If the unit and all these flexiducts are running through the hottest part of your house - 110+F - how the heck does it stay cool enough? Return is in 2nd floor(Looking to install unit for 1/2 finished attic and 2nd floor bedrooms). No installer will really explain. No commercial replies from SILA please!
The ducts are insulated. So the air inside them doesn't pick up as much heat as you might first think. A high velocity system is moving the air at a velocity between 1600 and 2,000 foot per minute(between a speed of 18 and 22 miles per hour). So its really not in the duct for very long.
A 2" duct with R6 insulation at a 70°F temp difference to the attic and the air in the duct, will only gain about 12 BTUs per linear foot of duct. So 200 foot of high velocity supply duct will only gain about 2400 BTUs(.2 tons).
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