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Question for the engineers re makeup air units
Ok, this has been bugging me for a while now. More than a few of the MAUs I've started have been recirculating-style, 80/20 split usually, for building pressure control. They often (but not always) have a stat on the return air to drive the damper to 100% outside air if the return air temp falls below set point; and/or a similar stat on the outside air that again drives the unit to 100% outside air. My understanding is that this is supposed to be some kind of oh-shoot-it's-cold response intended to warm the place faster or keep it from freezing up—but does it actually do that? It will certainly increase fuel consumption, but will it warm the space faster bringing in 100% outside air vs recirculating? These units control to discharge air, so until the unit is making its maximum ΔT it's going to put out the same volume of air at the same temperature regardless of how much it recirculates. In fact, I believe that going to 100% outside air will potentially limit the space temperature as the unit may not be able to make it's highest discharge air temp when it's trying to reheat potentially sub-zero air.
Or am I wrong in my assumption of the purpose of the stats, or my analysis of what will happen?