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  1. #1
    Join Date
    Nov 2005
    Posts
    1,350

    AAON head pressure trips

    Alright I am at the point I am going to rewire this unit, and wanted to see if anybody has run into the same problem.
    5 year old 40ton Aaon unit.
    4 ten ton scrolls and 4 stages of cooling
    6 condenser fans, 3 for the first 2 stages and 3 for the last 2 stages.
    No VCM just fired from DDC system with Y1-Y4 based on DAT set point.

    I get intermittent head pressure trips on the first 2 stages only. I have verified with gauges that they trip @ rated setting. I can run this thing front to back w/o any issues while on site, 2-20 days later they are tripped again. When I am there the head never goes above 200#. I have bypassed the cheap snap disk fan cycles and the time delays on the compressors. What is weird is on the wiring diagram the condenser fan motor contactor gets fired at the same time the compressors do.
    I STARTED WITH NOTHING, AND I STILL HAVE MOST OF IT!

  2. #2
    Join Date
    Mar 2009
    Location
    Mid-Mo
    Posts
    3,595
    Have any of the condenser fan motors been replaced? Maybe with non-oem motors and they are going out on thermal?

  3. #3
    Join Date
    Apr 2012
    Location
    lexington, NC
    Posts
    32
    have you tested the high pressure switches? Also could have a wrong motor like ryan said. People love to pick up universal motor which is ok I think on residential units but on the bigger stuff I always try to go back with OEM. What is the OA temp when unit is tripping off?

  4. #4
    Join Date
    Nov 2005
    Posts
    575
    This is why I love those data/temp/pressure loggers.
    Install them and see.
    Also can DDC system log any points you choose?

  5. #5
    Join Date
    Aug 2006
    Location
    Columbus Ohio
    Posts
    1,980
    Out of ANY units I have ever dealt with AAON units are by far the hardest coils to wash and keep clean.

    I have run into this exact issue. It took a fire hose to get those extra thick coils cleaned out properly and I was amazed at the ammount of dirt that we got out compared to a regular garden hose and pressure washer.

    I would bet a paycheck that your coils are plugged in the center. They may look clean, it's very deceiving.
    UA LU189

    10mm, because it's better than .45acp

  6. #6
    Join Date
    Feb 2004
    Location
    British Columbia, Canada
    Posts
    521
    Check the temperature differential of the condenser coil, if it's more than 30 deg TD then your coils need cleaning.

    How did you check high press cutout confirmation? You say you never saw it exceed 200#, is that from the ddc trend history?

    Check the temp on the air leaving side of the coil in multiple areas as airflow may be restricted unevenly.
    Last edited by koolkahuna; 04-26-2012 at 01:56 PM. Reason: Added extra info

  7. #7
    Join Date
    Nov 2005
    Posts
    1,350
    Quote Originally Posted by koolkahuna View Post
    Check the temperature differential of the condenser coil, if it's more than 30 deg TD then your coils need cleaning.

    How did you check high press cutout confirmation? You say you never saw it exceed 200#, is that from the ddc trend history?

    Check the temp on the air leaving side of the coil in multiple areas as airflow may be restricted unevenly.
    I put a stubby gauge on it and disconnected the condenser fan contactor and watched them trip.
    It doesn't seem to have a relationship with the out side air temp. Obviously they run more when it is hotter, but it just tripped with an osa of 70F.
    As far as the delta are you talking about air in and out?
    I STARTED WITH NOTHING, AND I STILL HAVE MOST OF IT!

  8. #8
    Join Date
    Nov 2005
    Posts
    1,350
    Quote Originally Posted by zw17 View Post
    Out of ANY units I have ever dealt with AAON units are by far the hardest coils to wash and keep clean.

    I have run into this exact issue. It took a fire hose to get those extra thick coils cleaned out properly and I was amazed at the ammount of dirt that we got out compared to a regular garden hose and pressure washer.

    I would bet a paycheck that your coils are plugged in the center. They may look clean, it's very deceiving.
    I know they are horrible, but why only once in a while a trip? I have watched them on some hot days and the head pressure is constant. It will literally run all day while I am there w/o a problem.
    I STARTED WITH NOTHING, AND I STILL HAVE MOST OF IT!

  9. #9
    Join Date
    Nov 2005
    Posts
    1,350
    I should say that this is a 100% osa unit on a bio-tech.
    I STARTED WITH NOTHING, AND I STILL HAVE MOST OF IT!

  10. #10
    Join Date
    Mar 2011
    Location
    East Coast FL
    Posts
    1,058
    Does this unit have dehumid mode with hot gas reheat?

  11. #11
    Join Date
    Mar 2011
    Location
    East Coast FL
    Posts
    1,058
    Like others said , you almost have to use oem cond motors , a 3/4 hp supply house motor is NOT up to the task and will get hot and go off. Even a one horse single phase is not heavy enough. (3/4 oem rating is very conservative)

    You bypassed the crappy fan cycles so it's not those , they kill the whole fan contactor and not individual fans. How is the fan contactor? The cond coil is only two row on that unit so it should be easy to determine its cleanliness. I'm still wondering about a possible reheat problem.

  12. #12
    Join Date
    Nov 2005
    Posts
    1,350
    I'll check for hot gas, the ddc system will fire cooling for de humidification. What's that angle on hot gas?
    I STARTED WITH NOTHING, AND I STILL HAVE MOST OF IT!

  13. #13
    Join Date
    Mar 2011
    Location
    East Coast FL
    Posts
    1,058
    Generally an outside air unit for make up will provide room neutral air temp (70*) and not actually contribute to cooling or heating the space. The cooling and heating would be staged to maintain supply air set point by the DDC controller. There are times when dehumidification is needed but running enough refrigeration to meet the %RH supply set point would overshoot the supply temp set point. At that time the hot gas reheat coil becomes active to bring the air back up to the neutral set point.

    The Aaon modulating hot gas valve controllers will receive a 24v command from the DDC controller or humidistat to the "reh enab" terminal. The hot gas controller will then begin to close the modulating valve feeding the condenser and open the modulating valve feeding the reheat coil as needed to maintain the supply air set point as dictated by the dip switches on the reheat controller. The setting should be similar to the DDC set point for supply air temp. The hot gas controller takes over compressor control generally running 2 stages of refrigeration. The first stage will have the modulating valves , if additional reheat is needed a 3 way valve will divert 100% discharge gas to the reheat coil for second stage.

    In your case it could be the cond mod valve closing down with the reheat valve not opening , or a problem with the 3 way heat reclaim valve on the other circuit.

    You'll have to go back and see exactly what you have and how it's wired on the controls.

    But that should be after all the simple fixes have been checked or confirmed.

    Hope that helps you.

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