I came across a job in Texas where the engineer says the preheat coil for a 75%OA and 25%RA VAV AHU must have a recirculation pump installed at a large stacked parallel piped preheat coil and is to run anytime preheat is required, discharge air setpoint is 50 deg F. Usually when I see a recirc pump installed at a preheat coil it will only run when the freezestat is tripped. So my question is, Is this the general installation and sequence of operation in the Northern States which have much colder weather than what we deal with in Texas? I thought that when the recirc pump runs anytime the coil is heating then the performance of the coil will decrease. It will take return water and feed it back to the supply making supply water colder when entering the coil. My thinking has always been that a recirc pump is the last ditch effort to stop the coil from freezing and should only run when the coil cannot keep up with the cold air. Am I thinking about this wrong?
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