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  1. #14
    Join Date
    Jun 2006
    Posts
    1,270
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    Pump down, inside or out if configured as described.

    Outside: pump down will keep the refrigerant out of the oil which will foam on start if saturated with it and can result in oil slugging as well as oil loss at the comp. The reduced lubrication will kill the comp prematurely.

    Inside: pump down will prevent start up with a full evaporator resulting in slugging of the comp with liquid refrigerant.

    In the configuration you say you have 3 evaps however you don't imply servicing three different spaces. typically with a system of that size with multi evaps it is good practice to put inverted traps at the evaps and pump down. If you don't have the inverted traps on the suction header and only have a single oil trap it is piped wrong and you can literally fill one evap with liquid from the other two. I don't have any idea, I can't see it from here, but these types of systems get piped incorrectly a LOT.

    Setting up the pump down is not too awfully tough but a lot will indeed do it wrong. set the cut out to about 5 psi unless you have a VERY low box and SST. not much reason to go below that with pretty much any gas in mainstream use out there. Don't go for the generic 15 psi differential, set your cut in a couple of degrees below your lowest operating SST. This will give you the most bang for your buck clearing the traps and promote good oil return without slugging it back.

    No reason to take my word for it or anyone else's, get your head into the RSES SAM manuals and they will set you free!

    Peace

  2. #15
    Join Date
    Jun 2018
    Location
    Bay saint louis ms
    Posts
    74
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    Thread Starter
    Thanks for the good info 98 percent of the info I get from this site is great!��

  3. #16
    Join Date
    Jul 2012
    Posts
    2,372
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    I have a couple 5 hp medium temp walkin coolers with no pump down. I do have a nice big suction accumulator with a crankcase rubber heater band around the bottom of the accumulator just in case. Works fine. No compressor failures. 25 superheat at the accumulator inlet.

  4. #17
    Join Date
    Jun 2011
    Location
    NE Ohio
    Posts
    2,892
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    recently replaced a 5 ton split system AC condensing unit, re-used the old R 22 fixed orifice coil that was compatible with R410a and also had an AHRI match ( new orifice ) with the new condensing unit. GO-TO technical support guy told me, due to the condensing unit being located 30 some odd feet above the evaporator, that I should install a liquid line solenoid valve, I thought he was wanting the system pumped down, he stated no, just to act like a hard shut off TXV to open / close with the thermostat, had a few other questions to run past him so did not ask the reasoning behind this just did as he recommended, it appears AC units, at least Carrier, come with hard shut off TXVs now a days. first I heard of installing a liquid line solenoid valve on a fixed orifice AC split system, not sure if I heard of installing one a zillion years ago in school.

  5. #18
    Join Date
    Aug 2019
    Posts
    26
    Post Likes
    Pumpdown is preferred but for an off cycle defrost is not manditory....if everything else is working ok. In extreme cold ambient temperatures, you may have a liquid migration problem, then pumpdown would be advisable

    Tim Parsons

  6. #19
    Join Date
    Jul 2012
    Posts
    2,372
    Post Likes
    I have a mom and pop carryout with an old Trenton walkin. 5 hp medium temp.
    No pump down. It has a big suction accumulator. I have a couple crank case heaters wrapped around the bottom of the accumulator and one on the compressor. No compressor losses.

  7. #20
    Join Date
    Aug 2019
    Posts
    26
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    To be precise....and that's an inside joke right now....that is called Liquid Line Solenoid drop....mostly to stop refrigerant migration in the off cycle. IN my opinion...every single piece of refrigeration equipment should have it. It's a cheap way of pump down but not.....but it does isolate the refrigerant into the high side of the system. That sure would prevent a ton of compressor failures and coil freeze ups, but they won't do it because of cost. Just my opinion......

    Tim Parsons

  8. #21
    Join Date
    Jun 2007
    Location
    Phoenix,Arizona
    Posts
    118
    Post Likes
    having the compressor just shutting off allows for shorter more even defrosts , you don't have the 5 to7 min pump out time and you also don't have the compressor short cycling on and off during the pump out phase . so if the coils are filled with gas ( not liquid ) the coils will become warmer faster and evenly . only recommended for +30 applications only .
    Last edited by AngryReferGuy; 09-17-2019 at 06:38 PM. Reason: 2c in wrong place
    Gun Control: Belief that violent predators willing to ignore laws against
    robbery, kidnapping, rape, and murder will obey a law telling them that they
    cannot do so with a gun.

  9. #22
    Join Date
    Mar 2015
    Location
    Medford, N.Y.
    Posts
    5,857
    Post Likes
    Sure refrigerant migrates to the COLD and causes problems if allowed to. But the thought that the comp being inside/outdoor makes a diff is not accurate. During an "electrical blackout" during a 95* day/night/day/night heat wave ,migration to the comp oil STILL takes place. And upon getting electric restored and upon the comp(s) starting up still causes dead/damaged comps.

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