Cold Air Return Vents: What They Are, How Many Do You Need? (+Furnace)

What is a cold air return vent?

Simply put, a cold air vent returns cold air back into the furnace. No furnace can adequately operate without sufficient return air supply.

We can think of a furnace as lungs. It ‘breaths out’ hot and it ‘inhales’ cold air. Air return vents are necessary for the ‘inhale’ process. They are located on the walls or floors; they are installed low on the wall because cold air has a higher density and it accumulates near the floor.

Installed and open cold air return vent delivers cold air to the furnace; furnace returns heated air via ductwork.

Here’s the deal:

There are many questions that may arise in connection with these cold vents. We will tackle a number of the most important ones regarding cold air return vents, including:

  • How many return vents should a house have? Do you need 1 for every room? It depends on the furnace’s cold air return requirements.
  • Can you use cold air returns in the basement? The basement is usually the coldest part of a house. How many vents would you need in the basement?
  • How to install cold air vents into walls? We include installing cold air return in a 2×4 wall example.
  • What will blocking cold air return vents do? Blocking cold air return vents in summer is standard practice for some homeowners.

Let’s begin with the most popular question:

How Many Cold Return Air Vents Do I Need?

A house may have anywhere from 1 to 10 cold air vents. Here are two ways how you can calculate how many return air vents you need:

  1. HVAC Engineer Calculation. HVAC engineers calculate the number of cold air vents needed based on the furnace’s CFM output and square footage. We will look into how do they do that
  2. Rule Of Thumb Calculation. This is a very simple method anybody can use.

Let’s first look at how an engineer would calculate the number of vents. This will give you an idea of which specification you should be aware of when doing these calculations. It will also illustrate how we come to the simple rule-of-thumb calculation (Method 2) we will show you further on.

Method 1: HVAC Engineer Calculating How Many Return Vents Should A House Have

The precise number of cold air return vents depends on the CFM output of your furnace.

That’s because we have to at least match the quantity of hot air furnace produces. To keep ‘breathing’, furnace cold air return vents have to supply enough air to the furnace. If we miscalculate how many return vents to install, the furnace will get insufficient airflow and it will start ‘suffocating’; operating at lower than 100% capacity.

How strong should air come out of vents?

Now, first, we have to figure out how much airflow (measured in CFM) does your furnace require to operate at 100% capacity. These are furnace cold air return requirements. You can check the airflow on the specification sheet of your furnace.

If you’ve misplaced that, or can’t find the furnace airflow on the furnace itself, you can use this simple rule:

“200 CFM for 10,000 BTU of heating output”

That means that a 60,000 BTU furnace produces 1,200 CFM airflow. Similarly, 80,000 BTU furnace produces 1,600 CFM airflow and 100,000 BTU furnace produces 2,000 CFM airflow. You get the gist.

Here’s what a cold air return vent looks like:

When we figure out how many CFM does our furnace output, we have to match that output with CFM input. Here is where the number of cold air vents comes in:

A typical return vent is 16 inches by 20 inches, according to Hunker. It includes the cold return air duct grille. If you consult the duct CFM sizing chart, you can quickly figure out that a 16×20 inch can handle about 1,500 CFM airflow.

Here’s where most people make a mistake. Common sense would dictate that if you have a 1,500 CFM vent and a 1,500 CFM furnace (that’s a 75,000 BTU furnace), you would only need 1 cold return air vent. That’s not the case at all.

A typical air return vent size (16×20) can provide airflow with a maximum of up to 1,500 CFM. To achieve that 1,500 CFM, we need a big pressure pushing air through the ducts and through the cold air return grille.

Even floor cold return air vents (the place where pressure is the highest due to cold air going down), we don’t get that kind of pressure. At most, you will get anywhere from 10% to 30% of that; that’s 150 CFM to 450 CFM per standard vent. The assumed average is about 300 CFM. There is a large range of CFM’s a standard-sized cold air return can let through, however.

If we take the 300 CFM assumption, we can quickly figure out how many cold air vents does a 100,000 BTU furnace, for example, need. 100,000 BTU unit requires about 2,000 CFM airflow.

7 standard-sized vents with each providing 300 CFM will allow for a 2,100 CFM airflow. That means that a 100,000 BTU furnace requires 7 standard-sized cold air return vents (with cold air return grilles, of course).

Note: The exact calculations are made with the help of manuals by the Air Conditioning Contractors Association. Those have charts for specific vent sizes, pressures, and allowed airflows.

You can use these assumptions to calculate the number of air vents needed for your furnace. This is how an engineer does it. However, most people prefer the simple-to-use rule of thumb method. Let’s look into you yourself can calculate that number much easier:

Method 2: Simple Rule Of Thumb Calculation

As we have seen, figuring out exactly how many air return vents we need to install in a house is not that simple. We can simplify that whole thing, however, with this simple rule of thumb:

“Install 1 cold air return vent in every room that is larger than 100 sq ft (10×10)”

That’s quite easy to understand. If you have a house with 7 rooms, you need 7 cold air return vents. If you have a house with 12 rooms, you need 12 return vents.

Example: How strong should air come out of vents for a 200 sq ft room? It should be somewhere around 200 CFM.

Now, we don’t count the small below 100 sq ft rooms. On top of that, we need to install 2 cold air return vents for larger rooms. Those are usually rooms with above 500 sq ft square footage.

This method will not give you the most precise result but it will give you quite a good estimate of how many cold air returns do you need.

Let’s look at how these vents should be used in basements:

Cold Air Return In Basement

The basement is usually where the coldest air in our house is located. Basement air has two distinct features:

  1. Basement air is cold.
  2. Basement air is humid.

What we usually don’t want is exactly what we have in the basement – cold and humid air. Sucking that air through cold air return vents into the furnace is the perfect solution. It will reduce the humidity of the air in the basement and increase the temperature.

On top of that, the basement air is heavier. This is a good thing; the standard-sized cold air return vents likely be able to pull more than 300 CFM airflow toward the furnace.

How many of these basement cold air return vents do we need?

Well, as always, it primarily depends on how big a furnace we have. Nonetheless, in many cases, we can estimate the number of cold air return vents needed in the basement by square footage.

You should install 1 return vent per 200-300 sq ft of basement. This is not an exact figure; it’s a ballpark estimate. For an exact figure, you will need to consult with an HVAC specialist.

Example: If you have a 500 sq ft basement, you will probably need 2 cold air return vents.

Installing Cold Air Return In 2×4 Wall (Example)

When you figure out how many cold air return vents you need, you will have to install them. Installing cold air return in a 2×4 wall is the most common practice; we will use this example to explain how air return vents are installed.

This is an example of how you can install a cold air return between two studs. Here’s how you do this step-by-step:

  1. Measure the centerline. Have a look at your ductwork and measure the centerline from the part of the 2×4 wall that is closest to the parallel wall.
  2. Mark the duct. Measure the duct. After you know how wide the duct is, measure 1/2 of that distance from the centerline. Add 1 inch to get it just perfect. Example: If you have a 16-inch duct, measure and mark 9-inches from the centerline.
  3. Compare marks with wall studs. This is to check if wall studs are adequately spaced (you might need to cut them).
  4. Cut the stud.
  5. Install the ducts.
  6. Fill the spaces between two studs.

This is a general principle. You can find more detailed installation directions at Duct Kings.

Blocking Cold Air Return Vents In Summer: Should You Do It?

Should you adjust your cold air return vents with seasons? Should return vents be open or closed in summer?

Some homeowners are already aware that you should adjust return vents with the season. The air return grille should adjust to different room temperatures.

Here’s what you do in the winter:

  • Block the upper vents (you don’t want to draw warmer air).
  • Open the lower vents (you want to draw colder air).

Here’s what you do in the summer:

  • Block the lower vents (you don’t want to draw colder air).
  • Open the upper vents (you don’t want to draw warmer air).

It makes quite a lot of sense to block cold air returns in the summer. If you use cold air vents only for the furnace (not for air conditioning), you should close all cold air vents. They serve no function in the summer.

So why should you do it? What will blocking a cold air return do?

Well, it will at least prevent the vents from accumulating dust, dirt, or even potential mold. With open vents and humid and hot summer air, the likeliness of mold infestation in your vents increases.

Shutting the vents down (you just simply block the cold air return vents with the grille) can protect your air vents.

Hopefully, all of this will help someone who is trying to figure out what cold air return vents are and how to handle them.

11 thoughts on “Cold Air Return Vents: What They Are, How Many Do You Need? (+Furnace)”

  1. You just made my life so much easier.I am a single woman and I take care of my house on my own.I moved in a year ago and have been at a loss as what to do as far as heating and the vents.I use window air units.But it’s an old house and there are return vents on the floor a couple on the wall that are covered up and I believe disconnected,also a floor one that has been disconnected.They put new heating vents that are on floor but just sitting on top of cut out holes not even attached.There is a vent on the duct downstairs also of heating so just confusion.But at least after reading this I finally have some knowledge of how it is supposed to work and why and maybe I can try to figure this mess out.I have a $400 electric bill every month and I just know something can be improved.Thank you so much for taking the time to post this.It was well written so as to easily understand but not be condescending and it did make a difference for someone,me! Thank you thank you

  2. I have a fujitsu 2 1/2 ton wall mounted mini split in my 3rd floor aprox 500 sq ft bonus room. I was told the unit is over sized because it is not pulling the humidity out of the room, I run a dehumidifier constantly. If I shut the dehumidifier off the humidity rises to 70%. Is it possible to run a 3-4 inch a/c supply line into this space off of my second floor unit as well as a return vent possibly 8×10?? Or would just a return vent help? Will either of these work better without negatively effecting my second floor unit performance?

    • Hello Steve, quite an interesting problem. That big Fujitsu 2.5-ton mini split has to cool a too small amount of air (I’m presuming 500 sq ft 8 ft ceiling height; that’s 4000 cubic feet of air). Obviously, you will see that the AC deals with temperature immediately but it doesn’t run enough time to adequately reduce humidity levels.

      The first advice would be to either put the mini split on ‘Dry’ mode to increase the dehumidification rate, or to run it on a low fan speed setting.. That in itself might help you reduce humidity levels below 70%.

      If not, you have suggested a good work-around. If there is not enough air to be cooled down, you can introduce more air in the 3rd floor room via a supply vent. In order to generate air circulation, having a return vent is also necessary. This may help the mini split on the 3rd floor to run for longer (having to cool the extra air) and will thus reduce the humidity levels as well.

      What you have to think about is your 2nd floor unit. If that unit is a bit big for the 2nd floor, you might have the same high humidity problem in the 2nd floor instead of on the 3rd floor. Now, if the 2nd floor unit isn’t all that big (it’s adequately sized or even undersized), sucking some air into the 3rd floor won’t have a negative affect on that unit.

      Hope all of this makes sense 🙂

  3. I have a 97% 80,000 BTU modulating furnace for 3 rooms with 11′ ceilings and 26 windows totaling 1400 sf. There are 3 – 7″ round flex returns with potential for 330 cfm, but in 2×4 walls (3″x9″ about 75 sq in total) and 13 round metal supplys 1625 cfm potential (limited by all supplys coming off a 14×8 main duct – max 490 cfm). Did my HVAC guy really mess this one up? One room also has zero supplys!

    • Hi David, the main thing here is how well are the rooms connected. If one room doesn’t have a supply vent, it is presumed that the supply vent from other rooms will do a well enough job to compensate. That means that you have to have open doors between the room without a supply vent and the room with supply vents.

      About the total CFMs, if you use the basic 400 CFM per ton rule, the total airflow for an 80000 BTU furnace should be close to 2670 CFM.

  4. Thanks for the very detailed explanation on cold air return. But I do have a question. Just bought a 1990s 1600 sq ft home and put in a new 98% efficient furnace with modulating gas valve and variable speed DC motor. ( as well as A/C ). It runs very quiet. Had this furnace in our last home. Could barely hear it. But the quietness is offset buy the voluminous sucking sound of the cold air returns. Especially in the primary bedroom. 5 basic rooms on main floor and 5 cold air returns. 1000 sq foot basement developed with 3 cold air returns. I want to block off the primary bedroom cold air vent so the 6am “ let’s get back to temperature” furnace run doesn’t wake me up.
    Can I do this? The door is always open to the hallway and two other smaller rooms (9’x10’ each) that each have a cold air return.

    Thanks again.

    • Hi Doug, blocking cold air vents is usually not recommended; the furnace requires that air. However, you have a specific situation; you can try blocking it off in the primary bedroom and see if everything works fine. If the temperature doesn’t vary and if you don’t see any signs of restricted airflow, you can keep blocking it. That sucking sound at 6 AM sure is a problem. It might be that you would need a bigger cold air return vent; that will lower the velocity of the air being sucked in, and thereby reduce the noise.

  5. Hi,
    I am using my front room as an office and there is a cold air return vent on the wall I where I am planning to build a built-in wall unit. The base cabinets would cover the vent, but if I were to use a perforated / mesh door would this solve the problem of ‘blocking’ the air from returning to the furnace? Thank you. Zeba

    • Hi Zeba, the key here is to enable free airflow from your front room office to the supply vents. Mesh door will restrict this airflow a bit, however, it will let quite a lot of airflow through. It is hard to exactly quantify if the mesh door will allow for sufficient airflow. The only way to know for sure is to try it. If you see that your furnace has problems, you will have a good idea that the airflow is not sufficient. If not, the air distribution is adequate.

  6. My basement is finished with a return vent on either side of a partition wall in the middle of the main room.(one return in the wall cavity, pulling from either side of the the same wall, at floor level)
    The HVAC system is in a small room under the stairs that doesn’t get much air movement. It’s warmer/cooler than the adjacent rooms, so I’m wondering if I should cut in a small return vent, in the return trunk, just before the filter. An HVAC/Plumber suggested that I cut in a pass-through vent in the wall to help even it out, but all that did was create a better place for fan noise to escape.


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