## Introduction

So you are having a new duct work installed. And you are still not sure what size you should use for return. Plus, you’ve got several tons of AC units. One of them is 3.5 ton.

Now you’re asking **what size return for 3.5 ton unit?**

**About 24/24 inches, 1400 CFM return is required for 3.5 ton unit. To calculate the size, you need to first know the way to size an ac return. Firstly, look for the size (ton) of your ac. Then multiply it with 144 square inches and get the square root. **

Still confused? Well, Don’t worry! Because in this article we have included everything you should know!

Keep reading to find out!

## How to Size an Air Conditioner Return Air Duct?

Appropriately sizing as well as **installing the air conditioning system** is crucial. Because this way you can make sure that the air conditioner functions expeditiously.

As per the United States Department of Energy, you need to remember one important thing. That is you should have a sufficient supply of vents that blow the air out.

Source: theurbanhousewife.com

Plus, you need enough return vents that suck the warm air back into the air conditioner. Moreover, incorrect duct installation is going to affect your air conditioner’s effectiveness. And your electricity bill.

So follow these given steps below. So you have the proper idea of how to size an air conditioner return air duct.

### Step 1: Look for the AC Size

First of all, you need to search for the data tag. It should be on the exterior of the condenser; probably on the outside unit. It is needed to infer the size of the air conditioner. For instance, whether it is 3 ton or 3.5 ton.

### Step 2: Select the Duct

In this step, you need to decide if you will be wielding a round duct. Or you are going to go for the rectangular duct.

### Step 3: Do the Calculation

After you have decided, now it is time to calculate. Evaluate your return duct size based on your air conditioner’s size. Also, take into consideration the kind of duct that you are utilizing. You can use the formulas given below.

If you’re dealing with a rectangular duct, first take into account the size of air conditioner. It can be 4 or 5 tons. Then multiply it by 144. Here 144 refers to 144 square inches. Then you will find the results for the **return air duct in square inches**.

Now, if you’re dealing with a round duct, then first take the diameter of the duct. Then multiply it by 3.14. Then you will get the return air duct results in square inches.

### Step 4: Get the Final Outcome

Then you need to calculate the square root of the total. For instance, 4 tons multiplied by 144 square inches. And you get 576 square. Now you have to square root it. So the square root of 576 will be 24. Hence, the return air duct and grill size should be 24/24 inches.

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## What Size Return Is Needed for a 3.5 Ton Unit?

So, following the rule, multiply the tonnage that is 3.5. Multiply it by the cumulative BTU per ton. Then calculate the square root of it as suggested. Then it should provide you the aggregate BTU rating.

Source: prismhomeinspections.com

So, using the previously mentioned formula, multiply the 3.5 ton unit by the 144 square inches. So the result will be 504 squared. Then calculate the square root of 504 which is approximately 22.5.

Hence, the return air duct and grill size should be exactly 22.5 by 22.5 inches.

Approved airflow is commonly 400 cfm per ton of an AC. Any 3 ton AC will thus require about 1200 cfm airflow.

When it’s just 423 square inch of return. The air velocity through the grill will be barely over 400 ft per min.

This number is decent as far as noise is involved. The velocity that is more than 500 ft per min is going to be too noisy. Inappropriate for any residential system.

As significant as the air velocity stands for noise, you need to consider the pressure drop. That is across the grille as well as the filter. If the pressure is too high, it will induce problems. Sometimes, you may notice cold air drafts from returns.

You might face issues sustaining the recommended airflow. Plus, it also accentuates the air shifting fan motor.

Hence, here we are taking 400 CFM per ton of an AC as instructed. So, the 3.5 ton is supposed to have around 1400 CFM.

## What Size Return Is Needed for a 4 ton Unit?

First, you need to know **how many CFM is a ton of return**?

**Well, the airflow is estimated in cubic feet per min which is CFM. And approximately 350 to 400 CFM per ton of cooling is needed. It is for adequate air conditioning system regulation. So, after calculation, it should be around 1600 CFM for a 4 ton unit.**

So, using the previously mentioned formula, multiply the 4 ton unit by the 144 square inches. So the result will be 576 squared. Then calculate the square root of 576 which is 24. Hence, the return air duct and grill size should be exactly 24 by 24 inches.

## What Size Return Duct for 5 Ton Unit?

So, **what size duct do I need for a 5 ton unit**? Using this technique, two 20 inches by 25 inches return grilles should be the perfect choice. That is when you’re using a **5 ton HVAC system**. So you can really deliver the 2,000 CFM back to that blower without inducing the return to hemorrhage.

You’re curious **what size should the return air grill be for a 5 ton AC unit**?

**The answer is, 500 square inches should be the total area of a grille. Two 20 inches are used employing this technique. Which is multiplied by 25 inches. Return grilles are surely the nicest choice in this case.**

## How Big Should the Return Air Duct Be?** **

You should know that usually if you’re utilizing a **Duct Sizing Chart** or possibly a calculator. For instance, a trane ductulator.

The size of return ducts should be 0.05 inches per 100ft or 0.40 Pa/m. This is based on the anticipated airflow via the return air duct. The cumulative amount of air from the returns should be a minimum of 250CFM per nominal ton. That is 33 l/s per kW.

Well, this sometimes differs according to which ton size you’re using. So, you can check the difference between 4, 3.5, and 3 ton AC. Just to be sure!

## FAQs

### How Many CFM is a 4-inch Duct?

Evaluating the size chart, any 4-inch pipe will disseminate 395 CFM at 4500 FPM. It says that a 4in pick-up on any machine will wield 395CFM from the filtering system. Or after gauging if you see that the machine will need nearly 400 CFM. That’s to eliminate the waste. Then you must use a 4-inch pipe for the purpose.

### How Much Return Air Do I Need Per Ton?

The typical square feet/ton is 400. There is nothing called too much return air. Suppose you’re operating an HVAC. Then you’d wish for no less than 144sq inches per ton.. So, if your house is around 16 sq ft, using a 4ton unit would work great.

### How Many CFM is a 5 ton unit?

A 5 ton system needs to have 2,000 CFM. Similarly, a 6-ton system needs to have about 2,400 CFM and so on. Gauging how much airflow distributions fluctuate from the math is very crucial. It is also a fast way to identify airflow problems in the residential duct system.

## Wrapping Up

Hope we have cleared all your queries about **what size return for 3.5 ton unit?**

Now you know how to determine the right size for your 3.5 ton unit. And 4 and 5 ton units as well. Not only that. With the instructions for sizing provided, you’ll now be able to deal with any tons size. We are sure of it!

That is all for now. Have a good day!