HVAC diagnosis

We survived Ike in extremely good shape compared to others in the vicinity. The only significant problem is the AC unit: the compressor won't come on.
My AC guy swapped out the start & run capacitors, bypassed the relay, and checked all the obvious stuff. No compressor action. He theorizes that power-line chatter at the storm's outset could have so confused the aged system that it committed suicide trying to keep up.
His best guess is a seized compressor piston on this 17-year old, 3-ton, Trane and he's checking with his buddies for a used condensing unit.
Observations and suggestions would be welcome.
Thanks.
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HeyBub wrote:

    Does the fan for the condenser unit start? Did he measure the voltage at the leads going to the compressor? Why did he bypass the relay (contactor)? Was it not latching up? I know these are fundamental checks, but you would be surprised at how some pros diagnose a unit.
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Ken wrote:

No.
Yes
Even if the relay latches, the points could be burned, preventing contact.

Well, the first thing he did was test the start capacitor and pronounced it "marginal." So he replaced it. No joy. That's when he went through the "try this, test that" mode.
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No.
CY: Unlikely to lose the comp and the fan both. Maybe bad relay.

Yes
CY: And got what results?

Even if the relay latches, the points could be burned, preventing contact.
CY: That's true. How about the power safety disconnect? Working properly? Sometimes those corrode.

Well, the first thing he did was test the start capacitor and pronounced it "marginal." So he replaced it. No joy. That's when he went through the "try this, test that" mode.
CY: Amp draw check is important. Also continuity across the compressor terminals.
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system you have is 17 years old and you will spend a

YOu can either spend that money now or in a few years spend the money for a new unit. Don't just get the condensing unit, but a whole new matching system. You probably will save money on the power in the long run to make up the differance. Just plan on spending around $ 5000 for whole new system.
If you are planning on moving out in a few years, then it may pay you to do just a patch job.
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Ralph Mowery wrote:

That will almost certainly never happen. Do the math. If the difference is $4k, and you spend $400/year on A/C electricity costs, you'd have to drive those electricity costs to zero in order to merely cover the interest on the $4K at 10%. It won't happen.
Just plan on spending around $ 5000 for whole new

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wrote in message >> new system. The

I think in the area the airconditioner is being used it will be much more than $ 400 a year on the AC. That is a 3 ton unit that is being installed. Not sure what kind of heat or how much would be used in the area, but getting a heat pump installed may be the way to go so the savings will be for all year.
But you are sort of correct, it will take a while to pay for it.
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Ralph Mowery wrote:

Of course one needs to do the analysis with numbers specific to the situation. FWIW, it costs me about $600/yr to run my four ton unit in Austin, Texas. I was guessing the smaller unit might be cheaper.
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Stormin Mormon wrote:

Yes, he had an ammeter. The compressor is drawing current, but doesn't compress.
And to correct an earlier answer, the fan does activate and run normally - it's just the compressor that won't go.
As an aside, I would think, in my neighborhood, there should be MANY available used condensing units due to Ike. Heck, there are subdivisions where the house is gone but the AC condensing unit remains, alone and forgotten.
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wrote:

By ohming out the windings, commont to run, common to start, run to start and common to ground. See here for further information: http://www.snipsmag.com/Articles/Feature_Article/aa545c2decb28010VgnVCM100000f932a8c0____
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NATE Certified Heating and Air Tech wrote:

http://www.snipsmag.com/Articles/Feature_Article/aa545c2decb28010VgnVCM100000f932a8c0____
How does an electrical resistance measurement reveal a mechanical seize-up?
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CJT wrote:

Checking the electrical resistance of the windings won't indicate a seized compressor. It will indicate a shorted or open winding. I also check for a short to the metal case of the compressor. What I look for is high current draw. If all the electrical components are OK and I measure a high current draw before the internal overload opens up, that's a good indication of a seized compressor. A three phase compressor can be checked by switching two of the leads to try to break it loose by running the motor in reverse. There are compressor analyzers available that can reverse a single phase compressor to break it loose if it is not severely damaged. Here's a link to one of them:
http://tinyurl.com/3nowtq
[8~{} Uncle Monster
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Uncle Monster wrote:

Precisely. The other poster had it wrong.
I also check for a short

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