Compressor Resistance - Replace?


Old 06-03-15, 03:01 PM
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Question Compressor Resistance - Replace?

Looking for advice on whether to replace a compressor or not.

In 2006 I redid the entire HVAC in the house. The installation company has been servicing the units annually for spring and fall for AC and heat, respectively. In general I do not think they are someone that would scam me but...

There are two systems for the house. One is a 4 ton the other a 3 ton.

During the recent spring checkup they ohmed the compressors. The 3 ton to ground measured 100-150 Megs. The 4 ton was 48 Megs. They indicated American Standard would replace the 4 ton under the 10 year warranty. They recommend checking the 3 ton next year before that warranty expires. While the compressor would be replace the labor is $1,400 for each unit. These units are using R-22.

I found an older report that the 4 ton was 100+ megs in 2013 and earlier.

They also quoted replacement but I really do not want to spend that money now.

I'm disappointed that these units are 'failing' in less than 10 year after spending some $16,000 to redo the system in 2006.

This megohm readings are new to me not having seen them used previously. I've had a number of houses previously and generally the AC lasted for decades without problems.

Is this really a problem? Should I just do the warranty replacement and hope the rest stays okay?
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Old 06-03-15, 03:24 PM
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Personally, I don't think it's anything to be overly concerned about. The 4 ton with 48 Megs (million ohms) resistance to ground would have ~4.5 uA (microamps or millionths of an amp) leakage current. The compressor should be grounded, so there shouldn't be any shock hazard.

Note: I should state that I'm not an HVAC tech, but I am an electrical engineer.
Old 06-03-15, 07:04 PM
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Welcome to the forums.

As far as I know.... that's not a routine measurement that is made in maintenance of home units but it is a valid test and does mean something. As the compressor gets older the insulation on the motor windings breaks down due to contaminants in the refrigerant and overall heat.

When the resistance decreases to a certain point.... the insulation is no longer doing its job and the windings arc or short out.

A single measurement wouldn't mean much but it would appear that your A/C company has been checking the resistance every year. That means they have an original or baseline figure and what it is right now. You said it was 100+ megs in 2013 and two years later it's now 48 megs. That tells me that the either they mis-tested this time or the compressor is failing rapidly.

I don't know at which point the compressor will arc or short out. My partner Houston may know the answer to that one.
Old 06-03-15, 07:44 PM
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This is and excerpt from the Supco megohmmeter manual:

100-150 Megohm Readings
Most electrical engineers and motor designers agree
that a measurement of 150 or more megohms across an electrical terminal to its ground would be considered excellent insulation, and a reading of 100 to 150 megohms is very good.

60-100 Megohm Readings
Measurements of 60 to 100 megohms would show a decline in the insulation resistance, either in an area
or a specific spot. This is a most
important measurement. It tells you that if you do
not take corrective steps, the insulation will completely breakdown. With open type motors
it is a matter of good housekeeping by cleaning the
dust and any grime from the windings, using a recommended solvent.

20-60 Megohm Readings
A 40 to 60 megohms reading in a hermetic compressor
is an indication that you can have any one of a fe
w problems; a winding that was
overheated, contaminated oil, or moisture circulating in the system. Sampling the oil for a burning odor will tell you immediately that the
winding was overheated. If the oil is clean and od
or free, it can still have some contamination that
is causing the low megohm reading. A 20
megohm reading shows severe contamination and failure of the system is likely. The best overall protective procedure is to dump and replace
the entire oil, change and install a new liquid lin
e drier. If the oil had an odor, a thorough check
of the condensing medium, air fans, cooling
tower and all motors for worn bearings should be made.
Now that you have discovered a low megohm reading and have taken the necessary preventative measures,
the follow-up is very important.
Take another reading in a few weeks, and if the reading remains the same or improves, take another measurement in a few months. If the
reading shows more deterioration, you can forewarn
the owner that there is going to be a major breakdown in his system.

One of the best maintenance programs for a new system is to log the
megohmmeter readings when the unit is first instal
led and compare the readings every
six months.
Old 06-03-15, 07:59 PM
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$1400 is a lot of money to replace a warranty part. a new 4 ton condensing unit is about $1000. Its very rare to meg a residential compressor. Sounds like either an over zealous company or a company looking for a sale at their magic ten year number. If it was me, I would do nothing.
Old 06-03-15, 08:30 PM
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Keep in mind.... part of that $1400 is a complete recharge of R-22 refrigerant.... although I agree that the price sounds like it's on the high side.
Old 06-03-15, 08:53 PM
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$1400 is a high price to save a 10 year old R22 unit.
Old 06-03-15, 10:27 PM
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Appreciate the comments

Thanks for the comments. I'm not concerned about it shocking anyone since it is supposedly well grounded.

The concern is the complete failure with the possibility of tossing a lot of junk into the overall system.

Is it feasible to replace the oil on the unit? I think it is a sealed unit.
Old 06-04-15, 04:14 AM
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In commercial circles if the compressor windings show a moderate amount of current leakage replacing the filter/drier and changing the compressor oil is where you start with trying to bring the readings down.
In a hermetic compressor changing oil is not that easy to do so just changing the drier should be all you need to do.

Most residential a/c uses a very small drier as part of the condensing unit.
Carefully sizing and adding a drier on the liquid line should not cost that much.
Old 06-04-15, 04:15 AM
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Changing the oil is even rarer than meging a residential AC unit.

PJ......They probably would just recover and reuse the R22. Either way the job is worth about half what he was quoted. And thats only if the compressor was actually bad. Preventive maintenance is a big seller for a lot of companies. Some of it is borderline legit. This IMO is not even borderline.

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