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Thread: unloaders

  1. #1
    Join Date
    Dec 2004
    Posts
    56
    could someone please tell me what the propper C.I. and C.O. is for an unloader on a 25 horse comp?

    also if it not to much trouble a brief explination of whats the purpose of the unloader. I iunderstand that it controls wheather your using all of your comp or half of it but i dont understand why you want to do that?

    is it for short cycling purposes?

    If the unloader settings are not correct could it cause blown fuses on the line feed between the disconect and the comp's contactor?

  2. #2
    Join Date
    May 2005
    Location
    Calgary, Alberta, Canada
    Posts
    92
    You want the compressor to keep running even when the load is small, unloading lets you shut down some of the compressor to keep your evaporator temperature constant. So yes it is for short cycling....to prevent it. The proper CI and CO is dependant on what evaporator temp you are trying to achieve. Is this for A/C or refrigeration? What refrigerant are you running?

    As far as this causing blown fuses who the hell knows.
    Yes we have air conditioning in our igloos.

  3. #3
    Join Date
    Dec 2004
    Posts
    56


    i appologize for not being clear on details I assumed everyone in this part of the forum was talking about A/C's. This is a Mcquay package unit on a roof with R22 in it. TY for help

  4. #4
    Join Date
    May 2005
    Location
    Calgary, Alberta, Canada
    Posts
    92
    Yes I suppose I should have thought that one through!!LOL

    Anyways, I would set your CI around 28-30 degrees and CO around 38-40. Avoid setting too close to avoid short cycling of the unloader. I would make sure the unloader control is actually switching at these setpoints as well.
    Yes we have air conditioning in our igloos.

  5. #5
    Join Date
    Dec 2004
    Posts
    56
    ok first ok ty for your input. secondly if im understanding you correctly basically what your doing by unloading is as your load decreasess your lessening your pump so your comp will run longer. which im assuming is more for the purpose of days when you only need a moderate bit of a/c so your big ole comp dosnt pull it down super fast then shut off before your stat satisfies and then you gotta wait for your back pressure to build back up just to have it do the same thing over n over again till you eventually satisfy yer stat. is that about the gist of what its purpose is?

  6. #6
    Join Date
    May 2005
    Location
    Calgary, Alberta, Canada
    Posts
    92
    It depends on how the system is controlled, but basically the unloader will keep the evaporator from getting too cold(freezing up) under low load conditions regardless of what the stat is calling for.

    The compressor shouldn't be shutting down until the stat is satisfied anyways. The situation you are illustrating would mean that the unit is controlling the compressor cycling off of pressure controls, which would pretty much make the stat useless. I doubt this is the case.

    As always with respect,

    websy
    Yes we have air conditioning in our igloos.

  7. #7
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,369
    Unloaders are for capacity control. That's it in a nutshell.

    Take a large tonnage semi-herm on a split system with VFD air handler and VAV's in the supply duct stream. The design of the system is to maintain 55 degree discharge air temperature off the air handler to feed the VAV's. The flow rate of conditioned air to the building through the VAV's is never a constant, nor is the heat load. So, a 40 ton system will most of the time not be taxed to its full capacity. Therefore it becomes necessary to "unload" the compressor so it does not overcool or, as you say, short cycle.
    Depending on number of compressor banks and manufacturer, a frequently encountered loading capacity is 33, 66, or 99%.

    I can't see unloaders out of adjustment being a common cause of blowing fuses but I wouldn't rule it out, either. But I'd really look for other causes first before reaching that conclusion.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  8. #8
    Join Date
    Dec 2004
    Posts
    56
    lol yes thats a good point i did not think that throughly, your absolutly correct about your stat keeping your comp on. ty for your explination that explained alot to me. the purpose of the unloader is to prevent the coil from icing up more so then to keep it from short cycling that makes perfect sense to me. im just a dumb refer guy who has no buisness working on a/c's, but am am being forced into it, which i guess is a growing experience fer me as a mechanic. anyhow ty very much for your assistance. its much appreciated

  9. #9
    Join Date
    Dec 2004
    Posts
    56
    ty for your input shop. i dont really think the unloader would cause that but im at a disadvantage by not understanding unloaders . i cant really accuratly diagnosis a system without understanding all its controls, which is what led me to starting this thread

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