What Size Return Duct For 3 Ton Unit? (Must Know It)

By - Ron Singh

Updated -

Textile ducts are designed to deliver and distribute cooled or heated air. The ducts are ideal for distributing fresh air effectively, consistently, and draught-free without creating dead spots.

A big question can be what size duct fits perfectly to your AC unit. Knowing the required return for your AC unit is essential because the aftermath of using the wrong duct on your AC will not be funny.  

An 18″ diameter circular return duct is necessary for a 3-ton air conditioner. A rectangular duct for a 3-ton unit needs to be 254 square inches, which is exactly the required size. If you can’t determine the actual airflow, use 400 CFM for each cooling ton. For example, the airflow rate of a 3-ton heat pump is roughly 1200 CFM.

What Size Return Duct Should I Use 16 Inch or 18 Inch?

What Size Return Duct For 3 Ton Unit

Choosing between 16 and 18 return duct sizes is based on the type of Ac you are using.

The return duct size of 16 inches is insufficient for a 3-ton AC. A 3-ton AC should have a return duct 18 inches in diameter.

For a 3-ton AC, a 16-inch return duct is too small. Ducts that are too small will increase static pressure, which will cause too much noise when air passes through the AC.

The heating unit will be overworked because inadequate ductwork is loud enough to be disturbing. Although 16 inches of duct return size will fit a 3-ton AC, 18 inches is advised.

What Happens if There is Not Enough Return Air? 

Your AC system won’t heat or cool your home effectively if there isn’t enough return air available.

Therefore, return air ducts need to be sufficiently big to transport enough exhausted air back to your furnace or air conditioner.

Bringing used air back to your HVAC system is the return air ducts’ primary purpose.

This evens out the airflow throughout the system and guarantees that there will be plenty of air returning to be cleaned, filtered, and pushed back out through the supply ducts. 

To stop pollutants and other particulates from being drawn into the airflow, return air ducts must be kept clean, securely fastened, and properly sealed.

The chances of pollutants entering your home’s indoor air are higher when introduced into the system through returns.

Additionally, these contaminants can overburden air filters, clogging them and decreasing their efficiency.

If the return airflow is less than necessary, you must determine why. Instead of measuring it, it is easier to look for the symptoms.

Below are some ways to tell if there is not enough return air. 

#1. Cold and Warm Spots

When there is insufficient return air, the hot and cold patches are the first problem most householders notice.

It is obvious when some rooms are warm and others are cool. This happens when there is not enough return air. 

The airflow will be more significant in the rooms nearest the AC system. There will be less airflow in rooms away from the AC system.

This is because the pressure will still be close to the device, and air will blow from the return vent.

But far from the unit, the pressure will drop, and you’ll notice that those vents aren’t putting out enough return air. 

In the summer, you’ll notice that rooms near the central air conditioning are cooler, and Rooms farther from the air conditioning unit will be warmer if there isn’t enough return air.

#2. Rooms Changing Air Pressure

A variation in pressure between your rooms indicates that return vents are not supplying enough air.

Rooms near the AC unit will receive more return air than those farther away, and You will experience various air pressures in various rooms.

There is insufficient return air if you notice various air pressures throughout your home. Air pressure can be measured with a barometer or your ears. 

#3. Using a Wet Finger

Your fingers are another useful human tool for determining airflow when wet. You should be able to determine whether there is enough or insufficient airflow by putting a wet finger close to the vent that produces airflow. 

You may feel the breeze by placing your licked finger next to the return vents. This is the simplest way to determine whether you are getting adequate return air in this situation or not. 

Only the speed or slowness of the air coming from return vents will be apparent. Since airflow is defined as the amount of moving air, it is impossible to know if there is enough.

Nevertheless, low airflow is typically indicated by low air velocity from your return vents.

Therefore, using this straightforward technique, you may determine whether you have sufficient return air or not.

#4. Warm Air from an Air Conditioner or Cool Air from a Furnace

You will experience issues with the air supply if the return vents are not supplying enough air. It is ideal for the supply air to be warm in the summer and the return air to be chilly.

The supply air will be warmer, especially at outdoor temperatures of 100°F or higher, if your ducts are not receiving enough airflow.

The AC unit may start pumping warm air instead of cool air, which is one sign you’ll notice. In winter, it will be the other way around.  

Duct Size Chart

For the same AC unit, rectangular ducts of different widths are acceptable as long as their total square inches are approximately equal.

For instance, the ideal 3-ton AC cold air return duct size falls between 16″x15″ and 26″x10″ and 40″x7″.

The math shows that these three sizes—240/246/280 square inches, respectively—are nearly 260 square inches. 

Below is a table for the required duct sizes respectively.

Should Return Duct Be the Same as Supply?

Return ducts are expected to be bigger than supply ducts to ensure that the air inside the house is balanced.

The necessity for larger return vents is caused by the system’s generally higher supply vent density. Maintaining an ambient environment requires ensuring enough air is provided and returned.

Return vents and ducts work together to pull air from your home and send it to your cooling system.

Usually found behind a grill, return vents are more extensive than supply vents and lack slats or louvers.

In addition, filters are typically located behind return vents to stop dirt, dust, and other impurities from damaging the AC system. But a filter is not necessary for supply vents. 

Can I Use a Flex Duct to Return Air?

You can use a flex duct to return air. Flex ducts are a viable option for return air.

However, to be effective, flex ducts must meet specific technical standards related to installations, duct size, position, pressure checks, particle filters, and insulating materials rating. 

Nevertheless, due to their affordability and simplicity of installation, flex ducts are an excellent option for return air.

In addition, they are considerably less expensive because of the material utilized, as opposed to other ductworks that employ more expensive steel.

As the name implies, it is very flexible, which makes installation simple. For example, if the duct needs to be installed in corners or via small gaps in your home’s construction, it is relatively simple to position it in these tight locations.

Flex ducts benefit your air conditioning system, which should never be underestimated. However, in contrast to standard ductwork, it is very technical.


If the supply and return duct is too tiny, your AC must operate longer, and it will be harder to cool your home.

Knowing your AC unit’s required return duct size is important because an undersized return duct harms your AC and causes noise pollution.

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