Should A/C Units be run below 60 degrees?

Submitted by DavidAndersen on Sun, 07/05/2009 - 13:50.

 These low ambient issues are about refrigeration migration.

Refrigerant migrates to the coldest part of the unit. When you run the heat in the winter time it increases the vapor pressure of the refrigerant on the indoor portion of the unit and it migrates to the cold compressor.
Refrigerant also has an infinity to combine with refrigerant oils. So the refrigerant and the oil mix together in the compressor sump.
When the compressor is turned on, the compressor crankcase goes into a low pressure and the refrigerant attempts to boil out of the oil. In the process the oil foams and gets pumped through the system.
When this occurs the compressor makes one heck of a noise!
It sounds like the compressor is going to jump out of the condensing unit.
As Charlie posted, I have seen a lot of equipment started up in the winter time in New England and I have yet to see a compressor failure associated with running the equipment below ambient temperatures.
Air conditioning equipment can be designed to operate in any weather condition, and is often required. Most of the manufacturing facilities that I worked at in New England didn't have heat. If it got cold in the winter time, they just shut down the air conditioning some.
The Coke machine outside of Wal-Mart has all of the equipment outside in the cold. Refrigerant migration is not a problem.
The heat of work, keeps the refrigerant on the cold side of the refrigerant circuit. This equipment also will frequently have a fan cycle control on the condenser.
The problem with oil return which Wayne mentioned, has to do with the oil being pumped out of the compressor and not returning.
This is the result from low refrigerant head pressure. If the condenser fan remains on, the temperature and thus the pressure is significantly reduced in the system. This lowers the velocity which is required to return the compressor oil back to the condenser-compressor. This condition will only damage the compressor of the unit continues to operate for extended periods of time. Compressor motors are submerged in an oil sump and the refrigerant is also saturated with oil vapor. This is generally sufficient lubrication to protect the equipment. Under low ambient loads, the refrigeration equipment does very little work therefore lubrication requirements are lower.
In theory, damage to the compressor can occur (especially if the equipment is aged and about to fail anyway) from this low ambient startup practice.
In reality, low ambient startups occur all the time.
As to whether the seller will hold you responsible for turning on the equipment on low ambient conditions, it is virtually impossible to determine why the compressor failed under these conditions.
Mark, you can tell if there is a heater installed if an extra set of small wires are visible at the base of the compressor (two small wires on top of the compressor is a high temperature overloaded on a scroll compressor and is not a heater). There are three large wires going into one terminal block on the side of the compressor normally. If there is an extra set of wires on the bottom of the compressor, it is a submersion heater. There is also a belly band heater which is an external heater that you can see around the base of the compressor. It is a bright silver metal ring with fins on it to displace the heat from the heater. There is a third type of heater which is usually only found in commercial applications where one of the compressor motor windings remain energized and generates a small amount of heat within the compressor. This type of heater is impossible to detect by the layman.
It is my recommendation that if you live in a cold climate and you are quite sure that the equipment was shut down for the winter and hasn't been started recently, that you do not run the equipment until things warm up. Just because the temperature falls from 70° to 60° overnight and the temperature has not risen back above 60° for 24 hours does not create the situation we are talking about here. Refrigerant to oil migration does not occur in a matter of hours.
As posted, use some common sense. It is more important to know if the compressor actually runs than worrying about low ambient startups.
If you are still afraid of starting the compressor after low ambient conditions, bring along a heat gun from the paint store and put it blowing on the compressor while you conduct the rest of the inspection. By the time you get around to turning on the AC unit, the refrigerant will be boiled out of the oil and you should not have a startup problem.
Again, the main concern is that oil should not get on the piston head as there is insufficient clearance for a non-condensiable liquid between the piston head and valves. The chance of this is possible but very unlikely as the oil must jump up to the top of the compressor head to be taken in on the suction side of the compressor.
The greatest cause of oil-lubrication failure in compressors is due to excessive oil levels in the system, rather than not enough. For those of you that have seen commercial compressors with slight glasses, you may have seen the oil foaming under low ambient or even normal conditions. The foaming oil does not give proper pressure differentials across the oil pump and the equipment generally will shut down before failure occurs.
In residential applications, excessive oil may occur after a compressor failure. Often, compressor failure is due to a refrigerant leak which results in low refrigerant pressures/velocity and refrigerant oil remains out in the refrigerant circuit. Adding a new compressor with a full charge of oil brings back the oil from the circuit and results in excessive oil levels in the compressor which cannot be detected in a residential applications.
Note: Low ambient startup is never a concern with a heat pump.
I recommend that those of you who are not trained or familiar with HVAC equipment that you do not go against standardized home inspection practices if you are uncomfortable doing so. I personally feel that home inspectors should have nothing to say about HVAC equipment, but we do not have the convenience of going against state laws. In this case the state law protects you from operating equipment below 60°F, so utilize it if need be.

Submitted by DavidAndersen on Sun, 07/05/2009 - 13:50.


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