What Happens If Air Filter Is Backwards? From Bad To Awful

“I know I have to point the air filter arrow in the direction of airflow. But what happens if the air filter is backward? Is it just a minor thing or can it blow up my furnace?”

Yes, furnace filter backward question. To be absolutely clear: don’t do it. Anybody who turned their air filter (be it AC or furnace filter) backwards can tell you that it’s not a good idea. That’s why you have to arrow on every filter to prevent this from happening.

Now, let’s just look at what does happen if you install the air filter into an air conditioner or a furnace backwards. Maybe just to illustrate why you should always put the air filter into the AC or furnace pointing in the right direction (we explained how to point that arrow in the right direction here).

Here’s the short version of the least unpleasant and the worst-case scenario:

  • Bad case: HVAC unit with a backward filter will not produce its stated capacity and efficiency. You have a 16 SEER 3-ton unit? If you put the filter in backwards, you can forget these numbers. For example, these can decrease to a 12 SEER 2-ton unit, and that’s just the least unpleasant start.
  • Worst case: Compressor floods. Heat exchange element cracks. Coils being superheated or frozen over. Eventually, those $1,000s air conditioners and furnaces can be worth a lot less, if anything at all.
putting air filter in a furnace backwards can overheat the hvac unit
Reversed furnace filter will overheat the furnace. This can eventually lead to a crack in the heat exchanger (shown above). This is devastating for the furnace as well as for our health (carbon monoxide can start forming).

It is important for us to understand that putting the air filter in backwards can start a chain of events that all have very unpleasant consequences. It all has to do with this one simple, but incredibly important thing:

  • Restricted airflow.

It’s kind of a scary awful rabbit hole something as simple as “filters backwards” or even “dirty filters” can lead us to, and we’ll explain fully why all that happens. That’s the main reason why HVAC experts always try to pound “regularly charge air filters” into our subconscience (but do it correctly).

Alright, let’s first explain why the wrong air filter direction restricts the airflow. Further on, we will see what happens to both air conditioners and furnaces with filters turned backwards:

What Air Filter Should Never Be Installed Backwards

In AC and furnaces, we have MERV 6, 8, and even MERV 11, and 13 filters (by Filtrete, 3M, you name it). These filters are cleverly designed to both allow airflow to HVAC units while also capturing damaging air particulates (to the HVAC unit, or our lungs).

Now, the key thing to remember is that all air filters do their job in one direction. If you flip them around and put them in backwards, they will not allow for that essential airflow. In short, air filters are not symmetrical in such a way; there is a clear right direction, denoted by that air filter arrow.

reversed air filter is bad
Air filter is designed to let airflow through in just 1 direction. If you flip it, you suffocate HVAC units.

Example: Let’s say you have a 3-ton AC unit that requires 1,200 CFM of airflow (400 CFM per ton) to function adequately. Here is what happens, depending on whether you put the filter in correctly vs. incorrectly:

  • Correct filter direction: You get 1,200 CFM. The AC unit works great.
  • Backward filter direction: You get less airflow (something like 500 – 900 CFM). The AC unit will struggle to “breathe” at first. Later on, all hell will get unleashed (coils will freeze, compressor will flood, you will get a $1,000 HVAC repair bill or even need a replacement).

To be more specific, let’s first look at what happens if you put an air filter into the furnace backwards. We will also look at the AC issues later on:

What Happens If Furnace Filter Is Backwards?

Alright, if you turn the furnace filter wrong – upwards in a downflow furnace, downwards in upflow furnace, or facing the left on a horizontal furnace – you will effectively restrict the incoming airflow. That means that the furnace – be it gas, electric, or any other – will still use a lot of fuel, but it will heat less amount of air.

We will cover 2 types of consequences of reversed furnace filters. The first set of 4 covers what you will see immediately (if you are wondering how to tell if the furnace filter is backwards, you will see all of these symptoms as well). The second set of 4 covers the serious damage you may face if you run a furnace with backwards filter for weeks, or even months.

Here is what you will see right from the get-go:

  1. Higher heating bill. With lower access to air to be heated, the furnace will work extra hard to achieve the output heating BTUs. The result will be that your gas bill (gas furnace) or electric bill (electric furnace) will increase.
  2. Thermostat may not achieve the desired temperature. Despite the furnace working extra hard, the heating output will be lower (due to a lower supply of air CFMs). With the right direction filters, the furnace will have no problems achieving 72°F. With the furnace filters installed backwards, the furnace may struggle to heat your home up to 72°F. It might not be able to achieve even 70°F.
  3. Cold spots. The return air from the furnace will be more heated than normal (more fuel, less air). That can make the rooms that have return vents hotter quickly, but other places will still be cool.
  4. Poorer indoor air quality. Even MERV 13 furnace filters will not make indoor air as clean as HEPA filters but they will have a bit of an effect on indoor air quality. With reversed filters, that effect is largely gone.
what happens if you put a furnace filter in backwards
For correct filter insertion, point the arrow on the filter in the direction of airflow. On photo: Horizontal furnace with left-right airflow (arrow should point to the right).

These symptoms of reversed air filters should indeed alert you that you should flip the filter. If you don’t and you still run the furnace for months on end, here are the awful things that can happen to the furnace itself:

  1. Crack in the heat exchanger. This is a major problem for any furnace as well as your health. If the heat exchanger cracks, you will lose efficiency (95 AFUE furnace goes down to below 80 AFUE furnace). Even worse, the air can travel into the crack, gets heated up, and produces dangerous gases like carbon monoxide (CO) that can endanger everybody living in the hose. Changing the heat exchanger is notoriously expensive as well.
  2. You will blow the fan blower motor. The fan blower motor will notice that there is less-than-needed air entering the furnace. That’s why it will try extra hard to deliver as much air as it can, operating constantly at 100% capacity. That is a sure way to blow the fan blower motor. At least replacing it isn’t as costly as replacing the heat exchanger.
  3. General furnace overheating. This can lead to all sorts of problems, including something as paradoxical as the furnace blowing cold air. The modern furnaces will alert you when the overheating starts.
  4. Reduced furnace lifespan. With all these parts trying to compensate for the lack of airflow, there is a lot of additional wear-and-tear going on. This will surely reduce the lifespan of your furnace.
flipped hvac air filter against the arrow
Downflow furnace has an up-down airflow direction. Make sure that the filter arrow points downward to avoid all sorts of serious problems down the line.

The thing to do here is to notice the milder 4 symptoms above, realize the filter is backwards, flip it, and avoid these awful and costly problems.

In a similar manner, let’s look at what happens in air conditioners as well:

What Happens If AC Filter Is Backwards?

Air conditioners with a filter installed backwards experience many of the same problems a furnace suffers from. All of these symptoms have a common root – the reduced airflow. Here is what can happen:

  1. Higher cooling bill. Due to reduced airflow, the AC unit will not be able to provide the same amount of cooling. That means AC will work extra hard to compensate for that, resulting in a higher electricity bill.
  2. Reduced capacity. The actual cooling BTU output AC unit will be able to produce will be decreased. That means that a 3-ton unit will only be able to produce 2 tons of cooling output, for example. Due to loss in capacity, the AC unit may have problems achieving the thermostat set temperature.
  3. Hot spots. Due to lower airflow, the amount of cooled air generated is reduced. That’s why the AC may have problems evenly distributing this air to all parts of your home. As a result, we will have rooms or part of rooms that will still be hot (and probably have high humidity levels as well); we will notice the so-called hot spots.
  4. Reduced air quality.

Now, these are just the first symptoms of a reversed filter insertion. Running an AC that is deprived of adequate airflow continuously can result in all sorts of unpleasant and costly failures, such as:

  1. Flooded compressor. This is the worst one; in most cases, if you have a flooded compressor, it’s cheaper to just buy a new AC unit than to fix the compressor. Due to lack of airflow, the superheat in the vapor line can lower to below 5°F. Once it falls to 0°F or below, the liquid refrigerant will enter the compressor, effectively flooding it.
  2. Blown blower fan motor. As in the case of furnaces, the blower motor will start to work extra hard trying to get as much air to the indoor evaporator coils as possible. Running a blower motor at 100% capacity for long periods of time can result in blower motor failure.
  3. Freezing coils. The incoming warm airflow over the coils warms the coils (filled with cold refrigerant). If there is not enough airflow (as is the case when you put the air filter in backwards), the coils may freeze over. The paradoxical result is that the AC unit will no longer blow cool air. Once you stop it and defrost the coils, you will also get that unpleasant AC leaking on the wall and on the floor.
  4. Reduced air conditioner lifespan.

All in all, we can see that putting the filters in backwards and thereby reducing the vital airflow, is a bad idea. At first, we will see HVAC units struggling (lower capacity, lower efficiency, higher bills). If you run units with wrongly inserted air filters for long enough, the HVAC unit itself can be damaged (even beyond reasonable repair).

We hope that this illustrates well what happens if you put air filters backwards in a furnace or AC unit. If you have any additional questions, you can use the comment section below, describe the problem/suggestion, and we will help you out.

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