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If you’ve ever looked at the return vents in different homes, you might notice that some of them have filters in them while others don’t.
Why is that?
Return vent filters are needed if there is no air filter in the HVAC unit. If there is an air filter in the HVAC unit, then return vent filters should not be used.
In this article, I’ll go over how return air vent filters work, and when they should be used. I’ll also cover the benefits of using return vent filters, and the differences between supply vent and return vent filters.
Should you use a filter in your return vent?
If your air handler does not have a filter rack in the unit, then you need to use a return vent filter.
If your air handler has a filter at the unit, then you do not need to use a return vent filter.
There are plenty of advantages of return vent filters. Here are some of them:
- The first line of defense – Return vent filters work as the first guard—preventing impurities from circulating in the air in your home.
- Easy to switch out – Due to its location, it’s usually easier to change out a return vent filter than the filter at your air handler. For example, if your home’s air handler is in the attic, you won’t need to crawl up there every time you need to swap out an air filter.
- Protection for ductwork – Return vent filters prevent particulates such as dust and hair from ever entering your HVAC ductwork. Notice dust building up in your ducts? The easiest way to get rid of this problem is to use a return vent filter.
- Promotes airflow circulation – If your home has multiple return vents, your air handler will have better airflow by using an air filter in each return vent versus a single filter at the air handler. This is because the filtration medium is spread out over a larger surface area, so your air handler will see less pressure drop and more airflow.
What if there is already a filter in your HVAC unit?
If you already have an air filter in your HVAC unit, then you do not need a return vent filter. You should only use an air filter in the HVAC unit OR the return vent, not both.
Why can’t you use a filter in your return vent AND your HVAC unit?
It’s all about airflow.
If you use an air filter in your return vent AND in your HVAC unit, then you run the risk of having low airflow issues.
Central air systems aren’t designed to run with the pressure drop from multiple air filters in series. So if you put filters in your return vent AND your HVAC unit, then your unit might start having issues.
Here is a diagram showing the locations of an air filter at the HVAC unit and return vent:
What if there is no air filter in your HVAC unit?
If there is no air filter in your HVAC unit, then you absolutely need a return vent filter.
I recommend using a MERV-5 to MERV-8 pleated air filter in your return vent.
In this case, the return vent filter will be the primary air filter. So it will serve as the only line of defense against dust and particles entering your HVAC system.
How do return air vent filters work?
Return air vent filters work by filtering out particles and impurities from the air in your home. As the air gets sucked into your return vent, the return vent filter blocks out dust and particles from entering the airstream to your HVAC unit.
Return air vent filters are sometimes preferred over using a filter in the HVAC unit for these reasons:
- Filters in return vents prevent dust from getting in your ductwork.
- Homes with multiple return vents can accommodate multiple filters—allowing for more airflow through your HVAC unit since filtration is dispersed over a larger surface area.
If you have a rectangular return vent in your home, it can use a regular, rectangular-shaped air filter.
I’ll talk about return vent filter sizing below.
How to find your return vent filter size
Return vents come in many different sizes. Depending on the size of your return vent, you may need a certain size air filter. The size of your air filter can be determined by looking at the old air filter.
If the old air filter isn’t available, then you’ll need three different measurements: length, width, and depth. They can be measured by opening the return vent and measuring the sides:
- Measure the short side. For example, the short side could be 15-1/2″
- Measure the long size. For example, the long side could be 24-1/2″
- Measure the depth. For example, the depth could be 7/8″
- Round up the measurement. The size of the filter that you need to order would be 16″x25″x1″
How to install a return vent filter
Here’s a quick tutorial for installing a return vent filter:
- Turn off the furnace or air handler. The first thing that you need to do when installing a new return vent filter is turn off your furnace or air handler. This is an important step because you don’t want your air handler to accidentally suck up dust from your old air filter when removing it.
- Open the return vent. To gain access to your return vent filter, unlock the hatch and open the screen. Most screens have hinges on one side of them, so it will flip open like a lid on a trash can. However, some screens come completely off—so be careful.
- Remove the air filter. As you open the return vent, the dirty air filter may fall out of its slot—so be prepared and keep one hand free to hold on to the air filter so it doesn’t slip out.
- Dispose of the dirty air filter. After removing the dirty air filter, throw it away. I like to keep an empty trash bag nearby to put the old dirty air filter inside right after removing it. Another thing to watch out for is dust build-up on the dirty air filter—be careful not to knock off the built-up dust from the dirty side of the air filter.
- Install the new air filter. When installing the new air filter, keep the filter direction in mind. The arrow on the side of the air filter should point toward the wall or ceiling. After putting the new air filter in its slot, close the return vent screen and lock it shut.
- Turn the furnace or air handler back on. When turning the air handler back on, you might notice the air filter move slightly towards the vent—that’s completely normal.
What if your return vent is an odd shape?
If your return vent is an odd shape or you can’t find a filter that will fit your vent, you can use a cut-to-fit air filter.
A cut-to-fit air filter comes in a roll of material that you can cut to conform to the shape of your air vent.
Cut-to-fit air filters usually come with velcro strips to hold them to your vent. By using velcro strips, your filters will remain in place until you’re ready to take them down for replacement.
For more information about cut-to-fit air filters, check out my article below:
Supply vent filters versus return vent filters
What are the differences between supply and return vent filters?
- Supply vent filters are used to prevent particles from blowing out of your HVAC system.
- Return vent filters prevent particles from entering your HVAC system in the first place.
If you are having air quality issues, then supply vent filters will just mask the problem without actually making it go away.
If there is dust blowing out of your supply vent, then there is a bigger issue with your system:
- Your system might have an air leak, and dirty air is leaking into your return duct from your garage or attic.
- Your HVAC system (particularly your ductwork) might be very dirty and require a thorough cleaning.
Return vent filters help filter out contaminants from ever entering your system in the first place.
A return vent filter will keep your air handler, ducts, and coils clean by filtering out particulates at the point of air intake.
For some homeowners, a return vent filter could be the solution that keeps their HVAC system running cleanly and efficiently.