Troubleshoot Air Conditioning Condensing Unit



This 'Troubleshoot Air Conditioning Condensing Unit' page will start off discussing some unusual problems I've run into, and then move on to troubleshooting the refrigeration cycle.


It might be a good idea to read our Air Conditioning Troubleshooting page to get a good idea of the first steps to take when troubleshooting the condensing unit.


The first unusual 'Troubleshoot Air Conditioning Condensing Unit' service call I'll discuss was on a unit with two compressors and two circuits, piped to an evaporator with two circuits.


No matter what the installing contractor did, on one circuit the head pressure stayed high, with mostly bubbles in the sight glass, high amp draw, and poor cooling.


I ended up drawing a picture of the end loops of both condenser coils and comparing them to each other.


They weren't piped the same.


Each condenser coil had two circuits; an upper and lower circuit.


The discharge line Tee'd to two headers, with distribution lines from the headers through the coil, out of the coil through distributor lines back to two liquid headers that Tee'd together before going out of the unit as the liquid line.


On the bad coil, a liquid distributor line and hot gas distributor line were switched and piped so that hot gas went straight into the liquid header and out the liquid line.


Pay attention to little details.


On another strange 'Troubleshoot Air Conditioning Condensing Unit' service call, the condenser fan wouldn't run, and the unit wouldn't pump down and shut off.


With the thermostat off, and the liquid line solenoid closed, it would run in a vacuum and keep on running.


This was a new unit, and on a quick visual inspection, there was a low pressure switch, attatched to a shrader marked "Low Pressure" on the condensing unit, and there was a high pressure safety and low ambient fan switch connected together on a set of shraders marked "High Pressure" on the unit.


We found that the factory had piped these shraders to the wrong sides of the system.


The 'high pressure' shraders were connected to the low side, and the 'low pressure' shrader was connected to the high side.

The low ambient condenser fan switch would never have sensed enough pressure to close and energize the condenser fan, and the low pressure switch would have never sensed a low enough pressure to open and de-energize the contactor.


On this same "Troubleshoot Air Conditioning Condensing Unit' service call, we also found the factory had brazed the TXV equalizing lines to the wrong circuits.


Pay attention to little details.


One last unusual 'Troubleshoot Air Conditioning Condensing Unit' service call involves a unit that had a control board of 7, 24 volt relays controlled by the thermostat and a simple logic board.


This relay board burned out several times.


No one was able to find a short or ground in the circuit, and a replacement board would work fine, the unit would run, service call done---for now.


I was asked to go and replace the board again, and when I found no grounds or shorts, I replaced the board and turned the unit on.


While I was taking amp draws and voltage readings the relays started chattering so I shut the unit down and was able to finally find a grounded wire.


It was the freeze stat lead, pinched between one of the evaporator end loops and the side panel of the unit, with a tiny cut in the insulation.

Condensation off the coil would short it to ground and fry the relay board, then would dry off before a tech arrived and tested it to ground.


On that 'Troubleshoot Air Conditioning Condensing Unit' service call I got lucky.


Ten more minutes and I would have been on the road, and I don't know how quickly that board would have chattered itself to death.


I didn't walk up to these units and find these problems right off the bat.


I wish I was that good.


I've described them to illustrate that on some "Troubleshoot Air Conditioning Condensing Unit' service calls you'll have to go through the unit wire by wire, connection by connection, and pipe by pipe to find the problem.


You might be working on a unit with one of these exact problems yourself.


Is your 'Troubleshoot Air Conditioning Condensing Unit' service call for a unit that's running but cooling poorly?


Is the condensing coil clean?

Is the fan the right size?

Is it running in the right direction, and at the right RPM?

Is it stopping intermittently?

Are the bearings worn out?

Has the run capacitor or start winding failed?

Take an amp draw on the run and start leads.
If you have no amp draw on the start lead of the run capacitor, either the run capacitor has failed, the start lead is broken, or the start winding is open.


Has someone else been working on the unit?


Is the condensing unit the right size for the evaporator?


Is the compressor the right size for the unit?


Verify the evaporator coil and filter are clean, and that the evaporator blower is the correct size and running in the right direction and at the correct speed, and that there is no air flow problem in the ductwork.


When you've finished the previous checks on this 'Troubleshoot Air Conditioning Condensing Unit' service call and have the unit running:


If you have more than 15° subcooling and less than 20° superheat, verify that the unit is not overcharged.


If you have less than 10° subcooling and more than 30° superheat, verify that the unit is not undercharged.


If you have more than 15° subcooling and more than 30° superheat, check your filter drier or metering device for a restiriction.


If you have extremely high suction pressure, extremely low head pressure, and a very low amp draw, check your compressor for mechanical failure.


For more tips on troublshooting the refrigeration cycle, take a look at our System Evaluation Manual.


I hope this page will help you on your next 'Troubleshoot Air Conditioning Condensing Unit' service call, and please, feel free to contact us with any specific HVAC questions you might have, including questions about air conditioning on Guam, or refrigeration on Guam.

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