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  1. #1
    Join Date
    Oct 2016
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    Delta T on 2 circuit evap

    Iím curious whether anyone knows of a general rule when it comes to taking an air temp reading across a 2 circuit evap. I have a 15 ton Trane unit that Iím only getting around 63 degree supply air with 73 return air coming back to the unit. Only 1 stage of cooling is running so Iím wondering if thatís an acceptable supply air temp given only half of the coil is currently cooling. My suction pressure is a little high at 76psig (r22) but my superheat is hovering right around 8-10 degrees and its a very humid day here so there is quite a latent heat load on the evap, so Iím thinking that the higher supply air temp is due to all the moisture being removed from the returning air. The other units on that floor Iíve looked at so far all have supply air temps of approx. 58 degrees with one stage operating, but those units have a relief damper in the mechanical room that are somewhat open allowing the supply air to recirculate back through the coil so Iím sure that has something to do with the lower supply air temps. The unit with the 63 degree supply air has the relief damper completely shut. If a normally operating a/c has a 20 degree delta T generally speaking, would you expect to see a 10 degree delta T on a two stage cooling unit with one circuit running or is there more to it than that?

  2. #2
    Join Date
    Jan 2010
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    What's up with the second stage not working ?

  3. #3
    Join Date
    Nov 2011
    Location
    East Side
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    Whatís your model number?

    If you have a 2 circuit system, with 1 running, youíre not going to see the same numbers as with both. What is acceptable is based on your pressures and temps.

    I agree with VPT, is the second circuit supposed to be on?

  4. #4
    Join Date
    Oct 2016
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    Thread Starter
    Sorry for the late response, the day got pretty busy. My understanding is that both stages come on when the room temp is considerably off from the set point. Itís not uncommon for me to see only one stage running on these units. Model # SCIH15052A01010. I have no control over what the set points are, thatís all controlled by the building manager, I can see a few parameters on the display, I believe itís a Tracer controller. Talking with the building manager he has most of the zones on that floor set at around 73 degrees so I had to override the first stage on just to keep it running long enough to get my readings.

  5. #5
    Join Date
    Jul 2019
    Location
    Central Massachusetts
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    Gotta love energy management right? Trane can be weird sometimes as it is. IE; the Voyager series will call for stage 2 by itself if it has been running for more than a certain time on just stage 1. (heat OR cool) just how this happens Trane will not say, but they have sensors in the supply, the return, between the heat and cool sections, mixed air, in the condenser, in the condenser air outlet and an outdoor sensor. When these run correctly, GREAT! When they don't, you're in for a wild goose chase. Why didn't you jump it out to call for both stages?

  6. #6
    Join Date
    Oct 2016
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    Thread Starter
    I didnít jump the second stage out that day because I already had a couple weeks prior durning May scheduled maintenance so I know with both stages I can get good supply air temps, my concern that day was the high supply air temp from just the one stage running. Now that you mention how the voyager series works Iím betting thatís how the tracer works as well; if the first stage canít pull the temp down after a given time itíll call for stage two, but with the space not calling for cooling the unit wasnít running long enough to bring the humidity down so it was feeling pretty muggy in that area.

  7. #7
    Join Date
    Jul 2019
    Location
    Central Massachusetts
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    Right, and running for extended periods on just one stage is the way that many of these systems regulate humidity. If humidity is a problem, be sure that the fans are set to cycle if possible. Sometimes you have to run the unit in "unOccupied" mode and set those temperatures to your usual temperatures. Otherwise, the condensation on the coils just evaporates back into the conditioned space when the compressors cycle out. If there isn't a humidity sensor connected, you can add one and set it up with Rover™. That would eliminate the "muggy" feel in the building. It can be set up to run heat and cool at the same time if absolutely necessary. It will maintain a steady discharge temperature to the point that occupants never even know it's doing it. It just stays comfortable!

  8. #8
    Join Date
    Oct 2016
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    23
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    Thread Starter
    Great advice thanks a lot!

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