What Happens If My Furnace Filter Is Backwards? (How Bad?)

Do you want to know what happens if your furnace filter is installed backwards?

Having a reversed HVAC filter may lead to system damage, mold issues, poor indoor air quality, and other issues.

In this guide, I will go over...

  • #1 worst consequence of a poorly installed furnace filter
  • Why mold may grow from increased dust
  • How exactly a reversed filter may damage your furnace or HVAC system

Let's get started with this guide!

what happens if furnace filter is backwards

What Happens If My HVAC Filter Is Installed In The Wrong Way?

As a licensed home inspector, I have seen many furnace filters over the last several years.

It may shock you that I have found extremely dirty filters, missing filters, wrong sized filters, and lastly — backwards installed filters.  AC filters have arrows on them that indicates the direction of airflow, so there really is a correct orientation.

The biggest harm that may result from a backwards installed filter is that it can damage your HVAC system and reduce its life expectancy. A backwards installed AC filter will greatly slowed down airflow, your furnace is going to have a harder time running. When the efficiency decreases, it increases the likelihood of a system failure such as a leaking refrigerant line or a failed compressor.

If your furnace or AC is 15 years or older (the typical life expectancy), you will be at a much greater risk of a system or part failure. A newer system will likely be better able to handle the reduced airflow and the subsequent harms.

Read Also: What Are The Best Washable Furnace Filters?

Short Cycling

A backwards installed furnace filter can result in something known as short cycling. This is when the HVAC turns on and off faster than it should.

It is turning off because it is overheating due to the extra work and energy from the impeded airflow. Especially with older gas furnaces, there is even a potential risk of a fire due to this overheating process.

The furnace is turning off because their is an overheat sensor or a high limit switch that automatically turns it off.

With the repeated short cycling of your furnace or HVAC system, you are putting much more strain on the system than with normal usage. Short cycling decreases the efficiency of HVAC systems by a sizable amount.

Read Also: Where Is My Furnace Filter Located?

Lower HVAC Efficiency

When the filter is installed backwards, and starts impeding airflow, the air may start actually go around the cardboard frame of the filter—which isn't that hard to do.

I have seen many bent, crushed, and warped furnace filters over the years. The cardboard frame isn't exactly made of titanium. And the air seal around the filter and the furnace slot usually isn't that great.

When air starts flowing around the filter, it can dirty up your system such as putting a coating of dust on the evaporator coil. The evaporator coil is basically a snake-like system of refrigerant tubing in your AC system—it usually sits on top of the furnace.

So if you are using your AC system, this coating of dust can affects its performance significantly as well as impeding air flow.

In addition to dust on the evaporator coil, when dust coats the blower fan, the blower motor, and other internal components of your furnace—this will also lower system efficiency for heating and cooling.

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

Indoor Air Comfort

Another aspect of lower HVAC performace is that you will have a harder time reaching your target temperature.

Basically, it means you won't be as comfortable in your home as you should be.

Since there is less airflow, the furnace can't manipulate temperature as quickly, so it may take up to 2-3 times longer to reach your target temperature. But you may never reach it at all because the overheating sensor shuts off the furnace prior to reaching your target temperature.

Read Also: Where Is My Furnace Filter Located?

Poor Indoor Air Quality

When there is decreased airflow, and with the short cycling of the HVAC system — dust, pollen, and dander particles may increase in your interior air. You may even notice that you require more cleaning and dusting of your home.

Especially with newer homes, they are designed to be virtually air tight for energy efficiency and equipped with high R-value insulation.

If your ventilation decreases, the trapped VOCs, dust, and other harmful particles increases in your home.

So even though newer homes are more energy efficient, being air tight can be hazardous to one's health if pollutants are trapped inside.

Read Also: What Are The Best Furnace Filters For Dust Control?

what happens if furnace filter is backwards (1)

Possible Mold Growth

As discussed earlier, a backwards filter leads to reduced airflow, and more dust getting into the system.

Well, dust is a food source for mold.

I can't even tell you how many furnace covers I have taken off only to discover a mold infestation. Sometimes it's not so bad, but other times I have to hold back the gag reflex.

Everybody responds differently to mold, so the homeowners may have been blissfully unaware. And so if your filter maintenance or installation is poor, a high dust quantity can lead to mold.

Mold only needs three things to grow...

1. Food source (usually in the form of dust)

2. Moisture (a lot of moisture in HVAC systems as a byproduct)

3. Darkness (very dark in ducts)

Mold can grow very quickly. Some species of mold can grow and take root in as little as 4-12 hours.

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

Balanced Distribution of Dust

When a filter is installed in the wrong direction, it can quickly get clogged with dust, pollen, dander, and other particles.

This is because the AC filter is designed in a way that allows an even distribution of dust inside of the filter material—rather than just the surface.

Basically, the section of the filter that faces the air first is more porous (larger holes), which allows it to filter out larger particles first, and then gradually filters out smaller dust particles.

It is very similar to portable HEPA air purifiers. Home air purifiers almost all have prefilters that are designed to catch large particles. Some then have medium-sized filters to catch the next size of particles.

And lastly, the air goes through the highly efficient HEPA filter to catch the smallest particles—up to a 99.97% efficiency.

But the same principle applies to your furnace filter.

When you install the furnace filter backwards, there isn't an even distribution of dust throughout the 1-inch filter material. Rather, the less porous side, the side with smaller holes, gets hit with air first.

Well, as you can imagine, a thin surface of dust builds up much faster than if the filter was installed in the correct orientation (side with larger holes first).

This will block airflow faster than normal as the dust builds up on the surface.

Read Also: What Are The Best HVAC Filters for Odors?

How To Correctly Install The Furnace Filter?

New homeowners or tenants who have a poor understanding of the HVAC system may have trouble with this one simple step of correct filter orientation.

The arrow indicates airflow direction, so this arrow on the side of the filter needs to point in the correct direction. But it really helps if you understand the main principle of how your HVAC system works — so you never make this mistake again.

The Return & Supply Duct System Loop

Airflow is always going from the return vents (usually one or three large rectanglar vents where air is getting sucked into it) back to the furnace to get re-heated.

The HVAC duct system works as a loop with a return side to the furnace and a supply side. The return side is when air is returning to the furnace after circulating throughout the interior of the home. The return side of the furnace is a much larger duct than the smaller supply ducts.

In a single home, there is frequently just one or two large return ducts, big rectangular ducts, that extends the length of the entire home, either vertically or horizontally.

The supply side is the smaller branching ducts that supplies air to the house through the air vents—usually at least one or two in each room. So if you look at your furnace, and follow the ends of the furnace, there should be one duct that stays large, and the other end should branch off into smaller ducts.

The Filter Is Always Before HVAC Equipment

In addition, if you look at where the filter is located, the air is coming from the side of the duct where there aren't any HVAC components.

The blower fan, evaporator coil, motor, and other parts are always downstream of the filter.

This makes sense because you want the air to filtered before it gets to the actual furnace equipment and coat it with dust.

Read Also: How To Clean Washable & Reusable HVAC Filters?

Final Thoughts

Installing a furnace or AC filter backwards is a really easy thing to do if you don't understand the return vs supply side HVAC loop. But there really is a good reason to install it correctly, with the arrow in the direction of airflow.

It basically boils down to the fact that the filter was designed to be more porous when air first hits the filter (to catch larger particles) and less porous on the exit side (to catch small dust particles).

If you reverse direction, and install a furnace filter backwards, you run the risk of a few things like...

  • Shortening the lifespan of the furnace
  • Harmful short cycling (turning on & off)
  • Reduced HVAC efficiency due to increased dust
  • Difficulty in reaching your target temperature
  • Potential for mold growth
  • Poorer indoor air quality

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Ask Me A Question!

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.