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.
The most common is the 'branch and duct' layout, but another option is the 'home run' layout. A trunk and branch system has a large main supply and a large main exhaust duct (the trunks) that are connected to the ventilation unit itself.
My colleague John Puryear teaches the simple “2 Foot Rule” for takeoff placement in his classes. When there is a change in air direction in a duct, it takes roughly 24 inches for airflow to restore its pattern. Because of this, John recommends keeping takeoffs 24 inches away from any turns, transitions, or the end cap.
The formula for calculating duct size often involves the airflow rate and the air velocity. The cross-sectional area of the duct (A) can be found using the equation A = Q / V, where Q is the airflow rate and V is the air velocity.
Just as oversized ducting can wreak havoc on your heating and cooling systems, so can undersized ductwork. When the ducting is too small for the specific HVAC system that you have, air flowing through it will start to back up. The air pressure within the ductwork will become higher.
The room's square footage will tell you what the CFM should be (12 x 12 = 144).
When setting up a ventilation system we typically figure a 7% loss in CFM for every 25 ft. when using flexible ducting (a 3% loss if using metal duct pipe). You should also factor a 3% loss for every 90 degree bend in your ductwork.
Cubic feet per minute: Calculating the CFM involves taking the HVAC unit's size in tons and multiplying it by 400. Next, divide that total by your home's square footage. That result gives you the CFM, which is used with the following factors to get the HVAC duct size you need.
The process includes calculating the home's heating and cooling demands based on factors such as square footage, insulation quality, window types, and the local climate. Moreover, the residential HVAC design guide process extends to determining the best layout for equipment and ductwork.
For example, for a standard residential HVAC unit, you'll generally need one supply vent and one return vent for every 100 to 150 square feet of living space.
In most cases, if you are installing ducts in each room where there is a supply, you will want to go with the same size as your supply vents. If you are installing just one or two return vents, you will want to go with a 12-inch return for 2-ton units and lower. 14- inches for 3 and up.
ASHRAE, the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, suggests in its Standard 62.2-2022 that residential buildings should have at least “0.35 air changes per hour, with a minimum of 15 cubic feet of air per minute per person” to ensure proper ventilation and acceptable indoor air ...
The supply plenum is an air distribution box. It's attached directly to the supply outlet of the HVAC equipment–that is, the outlet that releases chilled or heated air into your home. The air ducts that distribute the air into the individual rooms of your living space all connect to this plenum.
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.
In an equal friction design, all ducts should have the same pressure loss per unit length. Therefore, once the critical path is identified, the duct losses can be calculated by simply multiplying the design pressure loss (inches wg per 100 feet) by the total length of the critical path divided by 100 feet.
Since extractor fan vents aren't flat, the only way to obtain reliable air flow measurements is to use an air flow cone. The simplest set-up is to use a cone with a vane thermo-anemometer, although it's also compatible with a hotwire thermo-anemometer, provided the probe is centred in the middle of the cone.
Cubic Feet Per Minute Size Calculations
Since the size of your ductwork can increase or lower this measurement, you'll have to find the necessary CFM for each room before you can get the right duct size for each space. Cubic Feet Per Minute = (HVAC Unit Tons x 400) / total square footage of home.
Ductsize quickly calculates optimal duct sizes using either the static regain, equal friction, or constant velocity method. Data entry can be accomplished manually or taken graphically from Drawing Board. Duct sizes can be calculated on a round, rectangular, and flat oval basis.
One cfm is needed per square foot (1 cfm/sq ft) of floor area. This is the average air quantity required for a room or an entire building. This number is based upon an averaged heat load calculation for comfort cooling.
Sizes range from 4 to 16 inches in diameter, with 6 to 10 inches being the most common for main supply and return ducts. Their round shape allows for smoother airflow with less resistance, which can enhance your HVAC's efficiency.