The key takeaway here is that air moves from a larger to a smaller duct, the velocity increases. When it moves from a smaller to a larger duct, the velocity decreases. In both cases, the flow rate — the amount of air moving through the duct, in cubic feet per minute — stays the same.
It's doubtful it will change the balance on your system. Flex duct only has about 80% of airflow capacity as same size hard metal pipe. The NCI recommended airflow for a 6" flex is 75 CFM, and 8" is 160 CFM, lower volume in a larger duct will definitely lower noise.
In addition, a larger duct allows the air to slow down a little, which reduces noise. So, unless your duct length is under a few feet, having a larger duct is probably the right way to go.
Each section of ducting that gets installed in your home needs to be appropriately sized for your HVAC unit to perform efficiently. If your ducting is oversized or undersized, it could lead to premature wear out of your heating and cooling units and an uncomfortable living environment.
One of the most used methods is the HVAC duct sizing rule of thumb. In fact, even professional AC contractors might use this method to size the AC ducts in your home. According to this rule, you need to install an air conditioner of one-tonne capacity for every 500 square feet of the floor area of your residence.
Oversized Ducts
But your HVAC system may not be equipped to pump that much air. In fact, your system needs a specific air pressure in the ductwork to properly distribute air. Oversized ducts could cause your HVAC system to work too hard and limit the amount of conditioned air that reaches your home.
Ducts that leak heated air into unheated spaces can add hundreds of dollars a year to your heating and cooling bills, but you can reduce that loss by sealing and insulating your ducts. Insulating ducts in unconditioned spaces is usually very cost-effective.
Examination of this chart (or the Darcy equation) reveals several interesting air flow properties: (1) at a constant volume flow rate, reducing the duct diameter will increase the pressure loss; (2) to maintain a constant pressure loss in ducts of different size, larger volume flow rates require larger duct diameters; ...
Common issues like leaks, disconnections, or obstructions within the ducts disrupt the flow of cooled air, causing rooms farther from the HVAC unit to suffer the most. In some cases, inadequate insulation of ducts can allow heat to infiltrate, further exacerbating the problem.
Obstructed Vent
One of the most common causes for poor airflow is a blocked vent. To properly heat or cool your home, the air needs to be unobstructed. To see if there are any blocked vents, look by the baseboards or on the ceiling. Furniture, bedding, and the vent damper could all be possible obstacles to the vent.
If you increase the pressure, the CFM decreases, and vice versa: lowering the pressure increases the CFM.
Metal ducts also provide superior air flow, due to their smooth interior walls that reduce turbulence and air resistance. This promotes energy-efficiency of your overall HVAC system, especially if the ducts are regularly cleaned and professionally sealed.
Written by Michael Haines 06/07/2024 In 2024, air-source heat pumps emerge as one of the most efficient options for residential HVAC systems. This article explores the reasons behind their efficiency, the different types available, and the factors to consider when selecting a high-efficiency HVAC system for your home.
1. Adjust the dampers. If your vents have levers or dials, that means you can control the airflow by adjusting the dampers (the little louvers inside that move up and down to restrict or open airflow).
The key takeaway here is that air moves from a larger to a smaller duct, the velocity increases. When it moves from a smaller to a larger duct, the velocity decreases. In both cases, the flow rate — the amount of air moving through the duct, in cubic feet per minute — stays the same.
The HVAC Duct Sizing Rule of Thumb. As a general rule of thumb, the diameter of your HVAC duct should be no less than 125% of the width of your furnace outlet. This is especially important when it comes to square and rectangular outlets, which tend to severely restrict airflow.
Generally, a 12' x 12' room requires a 4” x 8” duct. The main factor in determining this size is airflow volume, measured in cubic feet per minute, or CFM. To adjust the CFM, you can change the speed of the blower or alter the ductwork size.
Another common mistake is using too small of a duct or underestimating the amount of airflow required. This can result in excessive noise and vibration, as well as decreased efficiency. If your ductwork is not properly insulated, it can cause the air temperature to fluctuate, resulting in hot or cold spots.
Restricted Airflow. Hot and cold spots are a telltale sign of air-distribution problems. With undersized ducts, you may notice that the vents nearest your HVAC unit blast air while the pressure drops substantially in other parts of your home.
Ductwork has a maximum lifespan of 20-25 years, and starts to deteriorate around year 15. If your ductwork is more than 15 years old, you may need to have the ductwork inspected to see if it is time to replace it. If it is older than 25 years, you almost certainly need to replace the ductwork.