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For duct designing the static depends on the fan internal and external pressures.
for size of duct supply air is normally design at .1"/100' and return is taken as0.08"/100'
ISP=internal static pressure on fan>>>>result from the intrenal fan filter + losses throgh the impller
ESP=external static pressure on fan>>result from the losses through out the duct and damper and the air diffuser....in AC catalogs you shall find the ESP only to tell how long duct can be installed on that AC .
EX :; if in catalog (ESP=0) That mean that AC is not design to work with duct (mini split units)
