Table of Contents
- Well-cared of and accurately-measured ductwork is a critical component of indoor air quality and home comfort.
- If you’re not into real estate or construction works, get ready to add a few new words to your dictionary. In this article, we’re dealing with several complex terms and equations concerning airflow, vents, and duct sizes in your house.
- Below, we’re going to discover the size of a duct needed for a 12×12 room, but also several ductwork-related definitions, such as CFM (an acronym for Cubic Feet per Minute), BTU, cooling and heating ducts, air flow velocity, and others;
Where to start the ductwork revolution?
You most likely know this problem from your own experience: some rooms in your house are always warm or cold regardless of what you do to balance this disproportion.
The reason for it is that many older homes have quirks in their heating and cooling systems. And what can we do about it? First, update the existing ductwork sizing.
Keep in mind that determining the correct duct sizing in all the rooms is the solution to most of the issues related to:
- heating,
- ventilation, and
- air conditioning
(HVAC unit) system;
If you need to improve or update the efficiency of these systems in your house, keep reading! Together, we’ll discover how to proceed with calculating the correct ducts measurements.
Main terms
Before we get to math, let’s break down the main definitions to know exactly what we’re talking about.
First off, what is the ductwork? It refers to the system of ducts used to transport air from a heating, ventilation, and air-conditioning (HVAC) equipment throughout your home.
There’s nothing as crucial as properly installed and well-maintained ductwork.
Wait a minute. We know what are “ductworks” but what are “ducts” alone?
A duct is an individual metal or synthetic tube, conduit, or passage used in heating, ventilation, and air conditioning (HVAC) to deliver and remove air.
The airflows include, for example, supply air, return air, and exhaust air.
CFM or cubic feet per minute measures the volume of air, in cubic feet, for each minute it moves. This measurement describes airflow velocity.
In the case of an air compressor, the CFM measurement indicates how much air can move per minute. For example, if your air compressor’s output is rated 30 CFM, it means that 30 cubic feet of air are flowing per minute.
Consider that you’ll need 400 CFM of airflow per ton of AC.
Another shortcut that’s useful to know is BTU (British Thermal Unit)—an international measure of energy used to describe the amount of heat needed to raise one pound of water one degree Fahrenheit. In our households, BTU measures the amount of heat that an air conditioning can remove from a room per hour.
In general, a cooling unit, or cooling system, is the apparatus, or a set of them, employed to keep the temperature from exceeding limits imposed by needs of safety and efficiency.
In other words, the primary job of such units is to keep the internal enclosure temperature at a constant level, independently of the ambient temperature.
Duct size calculations
Having the proper duct size is equally essential as having the right size HVAC system.
Just think about it:
1. If your unit and system are too small, your unit will continuously struggle and try to run to keep up with the demand for heating or cooling,
2. If your system is oversized, it will continuously cycle on and off, eventually prematurely wearing down many of its components;
Important: A more extensive HVAC system isn’t always better! The most efficient choice is to find the most suitable duct size for all the rooms.
A few things to know to calculate the proper size of the ducts for your rooms:
1. rooms size
First, get to know the total square footage of your house to determine how ample your HVAC system needs to be.
Measure the length and width of all the rooms separately and multiply them together.
Writing down the square footage for individual rooms will also help you understand the ductwork you need for each space.
2. CFM (cubic feet per minute) velocity
Every room will have its particular velocity with which the air should be delivered through the ducts to heat or cool it.
Let’s break it down: All the rooms have a velocity CFM measurement that correlates to the size of ducts.
It takes about 1 CFM of air to heat or cool 1 to 1.25 square feet of floor area.
How to calculate the cubic feet per minute (CFM) needed for individual rooms?
This is how to measure airflow: One CFM is equal to 1 square foot unless the room has many windows. If this is the case, then use 2 CFM per square foot. So, for example, if your room is 100 square feet, you will require 100 CFM.
3. duct friction
If it wasn’t enough counting, the air traveling through the ducts loses energy and speed because of the friction.
The longer is the way that the air has to travel (and the more turns it has to take in the meantime), the slower it will exit through the vents into the room.
If you’re very into math and physics, have fun tackling it, but it’s probably best to have professionals analyze these equations for you.
Tip: The equation to calculate we talk about is duct friction equals available static pressure divided by total effective length.
Duct sizing for 12×12 room
Let’s see what ductwork sizing is required for a 12 x 12 room.
We’ve just learned how to determine duct size; let’s try it out.
- square footage of the room (12 x 12=144),
- CFM of the room (144 x 1 CFM [or 2 CFM with windows]=144 CFM), and
- the friction loss of the ducts;
Mind that these factors will be used for a highly complex calculation. Search for an online chart or calculator to find out the duct size for your house.
Use the online duct calculator to assess your duct sizes properly!
You can also do this by using a manual that distributors sell.
For example, if a room is 250 square feet, the result will be a duct size of 8-inch round.
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