Service Hotline: 07/01/2002
By Wayne Shoemaker
Senior Research Maintenance Technician
I would like to add a comment to the Hotline question on residential units that appeared in the Dec. 3, 2001 issue of The News.
It is very important that there are no leaks in the duct of a unit located in the attic. If supply ducts leak, then conditioned air can escape into the attic. This leads to a loss in capacity. Also, warm moist air can be drawn into the supply duct at a leak.
If return air ducts leak, then warm moist attic air is entering the ducts and increasing the load demand on the unit. This load demand will increase as the heat of the day increases.
When both supply and return ducts leak, then it is unlikely a properly sized unit could maintain temperature in the conditioned space.
Any leak is, in truth, a large leak.
To Dale Bowman
Could you please answer a couple of questions concerning run capacitors?
In servicing 220 compressors, why are some run caps wired with two wires and some with three? I’ve also found 220 condenser motors like this.
Second, how are run capacitors used as crankcase heaters?
From Gene Silberstein
Both parts of your question are closely related. A split capacitance run capacitor with three terminals is commonly used on PSC compressors with run capacitance off-cycle heat. Basically, this means that during the compressor off-cycle, there is a small amount of current flowing through the start winding, generating heat. This heat serves to warm the oil in the crankcase, allowing the refrigerant to boil from the oil, serving the same function as the crankcase heater.
A simplified diagram of how this type of setup operates is shown here in Figure 1.
When the operation device (thermostat, etc.) is closed, the motor functions as a normal PSC motor. When the operational device opens, the run winding is taken out of the circuit and a small amount of current flows through the start winding and the high resistance bleed resistor. This generates the desired heat for the crankcase. The purpose for the split capacitor is to reduce the amount of capacitance in the circuit during the off cycle. The bleed resistor — being a low-wattage, high resistance device — keeps the amount of current flowing through the start winding very low during the compressor off cycle.
In my situation, the evaporator sits on the right side of a refrigerator. The right side holds the temperature. But the left side doesn’t. Both evaporator fans are working. The condensing unit is clean and the pressures are what they are supposed to be. These refrigerators are in the kitchen of a hotel.
I checked everything on the fridge but I can’t fix the problem. There is a 5 to 10 degree difference between the two.
From Gene Silberstein
From the information you provided, it seems like there is an airflow problem.
Assuming that the only evaporator is on the right side of the unit, the cool air must pass to the left-hand side of the unit via the small air ducts. These passages must be clean and in no way blocked or restricted. If they are, the thermostat, which is most likely located in the right-hand side of the box, will open its contacts before the left-hand side reaches temperature. Storing too much product in the box could very well cause the problem.
It is important to note at this point that the information provided in your question does not seem to be complete. If the box is in the kitchen of a hotel, chances are that this box is a commercial system. Commercial refrigerators are normally equipped with two evaporators, connected in either series or parallel with each other, each serving its own portion of the unit. Where in the kitchen are the units located? Providing the operating pressures, unit manufacturer, unit model, and other pertinent information will help us supply you with a more accurate and helpful response to your question.
Liquid Line SetsQUESTION:
By Chuck Desnick
I have a question regarding charging a residential split system with a 75- to 100-ft line set.
If this was a 25-ft set, I would have a wetbulb temperature of 70 degrees F, and an outside drybulb temperature of 90 degrees with a superheat of 20 degrees for a cap system, and a TXV set at 15 degrees.
Now what would I do different for a longer length? Or would I do the same thing?
From Dan Kramer
Patent Attorney and Specialist Grade Member of RSES
With a liquid line that long, I would install a liquid receiver. If there was a fixed restrictor, I would remove or bypass it and install a thermal expansion valve. I would provide a liquid sight glass ahead of the TXV and a liquid solenoid just ahead of the TXV wired in parallel with the compressor. I would simply charge by the liquid sight glass.
I would be concerned about the much larger than normal amount of refrigerant charge that might migrate to the compressor during off cycles and damage the compressor on startup. This would be of special concern with the new compressors that have a very small oil charge.
If there were to be high internal loads that might require the system to operate under non-summer conditions, I might provide an external discharge check valve, a low pressure switch, and set the system for auto-recycling pumpdown.
High Head PressureCOMMENT:
By Don Marino
I want to respond to a past question about high head pressure and low suction pressure on a split system air conditioner. The question concerned blowing out the system with nitrogen, pulling a vacuum, and replacing the drier. But it still had a head pressure of 325 pounds.
The answer concerned replacing the cap tube with an expansion valve. That would solve the problem. But I’m not sure it is necessary.
I think the problem is the wrong size orifice (too small) located in the brass fitting that the distribution fingers are attached to. These fingers are not metering devices. The disc or orifice does the metering. If the opening is too small, it will act as a restriction, starving the coil.
If you install a valve and don’t remove the disc, you have the same problem.
Cap Tube SystemsQUESTION:
By Art Hawk
I have a question about a refrigeration system with a capillary tube. I would like to know how to check the charge when the box is not at temperature.
I arrived at a customer’s business and the refrigerator was at 50 to 60 degrees F.
What troubleshooting steps should I use to determine if the charge is correct after checking the condenser and evaporator coils? I know I can remove the charge and weigh in a new charge, but R-12 is very expensive. I want to know if there is another way without weighing in the charge.
Also, I would be interested in the recommended cap tube system suction pressures for a freezer and refrigerator with R-12, R-409, R-404, and R-502, with the box warm and at temperature.
From John Healy
Jack’s Appliance Repair
Colorado Springs, CO
In regards to the first part of your correspondence, the first step is to locate the low or suction side of the cooling unit. If no valve is found, then add a tap — a line valve to the system.
Then check the refrigerant charge and consult a temperature pressure chart corrected for the altitude in which you are servicing the capillary tube system.
A correct refrigerant charge should have a pressure reading of +10 at the low or suction side of the compressor.
A lower reading indicates low refrigerant charge. In this case, add refrigerant until the correct reading is obtained.
A higher reading without proper cooling or with no frost on the suction line means that you have a low-capacity compressor.
In answer to your last question, the proper temperature chart is the place to go to find the correct answer.
Sidebar: Contacting The Service HotlineDo you have a technical question for the pros? Submit your Service Hotline questions directly at The News home page. You may also contact refrigeration editor Peter Powell at 847-622-7260; 847-622-7266 (fax); or firstname.lastname@example.org (e-mail).
Publication date: 07/01/2002