What size duct do you need? HVAC systems

By admin | Dehumidifier

What size duct do you need

HVAC systems are important for our homes as they help us in surviving the harsh winter chill and the torturous summer heat. A duct system paired with the HVAC system could assists you in channeling the air to different areas of the house where it is needed.

A duct system is a network of round or rectangular tubes designed wholly for the purpose of transporting the air from the central HVAC system to other parts of the house. ‘What size duct do you need?’ This simple question holds great significance when it comes to ductwork. In ductwork, size is considered everything. The right size duct will provide the optimal amount of air from an HVAC system.


For me, ductwork is the most complicated, significant and least appealing part of an HVAC system. If there are no ducts, the heated or cooled air will never spread through the entire house and stay trapped only in a single room.

What are the disadvantages of wrong size duct?

Most of the people think that large duct sizes offer good air flow. This thinking is utterly wrong. If you simply choose a large duct size without taking into account the cooling or heating capacity of your heater or air conditioner, your duct system might not be able to maintain the desired air flow. The duct work will cost more, strains the system mechanically and can be noisy, hence annoying.

Small size ducts put excessive strain on the HVAC system. To provide enough cooled or heated air for the entire home, the system has to run frequently otherwise it won’t be able to keep the house at desired temperature levels. The consumption of energy by the system will be greater and utility bills will be higher.

Therefore, selection of the right size is essential because it will not only distribute the air evenly but also makes a big difference in the bills.

How to calculate the duct size?

The disadvantages associated with the wrong size duct has made obvious that you should be very careful when making a choice because a wrong choice comes with consequences.

Required Calculations:

For calculating the correct duct size, there are a number of things you need to find out first. They are listed below:-

1. Size of the room:

Before selecting the duct size, you need to measure accurately the square footage for all the rooms you want to cool or heat separately. Note down these important figures.

2. Calculation of CFM

Another number significant for finding out the required size of the duct is CFM i.e cubic feet per minute. This term solely depends on the size of HVAC system you are going to install. For calculating the CFM, these steps should be followed carefully.

• Multiply the output of your HVAC system by the number of tons it weighs.

• Divide the result by the total square footage of your house. The resultant will help you in calculating the CFM for each room.

• To get the CFM for each room, multiply the square footage of that room by one. If CFM supplied to each room isn’t calculated separately, there will be an uneven distribution of air throughout the house that leads to uneven temperatures.

3. Duct Calculator

In order to determine the duct size, one of the easiest ways is to immediately visit a website having an online duct calculator. All you need to do is insert the values of friction rate and CFM. The calculator will display the result quickly. One important thing that must be noted is that the diameter of the duct depends directly on the area to be heated.

4. Installing the Duct System for Selected Rooms

Due to lack of money or some other reason, if your intent is to install the duct system only for a few rooms in the house, then you need to calculate the CFM of each room separately and add the resultants.

5. Installing Ducts

Duct installation is as important as calculating the size. A minor neglect could lead to undesirable results. All the joints in a system must be fit securely. Insulating the entire system is necessary too to avoid any leaks.

You should also be careful about the location of your duct system. If it is located in a place where the air is polluted and the connections aren’t sealed properly, chances are that the contaminated particles will mix with the hot or cool air and will be distributed throughout the house. So, make sure the ducting is well sealed.


In ductwork, calculation of the size is the most important thing. A proper duct size will maximize the efficiency of your duct system and keeps your house at the desired temperature by distributing the air evenly. Other than size, there are several issues needed to be considered for an effective duct work like the space available, appearance, capacity for expansion, noise level etc.

There are other far more complicated methods available to calculate the right size of the duct for your HVAC system. Therefore, it is advisable to hire a professional for the job instead of doing it yourself.

If you are interested in taking care your home air, maybe you like to read top 10 best basement dehumidifier or top 5 best dehumidifier for bathroom​

About the Author

Hello Friends, My name is Minnie Scott, the founder of CoolDehumidifier.com and I would like to take the opportunity to welcome you to our site. This site will let you know the absolute information about different aspects of various brands in terms everyone gets the right point. The information brought to your view here will help you make the correct decision before you hit the “buy” button.