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  1. #1
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    1 condensing unit with 2 different sized coils

    Ok I know there is a lot of you that are going to say "just put in a separate unit and be done with it!" or "why mess with it?" Well, where is the fun in that! I have an existing 3 ton condensing unit and furnace. The plan is to build an out building that will need some A/c in it. I am going "T" into the line set at the CU. I figured I will put liquid line solenoids and check valves at the CU for both coils. The furnace already has a TXV. The outbuilding will also. Here is another thing; the coil in the out building is going to be 1.5 ton.

    OK, the logic is going to be when the 3 ton furnace calls for cooling the solenoid will open on the 3 ton furnace, the 1.5 ton solenoid will be closed. When the 1.5 ton calls for cooling with the 3 ton, I will modulate the CFM across the 3 ton coil to an effective 1.5 tons. I will keep priority to the 3 ton furnace because it’s the house where most everyone is. So in saying that if the furnace cannot keep the house within 1 or 2 degrees I will turn off the 1.5 ton, closing the solenoid and close the air bypass at the 3 ton. Now what if the 1.5 ton needs cooling and the 3 ton doesn't? Hot gas by-pass? or... I dunno. I don't want to change the CU to a two speed or anything. The CU and the furnace is just a couple of years old. I just want to add the 1.5 onto this rig and see it work! I will have a DDC controller and be able to have pressure inputs and so on so it’s not going to be a "duct tape and relay" job.


    So here is what I have so far:

    1. Rheem 13 SEER furnace and condensing unit.
    2. VLC-853 Alerton controller
    3. VLC-651R Alerton controller

    So what do you think?

  2. #2
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    Sep 2007
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    Whacked!!!

  3. #3
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    Thread Starter
    Gees rotaryfrk, just the response I was looking. I guess I have seen something similar done before with four evap. coils with one condensing unit and I would think that with just two coils it wouldn't be that difficult.

    http://www.daikin.com/global_ac/prod...feature04.html

  4. #4
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    Jun 2007
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    If i understand your post correctly, you could get it to work. The DDC is overkill. Read up on the average install for a supermarket cooler system with multiple evaps(EPR's). Same difference. Take some finessing that's all.

  5. #5
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    as a project to occupy your mind I guess you could make it work, but it would seem to me that zonig the system would be much easier.

  6. #6
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    I have one cond unit and 2 ID system in my house. But the OD is 2-speed 3 ton compressor and 5 ton double row coil. No check valves -- just LL solenoid valve and TXV

    Upstairs is 3 tone evaporator coil with total 560 CFM
    Downstairs 2.5 ton coil with total 440 CFM

    If one calls -- low speed
    If both call -- high speed

    works wonderfully.

    If you're gonna go overboard for the funzzz, get an inverter board out of the mitsubishi to change the load capacity of your compresor. Hey if it dies you can always have a perfect reason to say "Honey ! Time to upgrade our A/C"

  7. #7
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    Thread Starter
    Well flange, the 1.5 ton is in a whole other building. So running a duct out to it wouldn't work.

  8. #8
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    Just_Opinion-is there any reason you used a 5 ton 2 row condensor unit with a 3 ton compressor? Efficiency-SEER? Someone had brought up that what would happen when the 1.5 ton coil was cooling and the 3 ton wasn't what would happen to the extra refrigerant? Is this why possibly?

  9. #9
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    Quote Originally Posted by xarralu View Post
    Just_Opinion-is there any reason you used a 5 ton 2 row condensor unit with a 3 ton compressor? Efficiency-SEER? Someone had brought up that what would happen when the 1.5 ton coil was cooling and the 3 ton wasn't what would happen to the extra refrigerant? Is this why possibly?
    Just just want the SEER. I put duck and chicken together to make a soup bowl -- funzz

    Heil coil, Lennox compressor, Trane variable speed motor, Lennox blade etc..

    I don't know your case will work with 1.5 ton in garage calling with 3 ton pumping capacity. Even though you try to compensate for the larger unit to run half load (I call it half A@#)

    You well know that the house is better insulate. Thus garage calls more often and loner runtime and the house may over cool.

    I am not worry about not having enough freon coming back to cool the compressor. I worry more about the large coil may freeze if you have running long enough. Or even your duct sweat. My house run about 23-25 delta T.

    Hey ! try it out. Worse comes to worse, you put a 1.5 ton condenser out next to the 3 ton. It ain't kill no body. We try to squeeze the dollar

  10. #10
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    Thread Starter
    So just_opnion, probably a couple of 3 ton coils would be easier. Run one or the other. If they both run, run with both in bypass so they split the tonnage.

  11. #11
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    Quote Originally Posted by xarralu View Post
    So just_opnion, probably a couple of 3 ton coils would be easier. Run one or the other. If they both run, run with both in bypass so they split the tonnage.
    It is not the coil size to prevent you from freezing

  12. #12
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    Yea, the TXV takes care of that along with the superheat. So I guess thats where I figure if I slow the CFM across the coils so that only an effective 1.5 tons are being removed from each coil when they are both in use.

  13. #13
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    Sep 2007
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    daiken

    daiken systems are designed to run up to three heads on one condensor.they they are low cfm systems designed to run all the time and maintain one temp regardless of outdoor temp.what you're wanting to do would be a clusterf#$%.

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