replace compressor or purchase new ac unit ????


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Old 09-08-11, 06:36 PM
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replace compressor or purchase new ac unit ????

My home has (2) 3 ton carrier units. Unit # 2 is problem unit. Yesterday the breaker for #2 was blown in garage. When reset, it immediately blew again.

Using the knowledge of my son, I think we have identified the problem. My son had a 3 years of HVAC repair in technical high school and actually even went to national competition for HVAC in the Skills USA competition, so he has some decent working knowledge of HVAC. But he does not do that for his occupation. But he still helped me ( actually I helped him) try and trouble shoot.


We first checked the capacitor and it is fine. Checked proper voltage via main line; ok. Checked contactor and all seems fine. Tested voltage from each lead on capacitor, and each wire connection screw on contactor; all seemed fine. I do not think there is a start or run capacitor, I think the triple lead capacitor we have does both functions.

We then disconnected the compressor leads from capacitor and then tried just the fan motor, it ran fine, no noise, and breaker did not blow. Then we disconnected the fan motor leads and tried compressor by itself. It tried to run, but made noise I would describe as very audible grinding for 10 seconds then breaker blew again. The noise made me think a fan blade was scraping something, but we had already removed motor fan from housing, the fan motor was d/c from wires, and noise was definitely from within housing. We also checked all wiring for breaks and or rub spot where it could possible caused a short, none found, all wiring insulation looked fine

As we tried this it was about 3:30 am in the morning, My son works weird hours and he just came home from work so we attacked the problem right away using numerous flood lights to illuminate the entire area to ensure safety. However. In a little semi darkness, my son said he saw a brief but noticeable electrical spark emit from compressor housing when we threw breaker on to see if compressor would turn on. I was standing50 feet away by garage to listen for the noise made if the breaker blew so I did not see the spark.

We reset breaker again and compressor ran another 10-20 seconds, again making loud noise, and breaker blew. Son hooked up his ammeter or ohm gauge ( i do not know proper name) and checked the three prongs on plug going into compressor housing. Two of the leads showed equal reading, one lead showed a lesser reading that he thinks is indicative of a problem (possibly electrical ??) within the compressor housing itself. Thus, we feel pretty confident the compressor went out on it.

So this unit # 2 is a 6 year 3 months old Carrier, 10 seer, 3 ton (carrier 38cks) unit that cools the upstairs bedroom and bonus room area. (Maybe 1500 sq ft.area to be cooled)
I have yet to have a professional HVAC company come on site. If possible, can anyone please give me a good guess-estimate of cost, including labor (including recovery and replacement of Freon) to reinstall a replacement compressor as opposed to the cost to purchase a newer more efficient ac unit, (maybe 14 seer 3 ton) and then have new unit installed to existing pad and connections. The unit itself only has a little rust so it may be worth replacing compressor, but I want to compare costs. Web search showed prices for compressor from 4 hundred to 19 hundred ( compressor is carrier p/n 739024-1602-00; model # H29B33UABCA oc-25 230/208v, 60hz, 1 ph lra 82)

If a new a/c is purchased, will is cause any conflict/ problem with unit # 1, which is a twin carrier 3 ton, that is also 6 + years old that cools the downstairs of my home.

I appreciate any and all insights.
 
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Old 09-08-11, 06:52 PM
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lets check some things first before we discuss replacing the compressor. How did you check the capacitor? Did you use a meter that can check capacitance? Does the capacitor look swollen on top (should be flat). During the resistance check of the windings did you check each leg to ground from the compressor terminals? The resistance check you did, do you have the numbers you came up with? 2 of the readings should add up to the third (or at least be close). At 6 years old it is possible (though unlikely) that compressor is still in parts warranty. Another inexpensive thing to try is adding a hard start kit before condemning the compressor.
 
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Old 09-08-11, 07:08 PM
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wew checked it with his Fluke meter I think he measured MFD's . Plus the capacitor has blown before and it was swollen on the top last time. so I purchased two, one for a replacement and an extra to have on hand. we also connected the new replacement just to ensure the capacitor worked .

I do not remember the actaukl numerical readings he got, but it was when he held the leads of the voltmeter to the three prongs that are sticking out from the compressor under the plastic cap housing that covers the compressor plug when it is plugged in. I just remember him saying that two of them were almost the same and the third was a readin of about 25% of the other two numbes. I just do not remember the numbers. he unit was warranted for 5 years only so I'm 14 months too late. We discussed a hard start kit but he was able to get it to turn just by unhooking the fan motor from the capacitor and leaving on just the , I think, 3 wires going to the compresssor. but when it did turn, he saw an arc or spark and it made a very loud noise, and blew the breaker in 10 seconds or so,.
 
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Old 09-09-11, 07:25 AM
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If the breaker took several seconds to trip it's unlikely the compressor is shorted but could have a locked rotor. Have your son check the amp draw on the common wire to the compressor. The compressor &/or unit data plate should list the locked rotor amp draw.
 
 

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