Are The Best Washable & Reusable HVAC Filters?

Does Not Changing My AC Filter Reduce HVAC Life Expectancy?

During my home inspections, one of the biggest things I tell my clients is the life expectancy of their A/C system.

Life expectancy is likely the most common phrase that I use in my home inspection reports, and I use it for major and costly items. For HVAC systems, the standard life expectancy is in the 15-25 years range — depending on the brand, efficiency, and scheduled maintenance.

So when I tell you that it can reduce the life expectancy by up to 12 years, that can be a significant part of the remaining life of the air conditioner.

Sometimes I even have buyers walk away from a home after I tell them that the HVAC, water heater, and roof are all at the end or past their life expectancy.

And if the AC system looks like it hasn't been maintained properly — such as signs of past leaks and dirty filters (and coils) — that can also have a big impact on a buyers attitude. Poor HVAC maintenance can sometimes be the nail in the coffin for a buyers decision to buy a house.

Read Also: How Often To Change Your 1-Inch Furnace Filter?

what happens when you don't change the ac filter

Why Not Changing AC Filters Reduces Home Comfort?

In addition to the harm to your AC system, not changing your filter frequently also just results in a poor living experience.

When the filter gets clogged with dust, this means that less air is flowing through your duct system. The ventilation in the home drops precipitously.

If I put my hand up to multiple air vents, and I feel weak or complete lack of airflow — the first thing that comes to mind is the furnace filter. And quite often, this simple neglected maintenance item is the culprit.

As soon as I change the AC filter, the air flow resumes to its normal level. When the air flow can't function normally, it makes it harder for the AC to heat or cool a home.

Basically, it means that you will be uncomfortable in the home and that your target indoor temperature will rarely be met.

In fact, during the peak of summer or winter, you should actually be changing the HVAC filter more often, not less. Since it is working harder during the peak season months, you want to stress it out as least as possible.

Read Also: What Are The Best Air Conditioner Filters For Allergies?

Why Not Changing Your AC Filter Reduces HVAC Efficiency?

When the filters are dirty, your AC system may actually turn off due to overheating. And when it turns off and on repeatedly, this harmful HVAC phenomenon is known as short cycling.

As you can imagine, short cycling uses a lot of energy and puts some serious strain on your air conditioner.

All of that extra friction and energy used is much greater than normal, reducing performance and lowering the life expectancy.

This extra work generates heat, and one of the safety sensors in the AC automatically turns it off. Especially with gas furnaces, this extra friction and heat could even lead to a house fire — fires are indeed the most common safety hazard in a home.

what happens when you don't change an ac filter

Dust On The Coil

And when your filter is clogged, that dust instead gets caked onto your evaporator coil (or A-coil) of your HVAC system  — which will reduce its efficiency tremendously.

This debris on your A-coil — mainly composed of dust, pollen, pet dander, and dirt —greatly reduces the efficiency of the heat transfer process. When the heat-absorbing capacity of the coil is disrupted, it increases energy costs, making it harder to heat & cool your home. 

Read Also: How To Clean AC & Furnace Filters?

Why Neglecting AC Filter Changes May Lead To Mold?

I remember I inspected a house once where there were multiple filters in the ceiling, and the owners who worked for a foreign embassy were totally clueless about maintaining the home — it appears that they never once changed the filters.

Well, that explains why there were numerous air vent covers with mold growing on them. I distinctly remember this house because it was disgusting. I'm pretty sure there was also mold growing in some of the wall cavities because of numerous water leaks.

I recently wrote a detailed on guide on the signs of black mold on air vents, check it out here.

what happens when you don't change a furnace filter

Mold growing on an air vent cover is frequently the result of not changing the AC filter.

Anyways, the reason mold grows on air vent covers is because a significant amount of dust was pooling on the vents, and mold eats dust as a food source. 

In addition, the furnace can become a breeding ground for mold when the filters aren't changed.

This is because it has three things going for it...

  1. Dust which is a food source.
  2. Darkness which mold requires to live.
  3. And lots of moisture as byproduct of cooling and heating.

Mold growing in your AC air handler, ducts, and air vent covers is probably the biggest potential health hazard of not frequently changing your AC filters.

Read Also: What Are The Best AC Filters For Odors?

How Air Moves Around The Filter When Its Clogged?

When dust clogs your filter, the air finds it easier to actually flow around the filter, rather than through it. Furnace filters aren't air tight products, they are loose in the slots.

And frequently, I notice HVAC filters aren't even correctly installed with missing filter covers and slots. I sometimes see filters half bent in the middle, or other signs of having a low air seal around the filter frame.

And as air flows around the filter, instead of through it, all of that dust gets spewed into your duct system, deposited onto your air vent covers, and also spread throughout the home. In fact, if you have to frequently dust your home, this is a clear sign that your filter needs to be changed or that it isn't installed properly.

Along with common dust, indoor air pollution may significantly increase including pollen, VOCs, and allergens.

If you are concerned about mold, I would also get a flashlight and inspect all of your air vent covers as well as the inside of your interior air handler. Some furnaces or air handlers have front covers that are easily removable with a simple latch, but others will require a few screws to be removed.

Read Also: Where Is My Filter Located?

Final Thoughts

Let's just say that even though changing your AC filter is such a simple thing to do, it can have an outsized impact on your HVAC performance, life expectancy, and possibly even your health.

I find it remarkable that some homeowners don't stay on top of filter changes.

I was recently at a buddies house, and I noticed his dirty filter. He told me that he usually changes it two times out of the year, or every 6-months.

This is bonkers. You should only be changing your filter every 6-months if you have 5-inch thick high efficiency filter, not the standard 1-inch.

Even the manufacturers state to change the furnace filter at minimum every 3-months. Personally, I always change my filters every 30-days, and recommend the same thing to my clients — the EPA recommends at least inspecting your filter every month.

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Arie Van Tuijl

Arie Van Tuijl

Arie is the founder of Home Inspector Secrets, an online resource dedicated to helping people understand how homes work. He is a licensed home inspector in two U.S states and owns a residential and commercial inspection company (read his full bio on the About page). To ask Arie a question, please use the comment box at the bottom of the relevant article.

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Home Inspector Secrets is an online resource for owners, buyers, and sellers to understand all aspects of home maintenance. We have detailed home guides, product reviews, inspection advice, and much more.