I have started work at an account that has a Trane RTU, larger one that is dedicated make up air. In it has a hot water coil and chilled water coil fed from the main plant. It has a in integral face and bypass upstream of the coils.
So intake, then filtering, then face and bypass then hot coil, then chill coil then fan.
My question has to do with the face and bypass. This face and bypass damper set up right now is modulating based off the discharge air set point of 55 degrees. I am pretty positive of this.
The hot coil is 100% actuated and flowing in this run sequence. The discharge air is able to hold set point real tight with this modulating face and bypass mixing raw outdoor air and a portion of that outdoor being preheated at the hot water coil. So we basically are bringing in outdoor air, some of that air goes through the hot water coil, some bypasses, those two air paths mix on the other side and walla we have our discharge air set point. As we trend up in discharge air or down, we will modulate those dampers. Like how mixed air does in an economizer sequence on a normal set up.
Is this the right way to be understanding this. I ask this because.
This unit feeds 5 different FCU's that then they both have chilled water and hot water coils in them. In the winter time, my assumption is, were gonna send down 55 degrees, if they need to reheat, then let it. The spaces that some of these 5 FCU's feed were not ever cooling down. Someone had the set point of the make up on this make up air set for 70 degrees discharge air. I reset it to 55, and life is good. Spaces being served by these FCU's cooling down, some starting to reheat. Others needing all that cool air. The volume of air is weak too, but that's because they need new bag filters and to clean the intake grills on the make up which will be done today.
When I reset the discharge air, the guy on site was freaking out because he was afraid the coils gonna freeze. For starters the chilled water coil is isolated and filled with glycol. There is a freeze stat, at the face of the hot water coil I tested and it works. And also, resetting to 55 degrees has given them cooiling in spaces they did not have before. They did not understand that we send 55, then if we need heat, we use the FCU hot water coil to reheat.
I was simply concerned about making certain the face and bypass is suppose to work to modulate this way since I am unfamiliar with that arrangement.
The stuff I am getting into on this account has been really amazing to me. They have had some real winners in this place working the system.