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Old 08-15-2011, 07:34 PM   #1
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Choosing HVAC Tech


I have some guys coming to give me quotes on replacing the condensor (I finally realized that the compressor is a major component of the condensor). Just talking with one on the telephone I wondered about his competence?

He said that the test to see if the compressor is good is to measure h to c, s to c and then add the two resistances. If the sum equals resistance from r to s it is good.

I told him I also measured from the teminals to the crank case and get readings at differnt times of day, when the temperature fluctuates, from 7 Mega Ohms to 9 Mega Ohms. Every thing I have read on the internet or seen on youtube includes ohming from the terminals to the crank case or the copper suction pipe. Should I run from this guy?

Thanks.

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Old 08-15-2011, 07:43 PM   #2
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Ask him if he will test with Megger.

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Old 08-15-2011, 07:54 PM   #3
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Ask him if he will test with Megger.
I will to see if he knows what a Megger is. The two small pocket size mutimeters I first had weren't sensitive enough to measure the short and I understand pros will usually use a Megger.
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Old 08-15-2011, 08:04 PM   #4
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The compressor can have 3 problems:
- Mechanical - Noise, pressure problems
- Isolation - when the windings are shorted to the housing
- Resistence, when the windings are short or open
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Old 08-15-2011, 10:08 PM   #5
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The compressor can have 3 problems:
- Mechanical - Noise, pressure problems
- Isolation - when the windings are shorted to the housing
- Resistence, when the windings are short or open
When the thermostat is turned on, the contactor closes and it immediately pops the 50 amp circuit breaker when the lead to the run winding of the compressor is attached to the load side of the L2 pole of the contactor.

The breaker opens immediately, without any humming that to me would indicate that the compressor was trying to start but was locked.

The windings aren't open as each one measures resistance. Doesn't the fact that I measure several million ohms of resistance from any of the three compressor terminals (resistance between the copper line and each of the three termnals is the same) indicatre a short to ground?

Pressure problems may have been a contrubuting factor as it hadn't been serviced in 15 years.
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Old 08-15-2011, 10:33 PM   #6
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Sounds like insulation problem.
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Old 08-15-2011, 11:11 PM   #7
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breaker "opening' immediately means compressor is shot, grounded out. If the breaker trips, one of two motors is grounded, the compressor or the fan.

If the condenser fan runs but the compressor does not and the breaker does not trip, open winding of the compressor, shot. Or the run cap but you're saying the breaker is tripping.

Bad compressor. A faulty run cap will not make a breaker trip.

Your compressor is shot.
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Old 08-16-2011, 02:50 AM   #8
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Thank you both for confirming that the compressor is shot. As for the run capacitor, it is part of a dual capacitor and the hermes side was bad, but I replaced it and it made no differnce.
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Old 08-16-2011, 06:37 AM   #9
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if you get any resistance number to ground it is shot with wires disconnected...burp the suction gas and smell it on your fingers might be acid smell confirming a burn out..how does the contacts look on the compressor contactor
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Old 08-16-2011, 06:38 AM   #10
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i always like it when a customer calls for service and then tells me what's wrong when I get there. Cust.-- I think it needs freon or the compressor is bad. Mech.-- Yes you are 100% right the compressor is shot. They feel better when signing the check because they were right.

Note: I am honest as the day is long but some customers insist on helping with diagnosing and fixing their equipment.

What people should tell the service man, it's making a noise here, it's blowing hot air, it's blowing the circuit breaker, it don't hold the temperature, it runs all the time, it keeps going on and off, etc etc,etc.
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Old 08-16-2011, 08:22 AM   #11
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i always like it when a customer calls for service and then tells me what's wrong when I get there. Cust.-- I think it needs freon or the compressor is bad. Mech.-- Yes you are 100% right the compressor is shot. They feel better when signing the check because they were right.

Note: I am honest as the day is long but some customers insist on helping with diagnosing and fixing their equipment.

What people should tell the service man, it's making a noise here, it's blowing hot air, it's blowing the circuit breaker, it don't hold the temperature, it runs all the time, it keeps going on and off, etc etc,etc.
You need to know enough to not be fooled, but you can not demonstrate you know ..... it applies to doctors, mechanics, etc
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Old 08-16-2011, 12:20 PM   #12
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i always like it when a customer calls for service and then tells me what's wrong when I get there. Cust.-- I think it needs freon or the compressor is bad. Mech.-- Yes you are 100% right the compressor is shot. They feel better when signing the check because they were right.

Note: I am honest as the day is long but some customers insist on helping with diagnosing and fixing their equipment.

What people should tell the service man, it's making a noise here, it's blowing hot air, it's blowing the circuit breaker, it don't hold the temperature, it runs all the time, it keeps going on and off, etc etc,etc.
I failed to make myself clear. The diagniosis has been made with help from DIY Chatroom Techs and I contacted this local technician to replace the entire condensor (as well as whatever else needs to be done such as replacing the evapotater coil). I told him that I had ohmed the terminals and he asked what they read. I also told him that Imt MM read 7 to 9 milion ohms between the terminal and the crankcase and he said that is not a test for a bad compressor.

Now this guy is coming out to give me a price on rrplacing the system, but if he doesn't know basic diagnostics I don't want him touching the system.

IIRC ohming for a short from the winding terminal to ground is a common procedure for testing a compressor. A megger is used by many pros since many muoltimeters don't have the juice or the sensitivity to find a short. My old GE pocket meter read infinite. A better pocket meter still did not read it. I got a larger Klein and it was capable of reading the several million ohms of resistance.
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Old 08-16-2011, 12:28 PM   #13
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You need to know enough to not be fooled, but you can not demonstrate you know ..... it applies to doctors, mechanics, etc

That is very wise. Before retiring I dealt with many different kinds of experts and the best way to tell whether they knew their stuff was to let them talk and listen to them. There is another contractor due here today to give a bid.
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Old 08-16-2011, 01:19 PM   #14
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Originally Posted by Klawman View Post
I failed to make myself clear. The diagniosis has been made with help from DIY Chatroom Techs and I contacted this local technician to replace the entire condensor (as well as whatever else needs to be done such as replacing the evapotater coil). I told him that I had ohmed the terminals and he asked what they read. I also told him that Imt MM read 7 to 9 milion ohms between the terminal and the crankcase and he said that is not a test for a bad compressor.

Now this guy is coming out to give me a price on rrplacing the system, but if he doesn't know basic diagnostics I don't want him touching the system.

IIRC ohming for a short from the winding terminal to ground is a common procedure for testing a compressor. A megger is used by many pros since many muoltimeters don't have the juice or the sensitivity to find a short. My old GE pocket meter read infinite. A better pocket meter still did not read it. I got a larger Klein and it was capable of reading the several million ohms of resistance.
Klaw, you made yourself very clear I was just throwing out a little bit of what I have experienced over the years it was not to insult you or jab at you, please except my apology if it did.
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Old 08-16-2011, 01:30 PM   #15
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Klaw, you made yourself very clear I was just throwing out a little bit of what I have experienced over the years it was not to insult you or jab at you, please except my apology if it did.

No problem, Cold, but thank you for the apology. I am just overly sensitive, but agree that the customer can waste a lot of time and confuse things, and increase expenses, when they don't let the pro do their job. Even in the rare situation where the customer/client knows the field as well, their work is often corrupted by a lack of objectivity.

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