Outdoor Compressor Overheat

Howie32703July 6, 2011

The compressor will run all day and never overheat if you keep the thermostat low. If set to normal say 78 degrees will run for one cycle. After system shuts down it will not restart

on the 2nd cooling cycle. Only outside fan runs. Here is what has been done and is driving my air conditioning guys crazy.

The entire outside unit was replaced as the system was 20 years old. High and low pressures are within specs, return suction line feels very cool back into compressor, out pressure line warm.

Heat exchanger coils inside cleaned and show no restrictions and shiny and blower motor cleaned.

When unit runs, vent air temp is between 50 and 55 degrees. Plenty of air flow out of vents.

What we cannot understand is why it will run for hours but not restart on next cycle. If you wet down the compressor with a hose and cool it it will start.

It did this with the old outside unit.

Anyone help ? Thanks

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From what you describe, the compressor is obviously overheating and the internal thermal overload is tripped until the unit is cooled down. This is normally an indication of a bad or wrong sized capacitor. Considering the unit is new, I would start looking at the power coming into the unit. Is it at the rated voltage? Brown-outs can cause this and also a faulty transformer from the POCO. Are you getting a clean 60 cycle supply? Are all the connections from the breaker panel to the outdoor shut off clean and securely tight? Is the wiring adequate? Is the breaker functioning adequately on both legs? Is the outside shut off a breaker, a pull switch, a 2 fuse box, a knife switch? I'm suspecting a high resistance connection develops after the unit has run. Once the unit initially starts, the required amperage drops and may not appear to be affecting the unit. Do the techs measure the amperage draw while running (it could be marginal), and also during a restart? How far is the unit from the electrical panel. Is this a 3 phase unit?

Although there are other possibilities, I'm going to assume the techs. are competent and looked at those issues already.

    Bookmark   July 6, 2011 at 9:22AM
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Something else you might check and see is what kind of metering drvice you have. I know this might sound crazy but the problem might not be in your outdoor unit at all. If your evaporator coil has an Thermostatic Expansion Valve rather than a orifice metering device, you will 9 times out of 10 need a Hard start installed in your outdoor unit. Here is what the thermostatic expansion valve looks like.

If you have one of these on your coil then you will need one of these for your outside unit.

What happens is that here is your a/c running along fine... cooling your house and when it reaches its set point, the thermostat shuts the a/c off. If you have a orifice metering device, the freon equalizes in pressure after a few minutes and on the next call for cooling,the outside unit fires right up. If you have an thermostatic expansion valve, it takes alot longer for the pressures to equalize and maybe when your outside unit trys to start back up, the pressures are not completely equal and the compressor starts cycling on its overload. It then gets hot from failed attempts at starting and then your house gets hot.

I would check to see if you have this type of valve on your evaporator coil and if you do, check to see if you have the hard start on your outside unit. If you do have the expansion valve and dont have a hard start kit, get ahold of your local a/c tech and have him install one.

Hope this helps

    Bookmark   July 6, 2011 at 10:13PM
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fluffy...To a service person, what you suggest doesn't sound crazy at all. I'm hoping the service guys checked that out already. It's already obvious the problem may not have been the condensing unit at all since the new one is doing the same exact thing the original was doing. You bring up a thought....hard start kit.

Howie....when the unit shuts down after running, do you hear hissing and gurgling (this is normal) at the evaporator coil? Is there any hissing and gurgling going on right at the time the unit starts back up?

    Bookmark   July 7, 2011 at 9:00AM
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Unit does have a thermostatic expansion valve exactly the same color too with the tubing taped to the suction line. no hissing or gurgling sound..Can the expansion valve be replaced instead of a hard start..

Also checked voltage at compressor on contacter( relay )..243 VAC when not running drops only a volt ( 242 ) when running. Current draw is around 12 amps. Also show 26VAC across the contacter coil when running.

    Bookmark   July 7, 2011 at 3:04PM
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Its alot less expensive to install a hard start kit on your outdoor unit than it is to replace a expansion valve. Reason being... hard start kit involves taking the outdoor units cover off, turning off the power, installing the hard start kit (5 minutes max), putting the cover back on, turning the power on and "Poof" your done.
Changing from a expansion valve to a orifice takes alot more time... pumpdown of unit, taking coil door off, removing valve, (coil might not even be designed for orifice, bleed line might be soldered in place insted of having a flair fitting which involves torches), new dryer, vacuum system... so on and so on. Pretty major service call not to mention $$$$$$$. You also loose some efficiency as well.

Did you check your outdoor units control panel to see if you have a hard start kit?

Hope this helps

    Bookmark   July 7, 2011 at 6:32PM
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Outdoor unit only has the round dual capacitor for fan and compressor.

I have been looking at different hard starts and don't know which one would be best..any suggestions ? its a 2 ton.

I read that they have to have a timer circuit built in to prevent compressor damage.

I found out that if I let the air cycle off, then shut the thermostat "off " it will restart after 15-20 minutes when putting it back to "cool" If left in normal mode will take about 45-60 minutes to restart..Guess it's the compressor trying to keep re-starting then goes into an overheat condition.

I am glad I have 3 8000 BTU window units installed that keep the house cool.. Put these in about 3 years ago as a backup..No fun in FL heat.

Thanks for all your input...don't know why my air guy has not figured it out..

    Bookmark   July 7, 2011 at 8:26PM
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Installed a hard start on the unit this morning ( ICM805 )..so far so good, no failed starts. Had to move and remount the regular cap to make room for the hard start.

    Bookmark   July 14, 2011 at 3:00PM
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