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  1. #1
    Join Date
    Apr 2014
    Posts
    149
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    Trane RTU dehum cycle

    I'm having an issue getting a thermostat that sends a 24-volt AC signal from the aux port to initiate a dehum cycle in the Trane rooftop unit.

    The rooftop unit expects a volts DC signal. There is a DC power supply in the rooftop unit. How can I take closed contacts from a relay and use them to start the Trane dehumidification cycle, using terminals 18 and 19 on the unit low voltage terminal block?

    They are the type labeled RH- and RH+. Standard Precedent unit.

  2. #2
    Join Date
    Jan 2010
    Location
    Sea to Sky
    Posts
    3,452
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    It used a standard 24 VAC signal last time I did one of those through the LVTB connections.

    2 out of 3 I did had the stake ones pulled off on the back for some reason. Worked once I fixed it.

    Are they using two compressors, one of which has a reversing valve??

    Sent from my SM-G965W using Tapatalk

  3. #3
    Join Date
    Dec 2014
    Location
    GA
    Posts
    29
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    If it is a reliatel unit, use a relay to bridge terminals 7 (24v+) and 12 (Hum) on "LTB"
    OR
    land a 4-20ma sensor/plc output on the RH- and RH+ terminals on "NLTB".


    whatever you do... don't wire a relay across RH- and RH+...
    (a little plume of smoke comes off the RTOM J8 terminal block when that happens...or so i've heard)

  4. #4
    Join Date
    Apr 2014
    Posts
    149
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    Thread Starter
    Thanks for the suggestions, I will be going on the roof next week. These are Reliatel equipped units. It's too bad they don't prepare for anything except a Trane communicating wall stat with a duct mounted RH sensor.

    PAB, I usually work with RIB relays, the U1C model. So pretty much I could use the stat to send coil voltage to the relay, close the Normally Open contacts and then wire the Comm and N/O wires to the 7 and 12 terminals? I can't find documentation in the manual that says if the 24Vac terminal outputs 24 or is expecting input voltage. Won't get there until next Tuesday to tag a meter on the terminals.

    I do see that the RTOM board J10 is a binary input for the reheat valve, so seems like I could also wire the relay there and it would take off. There are 5 Precedent units all the same setup.

  5. #5
    Join Date
    May 2018
    Location
    VA/DC
    Posts
    281
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    the 5 trane rtu I have at a site have the analog input, and you can set the points with tu or bacnet OR what im doing is using the RH dry contact input to initiate. I use a RIB to isolate the power supplies.

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