Measuring A/C Temp Solved, sort of...

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On 5/5/2015 I sent out a note about my air conditioner not cooling after being on for a long period of time. I had to drive up to the Sacramento area yesterday and meet an a/c tech during the late afternoon, when it got hot (but only 80 degrees outdoors), to demonstrate what was going on. He figured it out. I had kept outdoor/indoor/thermostat/vent temperatures for four days running, and noticed a pattern...cold air blew for a while, but after an hour or so, no matter the outside/indoor temperature, or thermostat setting, 74 degree air began to blow.
Whe the a/c first goes on, it blows 58 degree air and begins to cool the house. (I hung a digital thermometer on the overhead vent.) If the house isn't too hot, it gets cooled down properly and the a/c turns itself off. If the house is hot, the a/c begins to blow 58 degree air, but after a while (maybe an hour or so), the fan continues to blow, since the thermostat is still calling to cool more, but the compressor turns off. All the while, I thought the compressor was still going, because air was blowing out of the vent, but the air was 74 degrees.
Yesterday, I turned the a/c on at 2 pm, and the house began to cool with 58 degree air coming from the vent. The tech arrived at 3:15 pm, and just at that time, I showed him that the vent air was now blowing at 74 degrees. He went outside to the a/c unit, and noticed that although air was blowing out the indoor vent, the compressor was not running as noticed by the fact that the fan on top of the unit was not running. He measured all the amp/volts at the two main capacitors (start and run capacitors) and all was fine. He measured the freon, and it was up to capacity. He then measured the temperature of the compressor, and it was HOT, at 149 degrees. He called back to his company and also to the American Standard, the a/c manufacturer, for advice/trouble shooting. All were baffled.
He then took the garden hose and ran water over the compressor cooling fins, and the compressor started right up and the vent air went from 74 to 71 degrees in a minute, and continued to decline. He surmised that the compressor was overheating and was shutting down, as it should. But, no one knows why the compressor gets so hot. The outdoor temperature was only 80 degrees yesterday. Even if the outdoor temperature is 75 degrees, the compressor gets hot after running for a while and shuts itself off.
The a/c unit is an 18 year-old American Standard model 7A2042A100A1, R22, used to cool a 1700 sq. ft. single story home in the Sacramento area. This is my dad's house, who passed away in late 2013. Beginning in 2012, he used to call me up to figure out why the house didn't cool properly. We had techs out, and they replaced this and that, but still no luck. I replaced two thermostats, thinking that may be the issue, but still no luck.
A month ago, when the tech thought the problem was with the a/c fan, because it was VERY noisy, he replaced the fan and capacitor for about $500. But, warm air still blew.
So far, the tech can only explain this by saying that the unit is old, and just doens't run as efficiently. While I can buy this, I don't know why this would be. Anyone?
At any rate, to replace the compressor is about $2500, which I could do, as it seems everything else is running fine, and especially since I've just replace the fan/capacitor. To replace the entire unit is about $4500, but they suggest that if I replace the entire unit, I should also replace the perfectly running furnace, since a new a/c unit uses the newer 'freon', and they like to match that with the furnace. I don't understand this. But, to replace both a/c and furnace is about $10,000. The a/c unit is about a 3.5 ton unit. I prefer to get by with the $2500 compressor, as I don't know how long I'll keep the house.
Any thoughts?
TIA
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Is the outside heat exchanger dirty or blocked up? Spraying water on it lowered the temp enough for the compressor to restart.
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Do you mean radiator looking cooling thing, that's visible when taking off a cabinet side panel? Yes, it was clean.
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On 6/2/2015 1:04 PM, Boris wrote:

More than once, I've worked on radiator looking things that looked clean, but benefitted greatly from professional cleaning. I'm really amazed the tech and tech support didn't figure it out.
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CRNG wrote:

I wonder ODU fan is running reverse? Is the fan blowing out or sucking in the air? 3.5 ton for 1700 sq ft house? Mine is 2600 sq ft 2 story, 3 ton works just fine. 10,000.00 total cost for new system seems too high. If the compressor current draw is normal why does it OH? compressor in our unit is in a insulation jacket yet it never OH. Are the capacitors new for it? How about refrigerant oil?
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I guess I should have said that the tech said he'd replace with a 3.5 ton a/c unit. I assumed the current one is of similar capacity. He does know the size of the house. Is there a place online where I can calculate the size needed?
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snip

Well, I just looked around, and some sites give me 4.5 tons, and this one gives me 3 tons.
http://www.acdirect.com/ac-package-unit-learning-center-ac-sizing- calculator
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On 6/2/2015 4:55 PM, Boris wrote:

That site is giving vague averages. Fist step, did the old unit do the job? If yes,it is probably right or very close to it.
The "square foot" calculations just take a lot of averages, but may be way off on some houses.
Proper calculations consider: Exposed walls and direction they face. Insulation Ceiling/roof and insulation Window square footage and type
Too small, you won't be cool. Too big, you won't remove the humidity and it will be cold and clammy.
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Yeah, the current unit does the job just right. But, I haven't been able to determine it's capacity. I guess I can call American Standard. My dad was meticulous and detailed about keeping records/manuals for every bit of infrastructure in his house, except for the a/c. Go figure.
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Boris wrote:

Hi, Yes, Google it, you'll find "manual J calculation method" to calculate. Over-sizing is common mistake causing short cycling system. If you decide to go for new system, replace both a/c and furnace for improved efficiency resulting cost saving in power consumption and more comfort. I am just a retired EE but I went thru the system replacement at my home(~20 yo). I let the installer do mechanical part and electrical part I did saving some $$. Going into 4th year and not an issue yet. I just replace air filter twice a year(16x25x5 size, MERV 11) Just finished cleaning condenser coil and checked Puron level as cooling season begins.
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On 6/2/2015 4:47 PM, Boris wrote:

I noted inother messaged that you'd found an online heat load calculator. I think there is a misconception that if the AC isn't working, to replace it with a larger one. I had a customer say to me one time "well, if three and a half tons isn't cooling, maybe I need five?". The probelem was that he was not getting three and a half tons of cooling, he was getting zero cooling.
BTW, in that case, I checked the refrigerant, amp draw, temps, etc. A professional cleaning, and he was back to supply air vent temps about 53F, if memory serves. He was pleased for my simple minded, and totally correct diagnosis and treatment. He was ready to invest three grand in a system.
Freon (R) is a brand name, like Cheerios, or Sunoco. Refrigerant is the descriptive term, like breakfast cereal, or gasoline. I do have some Freon (R), and also some Forane (R) and maybe some Isotron (R) and some Genetron (R).
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On 6/3/15 7:11 AM, Stormin Mormon wrote:

So he wasn't getting great cooling for an hour at a time with a dirty condenser.
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On 6/3/2015 11:30 PM, J Burns wrote:

True, he was not getting great cooling, at all.
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On 6/4/15 8:32 AM, Stormin Mormon wrote:

I understand the OP starts out with great cooling, and it takes about an hour for trouble to become apparent. That makes me think something in the system changes in the course of an hour.
The site I found said the drier and the strainer can block the freon. If the drier causes a restriction, maybe it has absorbed all the water it can. A restriction would cause cooling as the refrigerant expands. If the restriction goes below freezing, ice could accumulate until the restriction is bad enough to overheat the pump.
If the drier/strainer is accessible, monitoring the temperatures on both sides would reveal a restriction, I think.
I hope you don't think I'm a troll, just saying that to get you all worked up!
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No thought of trolling at all. Never crossed my mind. Keep the ideas coming.
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It does sound as if the compressor is worn out after 18 years.
I don't understand why a furnace (you did not tell what type or kind) would have anything to do with the air conditioner. That statement abou tthe need because of the newer "freon" and a match seems bogus to me. I think I would be looking for a different company to do the work.
I believe I would replace the whole air conditioner. That way if it does not cool correctly it would be on the installer to make it right and he could not blame the problem on anything else.
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The furnace is an American Standard, up-flow, housed in the hall closet. There are copper tubes (carrying what, I don't know, but they get cold when the a/c comes on)going to the furnace bonnet. I don't know the interaction, if any.

You're probably right.
Thanks.

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Boris wrote:

Before you replace the unit , do as a couple of others have suggested and really clean the condenser coils - the radiator looking part of the outdoor unit . It can look clean and still have enough gunk on the fins to interfere with air flow and heat transfer . On the other hand , replacing a unit that old with a new more efficient one may be a good economic choice . Don't fall for that "we gotta replace the furnace too" crap . The systems are related in their handling of the air , but otherwise are independent systems .
--
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Yes, thank you. I'm sure I can do that myself, as long as I'm careful and pay attention not to disturb any other components.
It can look clean and still have enough gunk on the

Does sound like crap to me, too. Especially since he couldn't give me a good answer as to why, no matter how many ways I asked for clarification.
I was also reading about cleaning the evaporative coil in the top of the furnace.
http://www.handymanhowto.com/how-to-clean-air-conditioner-evaporator- coils-part-3/
I doubt it's ever been done.

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Boris wrote:

If you've kept up with filter changes it's probably pretty clean . Depending on how your system is put together it's probably a pure b***h to check .
--
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