5 Tips for Sizing Your Heating Ducts

An HVAC worker installs heating ducting.

Heating ducts route heated air throughout the home. When installing a heating duct, it should be properly sized in order to provide the home with the correct heating requirements. If you install ducting that is too small, your heater will have to run more frequently to keep your house at the correct temperature because it’s not supplying the heat it needs to.

Making the ducts too large will make energy costs a lot more as well. To make the duct work effective, the heating duct size should be calculated correctly. Read on for more information on doing this properly.

1. Measure Your Rooms

One of the most important numbers to have in trying to figure out the sizing for your duct work is the square footage for each room you will be heating. Be sure your measurements are as accurate as possible.

2. Know the Required CFM

The total CFM, or cubic feet per minute, is another number you will need and this will depend on the size of the heating unit to be installed. You will need to do separate load calculations prior to figuring out your duct sizing to find the right unit you need to install for your home.

The output of a heating and cooling unit about is about 400 cubic feet per minute, per ton, so to get the total CFM output, multiply this number by the number of tons of your unit. Divide the solution by the total square footage of your home to get the number you will need to calculate the CFM needed for each room.

For example, a two and a half ton heating unit will produce a CFM output of 1000. If your total square footage is 1000, you will get an answer of one when dividing the CFM by this number. Then, for each room, you will multiply the square footage by one to get the CFM needed.

3. Use a Duct Calculator

An easy way to measure the required ducting size for a room is to use a duct calculator, which can be downloaded online. Take note that the diameter of the heating duct will depend on the amount of area to be heated.

4. Combine Heating Requirements of Separate Rooms Together

If more than one room needs heating, yet not the entire home, then simply add the requirements of the additional rooms to your calculation. Since each room will have a different CFM depending on size, make sure to measure the entire area of each room accurately.

If one room needs 100 CFM and another needs 200 CFM, then your total CFM will be 300 for the two rooms, and the diameter of your ducting will be based on that amount. To make finding these numbers a lot easier, make use of the calculator.

5. Install the Appropriate Duct Size Connections

Once all the rooms are considered for duct size calculations, increase the duct size accordingly. Run the correct size from the furnace and be sure they are installed correctly. Installing the duct incorrectly will affect the heating capacity of the system as much as improper sizing.

In order to avoid the heating system losing its efficiency, fit all your joints securely, and consider insulating the system. It is almost fruitless to go through the trouble of calculating your duct size if you’re still losing a lot of heat due to lack of insulation.

There are more complicated, accurate versions of these calculations that will make for a more efficient heating and cooling system, but these are best left to professionals who should know the codes and manuals inside and out. If you have a home in need of complicated duct work, it would likely be best to hire one of these professionals rather than performing the work yourself.