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# How do you know what size ductwork to use?

## How do you know what size ductwork to use?

To calculate CFM, you need to know the size of your heating or cooling unit in tons. Multiply this number by 400, which is the average output of an HVAC unit. Then divide by the total square footage of your home. This will give you the multiplier for all your rooms’ CFM.

How do you calculate duct size for airflow?

To calculate duct sizes use the equation Quantity = Area X Velocity. By calculating duct sizes with this equation it is possible to change duct sizes and maintain the same quantity (CFM) and velocity (FPM) in the duct. However, the pressure loss changes each time the duct size changes.

Is it OK to reduce duct size?

Therefore, reducing ducting from the main trunk line to smaller ducting may help keep the cost of the heating system down. Beware though, reducing the size of the ducting too much will result in the air-speed velocity within the ducts to become too fast. This may cause problems with the pressure within the system.

### What happens if ductwork is oversized?

Using oversized ducts will erode the system’s efficiency, while undersized ducts won’t carry the necessary volume of air. Balanced airflow. To maintain neutral pressure in the HVAC system and throughout the house, the duct system must deliver and return a matching volume of air.

How much can I reduce duct size?

Just a 1″ reduction in diameter impacts performance by 32% (or more)! A 7″ duct (in place of 8″) will reduce 600cfm air flow to 400cfm.

Can ductwork be too big?

Ductwork is supposed to be sized based on the unique needs of your house, so it is possible that it could be either too large or too narrow. The old saying goes that bigger is always better, but that doesn’t necessarily hold true for HVAC systems. Using a handy chart, your HVAC expert can properly size your ductwork.

#### Can duct work be too big?

It is often loud enough to be distracting, so it will not be difficult to overlook and should be addressed, as undersized ducts overwork the heating unit. On the other hand, a duct that is too large will result in less heated or cooled air reaching the intended destination.

How many CFM can a 12 inch duct handle?

CFM Sizing Chart For Metal Round Ducts (50-2,000 CFM)

Duct Size (Inches) Metal Duct Airflow (CFM)
8 inches 180 CFM
9 inches 240 CFM
10 inches 325 CFM
12 inches 525 CFM