Bob is a service technician who is well trained and nationally certified. However, he sometimes suffers from the same confusion that all technicians occasionally do - the facts that he gathers may or may not point to the obvious cause of the problem or the best solution. But Bob has something that no one else has. He recalls his long-time HVACR mentor and imagines him accompanying him as "Btu Buddy," someone who reminds him to take time to stop and think before rushing to judgment, helping keep him on the right track, even with facts that are confusing.

The first cool weather of fall has prompted a service call to a commercial building that has a 100-ton split system with a single compressor. There are three evaporators, one on each floor and an air-cooled condenser in the parking lot behind the building. The building maintenance man called and reported that the oil level was low in the compressor. It was way below the sight glass where it had normally been so he shut the system off.

Bob arrives about 10:00 am and drives around back where the equipment room door is standing open with the maintenance man waiting for him. It is about 65 degrees F outside, but the maintenance man says the inside of the building is getting warm and complaints are coming in. He explains, "The oil level is real low in the sight glass, but the compressor was still running and cooling. It had been on all night, only shutting off when the thermostat was satisfied. It was running when I came in this morning and I noticed the oil level so I shut it off. I have a plumbing leak that I have to tend to now. I will be back in about an hour."

Bob uses a small penlight and looks in the oil sight glass, and he sees that the oil level is really low. He stands there for a minute thinking and starts for his truck when Btu Buddy appears.

Btu Buddy asks Bob, "What is your plan?"

"I am going to get gauges to put on the compressor, oil to add to the oil sump, and a pump to pump it into the sump," Bob explains.

Btu Buddy then says, "I believe that the low oil level is a normal function of this system this time of year. Let me explain. This compressor has cylinder unloaders that allow it to run at reduced load. Do you remember those terms from school?"

Bob says, "Yes, I remember them, but I can't say that I fully understand them. It seems that my schooling made me familiar with a lot of procedures, but I don't understand them until I have used them in the field. Would you mind reviewing this term cylinder unloading?"

Cylinder Unloading

Btu Buddy explains, "This is a 100-ton system. The only times that it runs at full load are in the middle of the summer on the very hottest days or when the system has been off long enough for the building to get warm and then probably for only short periods of time. This compressor has eight cylinders and is rated at 100 tons, that is 12.5 tons per cylinder. Cylinder unloading allows a certain number of cylinders to unload, or stop pumping refrigerant from the low-pressure side to the high-pressure side when the load is reduced on the system.

"It works like this: Each evaporator will have multiple coils that are solenoid valve controlled. For example, the building has three floors and three coils. The fan and evaporator coils for the first floor are right over there. Notice that the coil has two expansion valves and each has a solenoid valve before the expansion valve. This system probably has a return air thermostat that controls the solenoids. As the return air thermostat starts satisfying, it will shut off the top solenoid and stop refrigerant flow. This reduces the capacity to one-half capacity for that coil. What do you think that would do to the suction pressure at the compressor?"

Bob says, "It would go down because of reduced evaporator space."

Btu Buddy says, "That is correct. The compressor has a pressure control inside it that will stop part of the cylinders from pumping when the suction pressure drops, reducing the capacity and the power consumption. Everybody wins on that one. That is a simple process that works for all three floors of this building. A very important thing to remember is that a 100-ton (approximately 100 horsepower) compressor is not stopping and starting, it continues to run. Starting a compressor is harder on the bearing surfaces than keeping it running with the correct oil pressure. We will get to that in a minute. The point to remember is this compressor has capacity control. This compressor actually has three stages of unloading so it can be a 100-ton, 75-ton, 50-ton, or 25-ton compressor with the power reduced to the respective tonnage level it is operating at. Now, for the important statement about this job: This compressor operated at its lowest capacity for several hours last night and this morning before the maintenance man discovered the low oil level."

Bob asks, "What does that have to do with low oil level?"

Btu Buddy then asks Bob, "Do you see any oil on the floor around the compressor?"

Bob says, "No, but I haven't looked at all of the piping for oil."

Btu Buddy says, "Why don't you follow the piping and see if you find any oil? Meanwhile, I am going to sit here and wait for you."

Bob returns in a few minutes and Btu Buddy says, "You didn't find any oil, did you?"

Bob says, "No. Now I am really confused. There are probably at least two quarts of oil somewhere. What do we do next?"

Btu Buddy says, "If it is not out of the system, the only assumption must be that it is still in the system."

Bob asks, "Well, how do we find it? I think it would be risky to start the system up with really low oil."

Btu Buddy then asks, "Does the oil always stay in the compressor?"

Bob says, "I thought so."

Btu Buddy then explains, "All reciprocating compressors pump a certain amount of oil as a normal function of lubricating. The cylinders are lubricated with oil and when the piston rises in the cylinder, some of the oil is wiped off and pushed into the discharge line. Once oil leaves the compressor in this system, the only way back to the compressor sump is to take the whole route through the discharge line, the condenser, the expansion valve, the evaporator, and back down the suction line to the compressor where it migrates to the sump. That is a long route. What do you think causes it to move through the system?"

Bob says, "When I studied pipe sizing, it was explained that refrigerant velocity moved the oil."

Figure 1. This is an oil safety control. Notice that it has two connections. One will be fastened to the suction side of the system and the other to the oil pump discharge pressure connection. (All figures are from Refrigeration & Air Conditioning Technology, by William Whitman, William Johnson, and John Tomczyk, published by Delmar Publishers.)
"Exactly," says Btu Buddy. "Now what happens to refrigerant velocity when it is pumping less refrigerant through the same lines?"

Bob says, "The velocity would be less, so the oil return would be slower."

"Now you have it," says Btu Buddy. "Let's take the precaution of installing gauges on the system before we start it up. Get two sets of gauges from your truck and we will monitor the oil pressure and the high- and low-side pressures when we start it up. Bring an ammeter so we can tell what capacity we are operating at."

Bob installs the high- and low-side pressure gauges and turns to Btu Buddy and asks, "How do we install the oil pressure gauge?"

Btu Buddy shows Bob the oil test port for the compressor and says, "Fasten the high-side gauge from the extra set of gauges here."

Btu Buddy then explains, "This system has a very reliable oil pressure shut off control that monitors the oil pressure and will only allow the compressor to run for about 90 seconds in case of low oil pressure (Figure 1). Notice that it is manual reset and the maintenance man didn't mention resetting any controls. The actual oil pump picks up the oil in the very bottom of the sump, so the oil level can be low and there will still be oil pressure for full lubrication (Figure 2). We know this compressor has oil in the bottom of the sump because we can see it. Let's start the compressor and monitor the oil pressure. Do you understand the term net oil pressure?"

Bob says, "Again, I know the term but really do not understand how to apply it."

Figure 2. This oil sump diagram shows that the oil pickup point for the oil sump is low in the compressor sump. Also note the net oil pressure calculation. It is also different from the one in our example problem. Compressors have different net oil pressures.

Net Oil Pressure

Btu Buddy explains, "Oil pressure is a floating pressure that depends on the suction pressure. The oil pump inlet pressure is always the same as the suction pressure, which can vary from the off cycle to the run cycle. For an R-22 system like this, the system off pressure may be 140 psig and when the system is running, the suction pressure would be about 70 psig (see Figure 1). That is what I mean by floating, so we will use the suction pressure as the oil pump inlet pressure and the oil pump discharge gauge reading as the oil pressure. We will then subtract the suction pressure from the oil pump discharge pressure for the ‘net oil pressure.' The net oil pressure may run from 20 psig to 60 psig by different manufacturers. With this system we can expect the net oil pressure to be about 40 psig. With the suction pressure while running expected to be about 70 psig, the oil pump discharge pressure will be about 110 psig (70 plus 40 equals 110). Now start the compressor."

Bob starts the compressor with the system switch. The compressor starts up and sounds good. The oil pressure discharge gauge is reading 115 psig with the suction pressure reading 75 psig, so the net oil pressure is 40 psig.

Btu Buddy says, "There is plenty of oil pressure, so the compressor is safe from that point of view. Now, check the amperage."

Bob reads the amperage and it is 110 amps.

Btu Buddy says, "The name plate full load amperage is 115 amps and the meter reads 110 amps. The compressor is running at full load. Look at the oil in the sight glass."

Bob looks and says, "It is about the same."

Btu Buddy says, "Let it run for a few minutes while the system is pulling the space temperature down. Meanwhile, let's go back to a couple of statements that were made a while ago. I said that the oil must return through the system. There is such a thing as an oil separator that may be installed in the discharge line that will prevent most of the oil from leaving the compressor by separating most of the oil and returning it to the compressor crankcase. The other statement was that most wear occurs at startup of a compressor. Just like an automobile engine, it starts up with no oil pressure and the bearings are almost dry at startup. The only oil in the bearings is what did not drip out during the off cycle. Let's meet for lunch tomorrow for a review of what these are all about."

After about 15 minutes, the compressor makes a tone change and Bob asks, "What was that?"

Btu Buddy explains, "The compressor just unloaded two cylinders. It is now running at 75 tons. What does the ammeter read?"

Bob looks and says, "The amperage has reduced to 85 amps."

Btu Buddy then says, "Look at the oil in the sight glass."

Bob looks and says, "It is half full. It's a good thing that I didn't add oil, or there would be too much oil now."

Btu Buddy says, "You would be surprised at how many technicians would add oil and then remove oil when they only needed to load the system up to return the oil that is laying out in the system. You should explain to the maintenance man what you did and what he should do in case it happens again."

The maintenance man returns in a few minutes and Bob explains that nothing abnormal had happened, that the system had been running under reduced load for many hours and that the oil would have returned to its normal level in the sight glass when the load picked up again. He also explains that if it happens again, he could shut the system down until a load built up and restart the system and the oil should return. Bob also explains that he had put gauges on and verified that the compressor had plenty of oil pressure and lubrication, and all was well.

The maintenance man says, "I thank you for explaining the situation to me. That is very interesting."

Bob then says, "You can take classes at the local technical school that will give you a great education on these kinds of systems. Your employer would probably pay for any completed classes. It is a win-win."

Btu Buddy notes as they are driving away, "That was a nice added benefit that you explained to the maintenance man. Many people like him could raise their stock in life with more specialized education. Our profession needs more interested people. It is a wide open field."

Bill Johnson has been active in the HVACR industry since the 1950s. He graduated in gas fuel technology and refrigeration from the Southern Technical Institute, a branch of Georgia Tech (now known as Southern Polytechnic Institute). He taught HVAC classes at Coosa Valley Vocational & Technical Institute for four years. He moved on to become service manager for Layne Trane, Charlotte, N.C. He taught for 15 years at Central Piedmont Community College, part of this time as program director. He had his own business for five years doing installation and service work. Now retired, he is the author of Practical Heating Technology and Practical Cooling Technology, and continues as a co-author of Refrigeration & Air Conditioning Technology, 5th Edition, all published by Delmar Publishers. For more information, he can be reached at 704-553-0087, 704-643-3928 (fax), or

Publication date: 10/18/2004