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  1. #1
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    Adjusting charge in low ambient w/ head pres cntrl

    I am going to be troubleshooting a 5 Ton R410a AC only split system with poor cooling performance in a server room next week. The outdoor unit is a 13 SEER Goodman VSX model that is operating in 35-40 degree outdoor ambient conditions with the help of ICM325HN head pressure control (thermistor style). My preliminary checks will be evaporator air flow, TXV sensing bulb placement & insulation, & condenser coil airflow.

    My question is, with the low ambient head pressure control slowing condenser fan, how can I go about making sure the system has proper refrigerant charge? What data points should I collect? What R410a pressure should a head pressure control be running targeting?


    Thanks in advance - I’m a 3rd year apprentice looking to adopt best practices.

  2. #2
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    Oct 2016
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    Superheat and subcool would be the place to start.

    Making sure the head pressure is where it should be.

    Making sure the SST is proper for cooling as well as LST

  3. Likes CEAS-AC-TECH, R600a liked this post.
  4. #3
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    Thread Starter
    If the head pressure control maintains a head pressure that simulates a 96degree day (350PSI High Side with CTOA condenser split 20 degrees) should I expect the optimal 10-12degrees subcooling when properly charged?

  5. #4
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    Jan 2014
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    I would charge it by weight method ( at 35-40 degree ) if that’s what you have now and having a matched system, then when it’s hotter out and fan speed is not throttling, but running pretty constant dial in to subcooling if having a TXV metering device.

    BTW the VSX is a GMC brand of Goodman.

    If you have to open up the system, remember to remove the factory drier installed inside the OD unit.

  6. #5
    Join Date
    Aug 2002
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    Seasoned techs can sometimes get it close but still have to come back in cooling season to adjust.

    Just pull the charge and weigh it.

  7. Likes CEAS-AC-TECH liked this post.
  8. #6
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    https://www.google.com/url?sa=t&rct=...KoxYxPWIRVWCe5

    Page 31 shows the OEM low ambient kit, probably gives the exact location where the sensor is to be located. Looks to be a ICM control also.

  9. #7
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    Thread Starter
    Quote Originally Posted by Bazooka Joey View Post
    https://www.google.com/url?sa=t&rct=...KoxYxPWIRVWCe5

    Page 31 shows the OEM low ambient kit, probably gives the exact location where the sensor is to be located. Looks to be a ICM control also.
    Thank you very much for the link to unit manual. I had the broader range 181 page manual for multiple units of that family. The one you linked is way more useful for getting system performance up to par.

  10. #8
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    Quote Originally Posted by Nate Fryer View Post
    Thank you very much for the link to unit manual. I had the broader range 181 page manual for multiple units of that family. The one you linked is way more useful for getting system performance up to par.
    You mean like this,

    https://www.google.com/url?sa=t&rct=...pOv5subqViueDA

    following accessories must be installed on all condensers and remote heat pumps when equipped with low ambient kits.


    Crankcase heater must be installed if not factory installed.
    2. Hard start kit for single phase units. (See Spec Sheet for Accessories)
    3. TXV kit installed on indoor evaporator coil, if not factory installed.
    4. FSK01A - Freeze protection kit must be installed on indoor coil.
    5. Wind buffer must be installed for temperatures below 50F or areas with high prevailing winds. Wind buffer can be a wall fabricated from wood or masonry material that will prevent the prevailing wind from causing the outdoor fan to rotate. NOTE: When wind buffer is installed, it is necessary to use minimum 4" risers to elevate the unit off of the pad to provide better airflow flow.
    Last edited by Bazooka Joey; 01-17-2020 at 06:09 PM.

  11. #9
    Join Date
    Nov 2011
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    it's gonna be hard to tell what's going on. combined with the low ambient conditions, the change in fan speed will change all of your numbers.

    I have blocked 90% of coils before with plastic or cardboard to get a better idea....but bottom line is that you're only gonna get it close, which may leave you over or undercharged in the summer time.

    "best practice" is subjective.....sometimes you can't shut a unit down that provides process cooling (server room), which leaves you to do your best to get it cooling until it can be shut down for a full check out. in that case, "close enough" would be Best Practice!! LOL!!

    Here's what I would do.....first, see if it's a micro channel condenser, if it is, that's gonna cause me to slow down, and re evaluate the situation, as I REALLY don't like guessing with microchannels. Then, I'm gonna see if there is a second unit....if there is, I can do what I want.....if there isn't, I'm gonna ask the site staff A: How long can the unit be down, and B: if it is down, can we open the doors and get some fans, etc.?

    then I'm checking the coils....are they dirty?....they may not look like it, but they could be. air filters clean? registers open, nothing blocking returns, evap. coil clean? indoor blower clean, running right direction and correct speed? condenser fan running right direction, correct depth in housing? any oil on piping joints anywhere? no condensate pump that could be locking the unit out?

    if all that checks ok, and I can take my time, I'm pulling charge. I'll locate the operating charge for the condenser, find out how long of a line set the factory supplied charge is good for, then measure the lineset and calculate the actual required system charge. then I'll pull the unit down to about 3-5psi, and see what I have...if I'm close to factory, toss it back in. with charge eliminated as a possible cause, I'll move on to TXV....remove bulb, heat and cool while watching gauges. I use a cup of hot then ice water so I have time to get back to the condenser.


    That sounds like a lot of stuff, but it's really not. you can have all the preliminary visual checks done in less than 1/2 hour. You can pull charge in 15 minutes...it'll take longer to set up you gear. I use my G twin to pump the charge back in after I weigh it, so I can have it back in the unit is a few minutes...so 1/2 hour to 45 minutes. within 2 hours, you can go through the whole system.

    One thing I've encountered on units with constant loads is TXV's sticking. Your numbers will be off....high SH, High SC....then you'll hit that bulb with hot and cold water, and all of a sudden, everything starts working. I usually recommend a TXV replacement for those, as it will likely stick again. it will be hard to tell with the low ambient conditions though, as your SH/SC numbers are false.

    Good Luck young Jedi!

  12. #10
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    I don't find charging in low ambients to that big of a deal. But then, it doesn't get super cold here in the SF bay area. Where are you located?

    All I do is bypass the low ambient control, then charge to subcooling. Low suction pressure for a few minutes isn't that big of deal to me. If the outdoor coil is micro channel, I charge to 3*F subcooling. If the outdoor coil is conventional, I charge to 5-10*F.

    Then re-instate the head pressure control. Easy peasy.
    I do a triple evac with nitro to remove non condensables.

  13. #11
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    He said 35-40*. I bet what you consider low ambient is 60*!! LOL!!

  14. #12
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    35-40 is no big deal. Leave the fan on continuous. Pick your subcooling. Charge, then re-instate the head control and walk away.


    Quote Originally Posted by 71CHOPS View Post
    He said 35-40*. I bet what you consider low ambient is 60*!! LOL!!
    I do a triple evac with nitro to remove non condensables.

  15. #13
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    Quote Originally Posted by EnergyOasis View Post
    If the head pressure control maintains a head pressure that simulates a 96degree day (350PSI High Side with CTOA condenser split 20 degrees) should I expect the optimal 10-12degrees subcooling when properly charged?
    A 96*F ambient equates to a 350 psi head using r410a?
    With lower than a 70*F day what PSI head would you expect to see?
    Install a SG and verify that the LL feeding the metering devise is full.

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