I recently talked to a neighbor who’s still trying to figure out his HVAC. What vents he should open and close and when. What’s a damper? Why does his central air conditioner have two filters, and so on.

HVAC air filter
Confused why your HVAC system has two air filters? Read on!

Personally, I think it’s the realization they may have two filters on their system instead of one that confuses a lot of people. Because all the instructions we hear are about changing your filter. Singular.

So why does your AC have two filters? If you live in a large home, there’s a good chance your system requires more than one return air duct. And each duct requires a filter to stop particulates before they get to your blower motor. Or, you have a system that has multiple AC units.

Need that explained a little more? Read on!

Circumstances Where an Air Conditioner Needs Two or More Filters

As mentioned above, it’s not uncommon for an HVAC system to have multiple filters. Having said that, it might be a good idea to explain the components of your system. That should help you understand the need for more than one filter a bit better.

For any home that has central air, your system has the following. And please note that I’m focusing on HVAC components that relate to your AC, not necessarily your heat.

Air Hander

Air handlers have been described as the lungs of your system. They don’t heat or cool air, they simply manage the airflow across the evaporator coil which is housed within.

Air Ducts

If you have an unfinished basement, you know what your air duct system looks like. These large metal pipes and trunks distribute cooled or heated air throughout your home.

air ducts

Compressor and Condenser Coil

These are part of the outside unit of your air conditioner. The compressor cools your home by pulling the heat from the inside and releasing it to the condenser which releases it outdoors.

AC unit

Please note the above is an extremely simplified explanation that spares us from all the science behind it.

Evaporator Coil

The evaporator coil is found inside your air handler, which is mentioned above. Your system draws refrigerant over a group of small nozzles or expansion valves which in turn releases the liquid refrigerant as a gas. This process absorbs the heat from your home, lowering the temperature.

Blower Motor

Once the air has been heated or cooled a fan powered by the blower motor pushes it into your ductwork and ultimately out through the vents in your home.

Filters directly impact your blower motor. The filter is in place to stop any debris from entering your system and damaging the motor. However, high-efficiency filters can be just as detrimental to your blower motor as a dirty filter can be. Each will restrict airflow.

Restricted air forces your blower motor to work harder. This will impact its lifespan, but also make your system noisier.

So now that you understand your system a bit better, let’s talk about those two filters.

You Have Multiple Return Ducts

If you live in a larger home, it will impact the design of your HVAC. And there’s a good chance that multiple return ducts will be necessary,

A properly designed system needs to pull in as much air as it pushes out through registers and vents. The more rooms, the more supply vents. The more supply vents the greater the need for more return air.

Return air is air that’s pulled from the house. So it will have all the dust, pet hair, and whatever else is floating around in your air, some of it so small you can’t see it.

You don’t want all that debris entering your system and killing your blower motor, so there would be a filter for each return duct—but not each vent!

You Have Multiple Central Air Systems

This is something a lot of people wish for, although they probably don’t realize it.

What they do realize is their top floor is significantly hotter than the bottom floor and they wish for a solution. An expensive solution is a unit for each floor.

If you live in a home with multiple air conditioning units, you’ll need a filter for each.

When There are Two Filter Slots at Your Air Handler

This one seems to be a bit of a mystery. For a while, manufacturers were adding dual slots, but most HVAC pros recommend against using them both. But some don’t.

In many cases, the first filter was a looser weave that would catch larger particles and the second would catch smaller particles. Sometimes one slot would be for a 5” filter and the other for a 1” filter.

In either case, using both could seriously impact your airflow.

Interestingly, while the filter manufactures are pushing high MERV filters, most seasoned HVAC pros just call them furnace killers.

Do You Need Filters in Your Return Vents?

Honestly, probably not.

Yes. 9 out of 10 websites will say the opposite, but there might be some monetary gain for them to say that.

HVAC professionals who are simply about the tech and not part of some corporate entity trying to increase revenue advise against the practice.

Your main filter, the one closest to the air handler is the one doing all the work and catching debris right before it hits the blower motor. If it’s doing its job, you don’t need anything another filter.

Remember, your entire system is built around an amount of airflow. Every filter you add blocks air a bit more, and potentially destroys the very motor you are trying to protect by making it work harder than it is supposed to.

So, filters are absolutely necessary. Where they’re placed may be immaterial—have them at the air handler or the return vents—just make sure what you have isn’t reducing the required airflow that was designed into your system. And know that every system is, or should be, unique to the home it’s in.

Is Having 2 Thin Filters Better Than 1 Thick Filter?

Let’s have a lesson in Cubic Feet Per Minute (CFM) and as it pertains to HVAC. It will be brief because well, math.

CFM is a measurement of airflow volume which is determined by the amount of air in cubic feet that passes over a stationary point every minute. Each HVAC system will need a specific amount of CFM to work as designed.

So before adding any filter, you need to know that it won’t impact your airflow, because negatively impacting airflow will lessen the lifespan of your equipment—as in it will eventually kill your blower motor.

If you calculate your CFM and your two filters don’t impact your airflow, then sure, use both. Use whatever will allow the airflow you need for your system.

Is It Bad to Have Two Air Filters?

If you’ve read this far you’ve probably come to this conclusion on your own. In most many, if not most cases, two filters are not a good idea. Despite what all the people who would like you to buy a filter would have you believe.

As stated above, it’s all going to come down to the amount of airflow your system needs and not reducing it.

Conclusion

Depending on your HVAC system there may be legit reasons for having more than one filter. For example:

  • You have more than one return air trunk
  • You have multiple air conditioning units

The most important takeaways here are that you never want to have either a single filter or stacked filters that block out the required airflow for your system.

Also, while it’s typical to see filters at the air handler, you can choose to place them at your return air vents instead.

And yes, many, many sites you read will tell you you need more than one, or that more than one is just fine. That started because of companies wanting to increase sales, and now it just gets repeated as truth.

The truth is that too much airflow restriction, whether because of multiple filters or a single high-efficiency filter will reduce your system’s efficiency and eventually damage it, and increase your energy bills.

Hopefully, this answers your questions about an HVAC system that has more than one filter.

Thanks for reading. While you’re here, why don’t you check out our related topics below?