![]() ![]() Now take that potato and shove it in the tail pipe of your car so the exhaust only goes through the hole you made. Imagine taking a potato and cutting a hole through it so you can see through it.These are some issues with having a duct system that is undersized. ![]() Some customers ask if putting a bigger unit will help keep the house conditioned better and the answer is absolutely not! More BTUs means faster and better conditioning. There is a misconception that bigger is better. Also we have conducted evaluations where the entire duct system was undersized by at least 1-ton of air or 400 CFM! That’s the same amount of airflow you would use to supply air to four standard 10X11 bedrooms! Having a trunk line sized for 400 CFM of air for a branch line set that equals a combined total of 600 CFM will short those rooms almost a half-ton of airflow! This is more common than you may think. When sizing the duct work, the calculations need to start at the farthest point from the unit and work backward. Also trunk lines (think of major highways) need to be sized correctly to be able to supply more than enough air to the branch lines attached to it. If you measure the size of all the supply vents and add up all the CFM output for those vents, they should be very close to the total output of CFMs that the indoor unit produces. For example a typical 6” round flex duct which is common for bedrooms, kitchens, dining rooms, will produce approximately 100 CFM of air. The dimensions of the opening of that branch line equates to a certain amount of CFM. There are branch lines (either round or rectangular) that connect from the main trunk line to the vent. The duct system consist of vents which are in each room. ![]() So a 3-ton unit should supply approximately 1200CFM of air to the duct system. Typically an AC system puts out approximately 400CFM per ton that the AC unit is sized for. Indoor units supply so much CFM based on the size of the indoor unit (basically tonnage) and the manufacturer specs for the blower assembly of the indoor unit. Airflow in the HVAC industry is measured using CFM, or Cubic Feet/Minute. From our professional experience, homes in the Jacksonville area and approximately 80% of the evaluations we perform have had duct systems that were undersized to the A/C system that was installed. There is such a thing as a ‘ size’ for your duct system. ![]()
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