Anyone good with HVAC here ... I am adding a duct/fan to bring hot air from a loft area down to the floor of the main level, in order to help reduce heating costs (and possibly, in summertime, to make the loft area more livable. So this is separate of my main HVAC ductwork. I'm trying to figure the pressure the fan (probably a Fantech FR series) is going to be working against. The CFM of the fan drops off as the pressure head increases, but it doesn't get too significant until about 0.5" H2O, so I'm trying to figure out how much. The duct calculator says I should be below 0.2" or so, just based on the length and diameter of the flexduct. But since I'm sucking quite hot air downwards about 15ft, that's going to add some pressure over and above the friction loss in the ductwork. But I have no idea how to figure out how much. Thanks.