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What the heck is going on here?

The bottom of that evap coil is the head, so it's naturally the coldest part. The non-condensibles can't be seen when you look. If the suction PSI "sets" for a bit then rises quickly to balance with the liquid side, you have moisture freezing up in the distributor tubes.

I would first recover, install a f/d, nitrogen purge, evacuate and recharge. If it still happens, you may need to change evaporator and liquid line.


  • Steve Ebels_3
    Steve Ebels_3 Member Posts: 1,291
    Head scratcher

    Situation is this: 1 year old Nordyne AC condenser and evap. Ran fine when installed last summer according to the HO. Now the bottom 2" of the evap coil will ice up similar to a low charge or restricted airflow problem.

    When I arrived I checked the airfilter and blower performance, also pulled the front off the evap case and scoped out the bottom of the coil. All was well with those three items. Outside the unit was pulling 9.4 amps with a rated RLA of 13.6.

    Here's the strange part, after attaching the gauges I was puzzled to see the high side at 255 and the low side at 25 PSI on startup. As I watched the unit run it slowly pulled the low side all the way down into negative pressure territory. I checked for leaks around the condenser with a leak detector and visually looked for traces of oil and found nothing. Same thing inside and on the line set. No sign of any leaks.

    I reasoned that there had to be a major restriction at the orifice type metering device so I pumped the unit down, and pulled the union aprt to check it out. Clean as a whistle.

    The only other thing I could think of is that the condenser was WAY low on refrigerant but that really didn't make sense either due to the high side pressure. I have to say I succumbed to the urge to add refrigerant and hooked up a can of 22. I added slowly and had less than 6oz in the system when the highside pressure literally jumped to 325 from 250 and continues to climb slowly. The low side meanwhile crept up from around 10PSI to 25 again. With the highside at 330 and still climbing I shut off the can and watched to see what would happen. It continued to climb up to 380 while the low side slid once more to less than 10. When it hit the 380 mark I decided I was in over my head and shut it off.

    So here I am, seeking the wisdom of the Wall once more,

    What am I missing?

  • Mitch_4
    Mitch_4 Member Posts: 955
    with the symproms you describe

    I would lean to non condensibles, or a restriction.. htat is pretty much the only 2 things that will cause high head / low suction. Is there a filter dryer inthe condenser? is there a major temp drop across it?

    Did you install, and do you purge with nitrogen when you braze?

    possible that the high side valve is faulty and not opening fully?

    some thoughts.

  • clammy
    clammy Member Posts: 3,113
    same thing

    had this happen may years ago ,i found that part of the restrictor o ring had magically ended up blocking the restritor orfice .First i would un hook the liqiud line nd blow her clear with nitrogen ,chk liquid line solnoid if you have one and the line set for possible restriction caused by HO or other contractors it's possible also while apart braze a shrader on the liquid line and suction line at the coil and try her again if it's still running the same start checking with a digital thermometer on your liquid line for temp differences .Ps on the o ring restrictor it didn't happen all the time just every couple of cycles also how about non condensables and moisture icing up at the evap ,try a new dryer with larger capacity and a real good triple evacution with nitrogen bleed and charge her up could possible be the suction vales is bad on the compressor is it a scroll may be it's in by pass good luck peace clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
  • Steve Ebels_3
    Steve Ebels_3 Member Posts: 1,291

    I'm not the installing contractor. He won't show up when the HO calls him which leads me to believe there may be a problem in there somewhere that he knows about. Knowing the guy, he'd never tell me if asked him.

    There is no filter or filter dryer of any type on this puppy. I noticed no areas on the liquid line that are icing or changing temp indicating a restriction.

    The unit has a Bristol recip type compressor.

    I agree it sounds a lot like a restriction but everything looked clear when I opened it up and had a looksee.

    Still stumped
  • Mitch_4
    Mitch_4 Member Posts: 955
    if everything look clear then

    I'd say non condensibles are the culprit.
    What is the system pressure at rest, relative to the ambiant temp. the P/T should align, if not, your gas is contaminated
    Reclaim the charge,evacuate, and reload with fresh gas
  • Glenn Harrison_2
    Glenn Harrison_2 Member Posts: 845
    One other possibility.

    Many newer units have a strainer screen in the fitting/tubing right before the restrictor housing. I have found several units that this screen is plugged up, frequently with dirt from the lineset being dropped in the garden/flower bed next to the condensor. I would open the psiston housing again and look in the liguid line side of the housing and see if there is a screen, and if there is, I would bet it is restricted, especially since ther is no filter drier installed.
  • Eugene Silberstein 3
    Eugene Silberstein 3 Member Posts: 1,380
    Here's a quick lesson

    Well, maybe not so quick. When troubleshooting an air conditioning or refrigeration system, it often pays to think like refrigerant. When a system is operating correctly there is a certain amount of refrigerant on the high side of the system and a certain amount of refrigerant on the low side. When we gauge up on the system, we read the pressures and, if both the high side and low side pressures are within the desired range, we as technicians are typically happy and conclude that the system is okay. (Not always true, though).

    In the situation you refrer to, the high side pressure is high and the low side pressure is low. Since there is an obvious imbalance (too much refrigerant on the high side and too little refrigerant on the low side) between the low side and high side, the refrigerant charge is most likely NOT the problem. In a simialr situation, if the high side pressure is low and the low side pressure is high, the problem is most likely NOT the charge. Cool, huh?

    Now, back to your situation.

    You mentioned that the low side pressure is pulling into a vacuum and the high side pressure is high. Well, think about this. The system compressor pulls refrigerant from the low side and moves it to the high side, resulting in a lower suction pressure and a higher head pressure. The liquid line and metering device allow high side pressure to flow back to the low side. If the low side pressure is too low, refrigerant is being pulled from the evaporator, but no refrigerant is being fed back into the cooling coil.
    Your problem is a flow restriction between the high and low pressure sides.

    You can check for liquid line restrictions by taking temperature readings across the liquid line. Temperature drops in the liquid line result in pressure drops. Be sure to check any liquid line components such as liquid line driers.

    Now, to the issue about non-condensables in the system. If there are non-condensables in the system, both the high and low side pressures will be high, so thig is not the problem. More on non-condesables in a future post.

    Keep us posted and thanks for your input, guys.

  • TGO_54
    TGO_54 Member Posts: 327
    Hi Professor

    Could moisture in the system (a non condensable) be freezing some where in the system and causing a blockage?

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  • Eugene Silberstein 3
    Eugene Silberstein 3 Member Posts: 1,380

    That can happen, but it usually results in an intermittent problem. Consider the following true story.

    About 15 years ago I had a customer who called me and said that her window air conditioner was not operating. I told her to turn it off and that I would be there later on.

    When I arrived, I turned the unit on and it cooled perfectly. I waited about 15 minutes and the unit was still cooling well. I left the job. Two hours later, the woman called and said that the unit was not blowing cold air. Once again, I told her to shut the unit off and that I would be by later on. Same issue. Upon returning, the unit was cooling perfectly.


    Left again, and sure enough another no=cooling call from the now slightly pissed-off woman. This time I told her to keep the uit on. I rushed over and, sure enough, the unit was not cooling.

    The evaporator was warm and the compressor discharge line was hotter than a truck full of stolen stereos.

    This unit had a line tap valve installed on the suction line (a big no-no nowadays, but back then it seemed to be the way of the world), so I was able to take a suction pressure reading and, sure enough, the low side pressure was in a vacuum. The vacuum caould have been caused by moisture in the system, a blocked strainer or a blocked capiallry tube.

    Sure enough, there was air (and of course moisture) in the system. As the water vapor experienced the pressure drop in the cap tube, the moisture froze and blocked the capillary tube. Once the system was turned off, the ice melted and, when the system was turned back on, would operate fine for a period of time.

    The system was ultimately evacuated and recharged, after whcih time there were no further problems with the system.

    You can't make this stuff up!
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