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Making sure you use the correct size for your ductwork is important for the size of AC you are installing. If you mix this up you won’t be pushing the air properly throughout the home or office.

So, **what size return duct for 3.5, 4, 5 ton AC**?

**The right size of return duct for a 3.5 ton AC is 22 X 22 inches. In the case of a 4 ton AC, it’s 24 X 24 inches. And for a 5 ton AC, the correct size is 27 X 27 inches. Calculate the return duct size. And multiply AC tonnage by 3.14 for round and 144 square inches. This is for rectangular ducts.**

Now, we have all the answers and calculations explained in detail in this article.

## What Size Return Duct for 3.5, 4, 5 ton AC?

Here’s a quick little chart to sum things up for you!

Tonnage | Rectangular Return Duct Size | Circular Return Duct Size |

3.5 | 22 X 22 in. | 15 in. diameter |

4 | 24 X 24 in. | 16 in. diameter |

5 | 27 X 27 in. | 18 in. diameter |

## How to Calculate Return Air Duct Size?

If you’re curious about the right-sized return duct, fret not! We can help you out, it just requires a couple of simple calculations.

For instance, for a 3-ton AC, here’s how you can calculate the return duct sizes.

### For a Rectangular Return Duct

- Tonnage X 144 sq. in. = 3 X 144 = 432 sq. in.
- Now find the square root, √432= 21 inches
- For a 3-ton AC, you’ll need a 21 X 21-inch duct.

### For a Circular Return Duct

- (Tonnage X 400 CFM)/2 sq. in. = (3 ton X 400)/2 = 600 sq. in.
- Now, 600/ℼ =191
- √191 = 14 in. diameter
- For a 3-ton AC, you’ll need a 14 inches circular duct.

Return ducts’ right side will eventually provide the best results. And add up to the overall level of comfort that you’ll get through your AC.

Besides, of course, boosting your AC’s performance and increasing its energy efficiency.

### What Size Return Duct for 3.5 ton AC?

**For a 3.5 ton AC, the right size of the return duct is 22 by 22 inches. But how to calculate return duct cfm that’s optimal for any size unit?**

Well, we have a few simple steps through which you can find the perfect **AC return duct sizing**. Let us take a quick peek at these steps.

First, check the outside of your condenser for a data tag. There you should find that the tonnage of your AC is mentioned.

Based on that, let’s say your AC is a 3.5-ton unit.

The calculations will vary depending on whether you’re using a circular or rectangular duct.

So, for a circular duct, you will have to find the diameter of the duct. Just follow through with the formulas above, and you should be good.

For a rectangular-shaped duct, take the size of your air conditioner (3.5 tons in this case). Multiply this by 144 square inches, and you should obtain the required size of the return duct.

Finally, you might need to grab your calculator for this if you hadn’t already. Simply calculate the square root of the total obtained value determined in Step 2.

And so, your return duct size for a 3.5-ton unit AC is, thereby 22 by 22 inches.

Also, see my CFM calculator.

# What Size Return Duct for 4-ton AC?

Now, for a 4-ton unit, the appropriate return duct size is 24 by 24 square inches. So you have to follow the same steps you applied above.

So, to calculate the **return duct size for a 4-ton unit**, multiply 4 by 144. Find the square root of the obtained value, 576. And you’re all set!

Therefore, as long as you maintain the 144 sq. inches per ton unit, you are good to go!

### What Size Return Duct for 5-ton AC?

Finally, for a 5-ton HVAC unit, 27 by 27 inches is the right return duct size. And again, you can use the same steps that you used above.

This way, you can calculate return duct sizes for any tonnage unit of AC.

You can always use a sizing chart to help you determine the correct duct sizes. There are also duct-size calculators that you can use. And a good example is the VariTrane Ductulator.

Now, what happens if you use a wrong-sized return duct for your AC? Let’s find out!

## What Happens if you Use a Wrong-Size Return Duct?

Return ducts in AC should always be determined based on the tonnage. And the wrong size is also based on whether the duct is bigger or smaller.

So, let’s have a look.

### Case 1: Return Duct is Too Big

When return ducts are too big, it could cause the ducts to hemorrhage as air passes through. Meaning your AC will start bleeding air.

And if cold air comes through from the return duct, it will result in a draft. And that leaves room for discomfort.

What happens is too much pressure is put on your unit when air flows through the ducts. Therefore, it will be harder for your system to distribute air properly. And in turn, maintain the right temperature.

As a result of this, more energy will be required. And the energy efficiency will also be below. Which will also add up to a higher electric bill by the end of the month.

### Case 2: Return Duct is Too Small

Return ducts are too small or undersized, lowering your AC’s efficiency.

It won’t be able to return enough air back into the system. And even normally, not all the air that goes into the supply duct gets recirculated.

However, with a smaller return duct, lesser air will get recirculated. So, you’ll have to run your AC system for much longer than normal.

And only then might you be able to achieve the comfort you seek from it. But you’ll be promoting higher electric bills in this case as well!

It’s also a good idea to avoid having ceiling fans too close to an AC vent.

Now, if you still can’t figure out the return duct sizing, contact an HVAC professional. They’ll certainly be able to help you out.

## FAQs

### Can I modify the size of a cold air return duct?

Yes, you can modify the size of a cold air return duct. First, cut out pieces of wallboard. This is to vertically expand the sizes of your existing ducts. Then, you can install a fitting air filter and new cover accordingly. However, you can also horizontally expand the return duct size.

### Are supply ducts and return ducts usually of the same size?

No, supply ducts and return ducts are necessarily always of the same size. Typically, return ducts are larger than supply ducts. Similarly, you’ll also require larger return vents as well. This is because there are usually plenty of supply vents. Even compared to the number of return vents in the system.

### How many return vents should you have in your house?

It’s good to have multiple return vents at home. But you’ll need to have at least 1 return vent per room ideally. But, if you have 2 or 3 per room, that will certainly lead to better airflow. Otherwise, all the air pressure will fall upon the single return vent. And that will lower efficiency.

### Final Words

That’s all we had on **what size return duct for 3.5, 4, 5 ton ac**. Hope the article was useful for you.

Place supply vents and return vents on opposite ends of the room for more efficient airflow.

Good luck and have a good day!

HVAC tech with over 30 years of experience. Retired and doing repair work on the side around Madison County, AL.