Odd new record high ambient temp behavior, Trane XR12

I have a 7 year old 3 ton Trane XR12 heat pump unit (2-stage heat, 1 cool) servicing a 1500 sq ft single story house in North Texas. The outside compressor is located in a western alley where it gets direct sun about 2:00 pm for a couple hours but is otherwise shaded by my/my neighbors houses.
I like my AC cold, and have run this unit at 70 F for the past 7 years without a hitch. The one air filter is changed monthly, anf the outside coils are clean. Sight glass shows everything clear. The unit has performed flawlessly, like a champ.
Recently, though, corresponding with some record high temperatures
07-31 105 08-01 108 08-02 110 08-03 111 08-04 109 08-05 107 08-06 106 08-07 106
beginning about the time the sun hits the outside unit/and/or the outside temp hits 101-102 the unit stops produding cold air. Up until that point, works perfectly, hits set point 70 with no problem. Just updated thermostat to Honeywell 7-day, same behavior, that is, later in the evening, about 8:pm or so, when the outside temp drops down below 100, the cold air starts up again and the house cools down as set on the thermostat. Ice cold, so freon seems good. Rinse and repeat with each 102-3+ F day so far.
Is the unit, which outside of these extremes seems to work flawlessly/no differently than last 7 years, protecting itself somehow with some type of automatic limiting switch? Or is this indicative of some breakdown that would require technician service?
Thanks for any insights. In the meantime, I'm warm now, I'll be cool tonight and tomorrow morning, and mid afternoon forward tomorrow I'll be warm again, unless something changes.
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wrote:

    The unit does not 'blow air of a certain temperature', it blows air of 20 - 25 degrees colder than the air going into it.
    It does so with a certain amount of air, perhaps 1200 CFM for this unit. Do the math and that's the amount of heat load it can handle, IOW heat it can take out ( not accounting for himidity here ).
    If the total heat coming in is more than that, the building will warm up.
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.p.jm.@see_my_sig_for_address.com wrote:

Poor 'stat took the blame. It is called law of physics. aka Thermodynamics.
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On Sun, 07 Aug 2011 20:28:40 -0400, you wrote:

Thanks, Paul. That was my initial theory, too. But when I set a thermometer in a bathroom register, it read a constant 80 degrees - until the compressior finally kicked back on and the temp then started falling rapidly, so rapidly that I could see it drop.
One time that happened after resetting the thermostat, the other after installing the new thermostat when I had cut the breaker off. That's when I began to suspect some type of switch that cut the compressor or condenser or whatever out of the system entirely. Since when it reengaged it proceeded to work like a champ, once again pushing the house temp back down to 70 overnight, I assumed I didn't have a freon problem.
What I guess I don't understand is the rapid, measurable cooling after resetting the thermostat. If what you're talking about is all that's going on, I'd expect to see very gradual warming all day, not the case, followed by just as gradual and imperceptibly measurable cooling.
But obviously I don't know enough about how an AC/heat pump unit like this Trane XR12 differes from a straight AC, and none of my user manuals talk about any protective switches or cutoffs or anything.
Thanks again for the help.
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up.

Any PM done in the past 7 years on the system? Any sign of ice forming on the evaporator coil?
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On Sun, 07 Aug 2011 20:32:18 -0600, you wrote:

PM...preventative maintenance? No, only by me, keeping the drain line clear down through the trap it empties into, keeping the outside unit clean, but it doesn't really accumulate anything but a little dust, no leaves, etc. No freezups, sight glass always clear, no bubbles, no colors.
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wrote:

Possible - compressor overheating
Possible - high limit switch on compressor tripping ( this is very often a manual reset, but not always ).
Try - cutting power to the condensor and restoring it IOW flip the breaker. If that restores cooling immediately, it's one of the above.
Disclaimer - there may be a minimum cycle time at the condensor.
Time for some gauges and a good servce tech.
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First off, replacing the thermostat is like saying "my car isn't working right, maybe if I replace the ignition key...
Thermostat is nothing more than a temperature controlled ON/OFF switch.
I am curious as to the design temps there.... Correctly sized systems will not keep up in record heat. If the design temps are 75F with 50%RH inside, at 98F outside, when it hits 98F outside, it will be running continuous to maintain that 75F inside. if the outside temp is higher than that, the inside temp will rise commenserate. If your able to maintain 70F @ 102F outsied, I would be willing to bet that the system is grossly oversized. This is not a good thing either.
COLD air is not good, it should be cool and dry. You need to call a competent tech to check and service the system. IF its low on refrigerant (FREON), the first thing the tech should do is find out where it went. Refrigerant is not "consumed". If its low, there is a leak.
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On Mon, 8 Aug 2011 08:13:10 -0500, you wrote:

Thanks, Steve. While this may be true, I nevertheless felt I had the personal freedom and budget to upgrade my 7 year old TayStat 540 to a Honeywell 7 day unit in order to begin controlling my usage more finely than I had done in the past, since I will be changing electricity providers next month as well.

Since I am not able to maintain 70F @ 102F and never claimed I was, this is not a problem. The unit is a 3 ton Trane XR12 Weathertron heat pump matched to a 3 ton air handler. The unit it replace was a 2.5 ton outside unit matched to a 3 ton air handler. My house is one story, 1500 square feet.

I enjoy my air cool and dry at 70 degrees F, although as electricity costs keep rising I may find myself raising the temperature of the cool and dry air I enjoy accordingly.

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    Where the hell do you think you are, America or something ?
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While all above maybe correct and according to you, condenser maybe clean you must take in account corrosion which can cause condenser be coming inefficient and does not transmit/pick/discharge heat as it has being at that temperatures afternoon compressor gets overheated results in shut down which you perhaps did not notice. Also once compressor shuts down at high temp it may take an hour or longer before it cools down to come back on.
Remedy buy new unit with little more capacity "OR" put sprinkler on it after noon's. Sorry that is life!!!
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On Sun, 07 Aug 2011 18:44:13 -0500, you wrote:

Thanks, guys. So, one, two, three...five respondents, and none of you is directly familiar with a 13 SEER Trane heat pump? Does Turtle still come around these parts? Or maybe they just sell more of these odd Trane systems around where I live than elsewhere for some reason.
Except for this novel anomaly during a string of record heat days we haven't seen since 1980 I've just started having (I'm home all day), since my system continues to function perfectly otherwise, I was trying to determine if the anomaly was a normal function or not, since A/C calls are currently running at a premium around here. I had called my guy who put the unit in the first and second years after installation and he told me that, unless there was noticeable debris or dirt in the outside unit, there was really no worthwhile maintenance he could perform on a unit so new. Since then, whaddya know, it's been 7 years now.
Christopher, the cleaner you're referring to would seem to be this Virginia stuff sold by Grainger:
http://www.grainger.com/Grainger/VIRGINIA-Condenser-Coil-Cleaner-2NTK6
but now I'm thinking that, rather than spring for 20 bucks plus and do it myself, I'll just put that $20 toward a full maintenance call, even if this episodic condenser/compressor cutting out phenomenon is normal for these Trane units in our current record outdoor heat.
Thanks again for all your responses. If anyone does know authoritatively about protective Trane switches and how they operate, though, I'd still like to know.
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It most likely does have high and low pressure safetys in it... if its tripping in high ambient heat, its most probably tripping on high pressure. The tech needs to find out why and correct the problem. FWIW, most problems either show up during extreme heat or extreme cold. More often as not, its due to lack of being serviced, and/or it wasn't serviced correctly the last time, and/or improper installation.
Another thought.... keeping the thermostat @ 75F for cooling instead of 70F will most likely drop your energy bill as much as 30%.
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