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Lennox XC13 shutting off when thermostat still calling for AC / Unico fan blowing

I have a Unico high velocity AC system w/ Lennox XC13 condenser which has been giving me grief since it was installed 6 years ago.  The AC company that did the install has been super responsive and has spent a lot of money out of their pocket to try to solve my problems, but alas, the problem is still not corrected and I am getting tired of not having reliable AC year after year.

The problem I am having right now is that my Lennox XC13 runs for a while when the Tstat calls for AC,  during that time, I do get some level of cool air through the Unico air handler.  Within 15-45m, the condenser shuts itself off even though the tstat is still calling for AC and the fan is still running. Then within about 10m, it decides to turn itself back on again and goes through the same cycle, depending on the temp outside, this means that the room just never cools down enough because of the cycling. When the system was first installed, the XC13 never turned off when the tstat was calling for AC, so I am confident that any time the XC13 turns off when the tstat is still calling for AC, something is busted. (Previously I did have a leak somewhere in the line that resulted in short cycling)

Recently they replaced the entire Unico air handler,  replaced all the refigerant lines and recharged with R410A. The XC13 has been cleaned (Not clogged) and the air filters inside are both clean.  Last time they came back they checked the refrigerant and it looked good (claimed there was no longer a leak). It seems like the common causes for a freezing coil are not in place here (refrigerant problems or clogged condenser / fitters), yet the XC13 is still turning itself off intermittently for some reason.

What steps would you take to finally get to the bottom of this problem?


  • clammy
    clammy Member Posts: 2,637
    unico issues

    Personally i would check the freeze stat which is located on the unico suction line assembly on the coil .Sounds  like the coil may be icing up which shuts the outdoor unit while the unico fan contuines to run .If this was a constant issue from when the unit was first installed i would venture to say either the refregerant charge is off or that the manafactures lay out and installation instruction for a unico system where not followed.I have been installing unicos for aboout 20 years and have not had these issues which any systems i have installed.Check and see that you hae a mininun of 6 supply regesters per ton and at least 1 sqare ft of return per ton for return.Aside from all the other uninco specs for instalation i would bet it is a issue which air flow /charge and the outdoor unit cycling on the freexe stat .Usua;lly when this happens it is a air flow/charging issues if your a/c contractors has been trying to iron this out for 6 years i would believe it is time to find another ac guy,this issue should not have been going on so long and is usually connected with not following the manafactures duct layout and air flow issues .Peace and good luck clammy 
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
  • don_9
    don_9 Member Posts: 395
    some question?

    When the condenser turn off do you mean nothing runs at all? No fan or compressor?

    If the unit shut off completely you would have to look into any safety that could be taking it out and, even a voltage problem.I have seen a many of slow drains with safety float switches that will take out the condenser before the call for ac was over.Then once she drained the float would drop back down and the unit would come back on.

    I know at one time lennox used a non-bleed txv and the compressor was starting under a load and would need the help of a hard start kit to kick in the butt if i may used that terminology.The outdoor unit would be calling and the fan running but the compressor would not get going without the kit.

    We would check all that applies here...voltage,safeties,tstat to see if it is setup correctly.The refrigerant circuit along with airflow across the system.Heck sometimes one just have to sit around for hours and monitor the system over time to catch what  is assume to be an intermittent problem

    I hope your contractor finds it soon sound like he is in the red with this one


  • Pete_18
    Pete_18 Member Posts: 197
    Drops & Size

    4.0 Ton

    26 drops

    split return

    14x25 - 2nd floor

    14x20 - 3rd floor

    Door between 2nd & 3rd floor is open
  • Pete_18
    Pete_18 Member Posts: 197
    Condenser shutting off

    The outside XC13 goes completely dead.

    The inside Unico air handler keeps going (fan is running).

    Yes, they are definitely in the red and they have tried very hard to fix the problem, replacing lots of parts and recharging numerous times, they just seem stumped as to what's next to get this fixed finally.
  • Pete_18
    Pete_18 Member Posts: 197
    Water drain / float switch

    I am not sure if this means anything, but while it is running, the condensate line does drain, but slowly.  Then once either the condenser and/or entire system shuts off, there is a brief period of time where this a much greater flow of water from the condensation line. I don't believe there is a trap on the condensate line.

    Where would I expect the float switch to be?  They mentioned there is one, but as I look at the condensate line coming out of the Unico, I am really not sure where it is.  I can take a picture if it would be useful.
  • EddieG
    EddieG Member Posts: 150
    It's a safety

    It is a safety switch problem. The question is, is a bad switch or an issue causing the switch to trip? The float would be located in the condensate line, you would see wires connected to it. My guess is a pressure switch or freeze stat. TXV could be acting up. If the tech. can't seem to find out what is tripping when he is there, I would suggest re-wiring the freeze stat to shut the entire system down. So at least when it happens again, you can inform him that everything shut down. But if just the outdoor shuts down you will know it's a pressure switch. This is not a FIX, but when you have problems you can't catch: Process of elimination!!!
  • Pete_18
    Pete_18 Member Posts: 197
    Safety problem?

    I asked them about the float switch and they don't believe that's the cause since it would not then result in the condenser turning itself back on for a very long time (they claimed that the water in the float would need to evap before it would re-start.  The float must be located internal to the Unico itself since it is not visible on the outside air handler condensate line.

    The TXV is new and was replaced with the entire Unico system. 

    They are checking in with the mfr on any additional suggestions before planning a longer period of time to camp out and see if it's the freeze stat triggering.

    Good suggestion on rewiring the freeze stat for a total shutdown for troubleshooting.  Part of the problem with the issue now is that from inside of the house, it's not really that obvious when the condenser shuts down since it's not very loud and the air handler fan is still blowing.  It becomes more obvious when I am staring at it outside no longer operating and wondering why the cool temp has not reached and the air handler fan is still blowing.

    I really don't believe it is a Tstat problem, but for troubleshooting, should I bother to put RC, Y and G together  and watch for an hour or so to confirm that the condenser is still turning off on its own? This would then at least rule out that the Tstat isn't the cause (which I doubt it is).
  • EddieG
    EddieG Member Posts: 150
    I doubt...

    it's the t-stat or condensate. I just wanted to give you a description of the float switch. I wouldn't rule out the TVX, just because it's new. If the indoor unit was replaced and the other components were cleaned, there is no way of knowing how well it was cleaned. For that matter, I've seen bad TXV's out of the box. Based on the timing of on/off operation, my money is on the freeze stat. Which could be bad or antoher issue is causing it to do it's job. Such as airflow, charge, or bad TXV. For that matter a kink in the lineset, that nobody see's.
  • Pete_18
    Pete_18 Member Posts: 197
    Multimeter testing

    Will I be able to test if the freeze stat and/or float is activated by testing the C and Y with a multimeter at the Tstat or do I need to test somewhere else to know for sure?
  • Pete_18
    Pete_18 Member Posts: 197
    Low ambient control kit

    I found something in the installation manual which may be describing what I am experiencing. Do any of you use a head pressure control as part of your installs? If not, do you have any problem running the AC when it's 65-68F outside (inside temp was about 69-70F and humid).

    Using a Low Ambient Control Kit

    Since the Unico System operates at colder coil temperatures

    (in cooling mode), an anti-frost switch is installed

    on the coil to prevent coil freeze-up. In certain instances,

    such as when the outdoor ambient temperature is low,

    the condensing unit will cycle on the anti-frost switch.

    This may reduce the cooling capacity at a time when the

    cooling load is still fairly high. To provide better control

    and comfort, install a low ambient control on the condensing

    unit. Typically, a low ambient control is necessary

    when operating the unit at outdoor temperatures

    below 80°F (26.6 °C), especially for a 5-ton nominal

    capacity system.

    These controls come in different configurations such as

    the Hoffman Controls Corp. series 800AA-head pressure

    control. This control modulates the outdoor fan to maintain

    a minimum liquid line temperature. Other controls

    may cycle the fan on/off. In either case check with the

    condensing unit manufacturer to determine what controls

    are compatible with the condensing unit.
  • Empire_2
    Empire_2 Member Posts: 2,343
    Best bet:

    See if you can get a professional over to give you a hand.  I know you have questions, but without being there, only more questions will arise. 

    Mike T.
  • Techman
    Techman Member Posts: 2,144
    edited May 2012

    You said that more water drains out of the condensate line after something shuts down the cond unit. I agree w/ the others. It sounds like a "freeze-stat" situation. A set of refrigeration readings is required to see if it is an air flow problem or what! If you noticed an uncomfortable condition in the house due to the freezestat cycling then you have a problem. If an AC unit needs to be run in any "low ambient" condition  then some sort of head pressure  control device is needed and  is considered normal.Is there a drain line trap?
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