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  1. #1
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    Mar 2016
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    Help on low voltage 10 ton units

    I am working on a Church built in the early 1900's. They were given some 10 ton gas packs 30 years ago and converted them into straight condensing units by disconnecting the blower and evaporator coils then piping to air handlers inside the building.

    My first problem is this. I have a 20 ton air handler with two 10-ton condensers. After looking at the low voltage wiring I am not sure if the systems were ever wired correctly. Anyway I noticed that the low voltage wiring was old and the insulation was falling off. I cut the wire back at a good point. When I energized the thermostat I found 24 volts where I cut the wire. I then ran new low voltage wire to one of the converted condensing units and it fired up. I now need to get the second 10-ton condenser to fire up with the first.

    My thinking is to add a relay that when the system calls for cool and energizes unit (1) one it also energizes 24 volt contacts on the relay and takes 24 volts from unit (2) transformer and makes the connection to the contactor on unit (2).

    Am I correct in my thinking? I want both condensing units to come on at the same time. Any advice would be appreciated.

  2. #2
    Join Date
    May 2004
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    SE Pennsylvania
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    Has it worked for the last 30 years thinking something was correct at one point? If they are two individual condensers and evaporators stage them.

  3. #3
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    Apr 2016
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    Yes use a 2 stage stat and have Y2 work the 2nd condenser.
    You may want to upsize whichever transformer you have connected to the stat since it will be powering two circuits.
    Staging is for sure the right way to do this.
    If you insist on starting them both together every time, run a wire from Y at one condenser to Y at the other. If you don't want to upsize the transformer check your control voltage under load (preferably as both compressors start and are running) to make sure you don't drop it too low.

    No need for additional relays

  4. #4
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    Quote Originally Posted by kylefield View Post
    I am working on a Church built in the early 1900's. They were given some 10 ton gas packs 30 years ago and converted them into straight condensing units by disconnecting the blower and evaporator coils then piping to air handlers inside the building.

    My first problem is this. I have a 20 ton air handler with two 10-ton condensers. After looking at the low voltage wiring I am not sure if the systems were ever wired correctly. Anyway I noticed that the low voltage wiring was old and the insulation was falling off. I cut the wire back at a good point. When I energized the thermostat I found 24 volts where I cut the wire. I then ran new low voltage wire to one of the converted condensing units and it fired up. I now need to get the second 10-ton condenser to fire up with the first.

    My thinking is to add a relay that when the system calls for cool and energizes unit (1) one it also energizes 24 volt contacts on the relay and takes 24 volts from unit (2) transformer and makes the connection to the contactor on unit (2).

    Am I correct in my thinking? I want both condensing units to come on at the same time. Any advice would be appreciated.
    Make sure you only have ONE transformer powering your control loop! There should be nothing attached to the thermostat side of the R terminal in the other condensing unit. If there are no control boards in the "slave" unit you could disconnect the transformer entirely

  5. #5
    Join Date
    Mar 2016
    Location
    East Texas
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    I've only have 2 conductor wire feeding condenser and it would be really hard to run new low voltage wiring. What's the benefit to staging? I could install a time delay to stage in the slave unit if it is better to be staged. What would be best time to bring on slave unit?

  6. #6
    Join Date
    Jul 2012
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    WA
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    Staging will provide better control and comfort during low load. Why are you having to redo the control wiring? How was it set up?

  7. #7
    Join Date
    Mar 2016
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    East Texas
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    Quote Originally Posted by Red Man View Post
    Staging will provide better control and comfort during low load. Why are you having to redo the control wiring? How was it set up?
    The old man who has been overseeing the church for the last 30 years said he's gone through a few contractors over the last two years and that everybody acts like it they don't understand what's going on.

    After going through the Church I can tell you that it is anything but typical. The started with a chiller system and a boiler and now have air to air and a boiler.

    Anyway the 24 volt 2-wire wiring originally came into the now slave condenser then went over to the other condenser (This wire was not connected at the first unit it was connected to contactor at other condenser. they still have components wired in that are not needed like Transformers from when they where gas package units.

    My question is this. If I only have 2 conductor low voltage wire, What is the best way to stage these units?

  8. #8
    Join Date
    Jul 2012
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    WA
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    If you have 2 wires going to each, then I don't see an issue. Are you saying you have only 2 wires from inside going to one unit, but they aren't connected at that unit, and continue on to the other unit? If you're short on wires, you could go wireless to an EIM.

  9. #9
    Join Date
    Mar 2016
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    East Texas
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    Quote Originally Posted by Red Man View Post
    If you have 2 wires going to each, then I don't see an issue. Are you saying you have only 2 wires from inside going to one unit, but they aren't connected at that unit, and continue on to the other unit? If you're short on wires, you could go wireless to an EIM.
    One 2 conductor wire coming from the building to turn on both 10 ton condensers. The wire was ran into first unit where there was a jumper wire going from the first unit to the second units low voltage side of its contactor. The wires were not connected in the first unit when I first arrived. I think the guy before me started to work on it then got frustrated and left.

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