Panasonic AC window unit short-cycling

[fred_perkins]

In 2004, consumer purchased a large Panasonic window unit (24000 btu, model #CWXC243HU) and had it professionally installed in a NYC loft space.  It cooled the space well for the first four summers, but did not seem to work as well last summer despite annual cleaning of evaporator and condenser fins, replacement of the e+ filter and cleaning the larger screen filter every two weeks.

Refigerant levels are fine (no leaks, no icing anywhere) and when the compressor is on, the unit cools very well.  BUT it is short-cycling -- compressor only stays on for two and a half minutes (2:30) and usually dosn't cycle back on for four or five minutes -- regardless of the temperature setting.  Replaced thermistor assembly (it plugs in to the main PC board) but the unit is still short-cycling.

Is there anything else that I should check before ordering a new PC board (cost is about $60)?

[RegUS_PatOff]

model number ?

Is the Compressor hot (over-heating) ?

[Pegi]

Well, I would be checking for voltage to the compressor first to see if it could be a weak overload perhaps or low voltage to the compressor or to the unit itself..

[fred_perkins]

Sure enough... the voltage to the compressor, measured between R (black) and S (red) leads, does seem high.  For the first 10 or 20 seconds as the compressor is firing up, the voltage fluctuates between 210 and 250 -- then it eventually goes steady at about 255 VAC.  

The compressor housing is hot to the touch -- I'm guessing it gets up above 200 degrees F.  

The voltage at the wall plug checks out fine: 220 VAC.  And the breaker never trips.  I already listed the model number in parentheses in the first sentence of the original post. 

[Pegi]

Sounding like a bad compressor or capacitor.....betting on bad compressor tho...

[fred_perkins]

So I should try replacing the capacitor ($35 from Panasonic's parts service) before recommending replacement of the entire unit?

I don't think a replacement compressor is even available.  And even if I managed to find a new compressor, I wouldn't want to tackle that sort of job.

[dkpd1581]

Measuring voltage across R and  S is showing you the counter EMF off of the Start winding and or/capacitor so you will get an odd ball reading doing it that way that is in excess of the true applied voltage. Measure between R and C if you want to measure the true applied voltage.

An amp draw reading on the common leg of the compressor will tell you if the motor is pulling locked rotor or unusually high amps.   This can be caused by defective capacitor or poor terminal connections at the compressor face or through a  switch/contactor.

If the amp draw is normal to low and the unit still over heats you may have a unit that is running but not pumping properly due to defective valves.  You may also have a defective internal over load switch either internal to the compressor or external mounted on the head of the compressor.  Defective valves will show little to no temperature drop across the evaporator.  Defective overload will show a normal temperature drop across the coil but shut off for no apparent reason.

The head of the compressor will get warm/hot regardless.  The issue is the degree of how hot it gets (normal condensing temperature of R22 is 120F +/- on design condition day 0f 95F outdoor and 75F return air.  Add in the heat of compression and windings and the head could be as much as say 150F).

[RegUS_PatOff]

[user=53122]fred_perkins[/user] wrote:

The compressor housing is hot to the touch -- I'm guessing it gets up above 200 degrees F 

may be nice to have

typo: should be 12:1 ratio

click on picture  $ 43.99 w/free shipping

[fred_perkins]

OK, I'm now measuring between R (black) and C (blue) leads, and it's steady at exactly 220 VAC.  So please ignore post #4 and my references to 250 and 255 VAC.  

I have not disconnected any of the three leads -- these mesaurements are being taken with all leads connected while the compressor is running.

Will consider the recommended infrared thermometer.  Thanks for the suggestion.

[RegUS_PatOff]

take print-out to store

one link mentioned a "one day sale" 

click on picture reg $ 19.99   sale $ 9.99

20% OFF Coupon 7/03 - 7/09 good on sale prices

[fred_perkins]

Thanks, but I already have a great VOM/multimeter.

Temperature drop across the evaporator is significant.  

I'll try to measure the temp on the compressor housing.

[RegUS_PatOff]

w/ clamp on amp meter ...

[dkpd1581]

When you state SIGNIFICANT what specifically is the dry bulb return and supply temperature difference and what is the specific amp draw.  If there is some type of airflow issue where the TD is abnormal then you may be shutting down on a frost stat. Just because the coils were cleaned doesn't necessarily mean that they are unobstructed and the air flow is sufficient. High TD and low amp draw is a good indicator

If the unit is over charged and using any type of TXV meter device then you will have a backed up condensor and an otherwise normal looking evaporator and will definitely shut down the compressor as it cannot overcome the excessive head pressures.  If the TXV power head, sensing bulb, or cap tube is bad then you will have a backed up condensor because the valve will fail in a closed position. 

Again you could simply have a defective internal overload or a defective overload on the outside/head area of the compressor.  A voltage check across the external one while running will show if it opens and is shutting the compressor down.