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Thread: Trane RTU

  1. #1
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    Trane RTU

    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.

  2. #2
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    I am going to re-read what you said but just first glance it sounds 100% right,

    and yes send 55* air to the FCU's and then let their own hot water re heat as needed.

  3. #3
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    I have a lot of Trane RTU's setup just as you described.

  4. #4
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    sounds perfect. the sole job of the rtu is to send 55*air, conditioned and with required fresh air to the FCU's

    your hired

  5. #5
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    so in this arrangement the face and bypass are modulating to achieve the discharge air. I thought so, just wanted to make sure. And so we keep that hot water coil active 100%. Let the face and bypass decide position, take heat and mix it.

    Well. Another day.

  6. #6
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    With stand alone controls, you are stuck with supplying the 55* as previously mentioned.

    However with the proper controls in place and all the items networked, you could reset based off of demand from the served zones.

    You would be surprised as to just how much you save when your not going down to 55* when all is calling for heat

  7. #7
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    Quote Originally Posted by Chris_Worthingt View Post
    With stand alone controls, you are stuck with supplying the 55* as previously mentioned.

    However with the proper controls in place and all the items networked, you could reset based off of demand from the served zones.

    You would be surprised as to just how much you save when your not going down to 55* when all is calling for heat
    that was my next thing. I can do this and will be going back next week to do.

    The 5 FCU's will be resetting the make up unit discharge air based on there demand. If everyone is satisfied we will float up. I will float the dampers and the actuated hot water. I am gonna play around with a few different options. But indeed, trying to float up will be a huge energy save.

    I wanted t make certain I understood the face and bypass, and the fact that 100% hot water in this sequence. Now that I know this was how it was intended, I can now do some floating and stay with the original intent. I have 2 FCU's that need cooling a lot. But maybe I will be successful floating to 60 degrees DA on those guys when everyone else is stylin. I got the controls. I am gonna plan a few different scenarios and start with one and go from there. I just got off the phone with the Superintendent of the building. They have not had cooling in those 2 trouble zones for 10 years with out bringing on chiller or being uncomfortable. So he's now thinking I am a genius. Little does he know. LOL.

    Thanks guys. It's nice to come on here and talk shop and get answers.

  8. #8
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    Quote Originally Posted by Dowadudda View Post
    that was my next thing. I can do this and will be going back next week to do.

    The 5 FCU's will be resetting the make up unit discharge air based on there demand. If everyone is satisfied we will float up. I will float the dampers and the actuated hot water. I am gonna play around with a few different options. But indeed, trying to float up will be a huge energy save.

    I wanted t make certain I understood the face and bypass, and the fact that 100% hot water in this sequence. Now that I know this was how it was intended, I can now do some floating and stay with the original intent. I have 2 FCU's that need cooling a lot. But maybe I will be successful floating to 60 degrees DA on those guys when everyone else is stylin. I got the controls. I am gonna plan a few different scenarios and start with one and go from there. I just got off the phone with the Superintendent of the building. They have not had cooling in those 2 trouble zones for 10 years with out bringing on chiller or being uncomfortable. So he's now thinking I am a genius. Little does he know. LOL.

    Thanks guys. It's nice to come on here and talk shop and get answers.
    Sometimes it also better to serve a nuisance caller with a mini split or something, instead of firing up the main plant to satisfy "a" zone.

  9. #9
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    Quote Originally Posted by Chris_Worthingt View Post
    Sometimes it also better to serve a nuisance caller with a mini split or something, instead of firing up the main plant to satisfy "a" zone.
    if I had it.

  10. #10
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    You need to explain to your customer that face/bypass systems were designed specifically to prevent coils from freezing while maintaining a constant volume of air.
    UA LU189

    10mm, because it's better than .45acp

  11. #11
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    The hot water valve can't react quick enough to maintain 55* without tripping the freeze stat if it is allowed to close.

  12. #12
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    The hot water valve can't react quick enough to maintain 55* without tripping the freeze stat if it is allowed to close.

    Hey Chris, Did you get your Voyager blower to stop blown?

  13. #13
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    Aug 2007
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    If no nuisance freeze stat trips on them really cold days then all will be fine.


    Expect nothing, yet expect the unexpected.
    Press on Regardless, Endeavor to Persevere.

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