When Bob and Tim arrived, they talked to the building manager and determined that it was too warm in the building. They looked at the thermostat and noticed that it was set at 72°F but the thermometer was reading 78° — not good.

Tim said, “The fan is running, so there is control voltage and the thermostat seems to be calling for cooling.”

Bob said, “Tim, that is a good observation. It is good to notice these things right away so that we don’t have to start looking for control voltage. What do you think we should do next?”

Tim said, “Let’s go to the roof and see what is going on up there.”

They went up through the roof hatch to the roof and walked over to the condensing unit and Bob asked, “What do you see, Tim?”

Tim said, “Nothing is operating up here. Let’s check the breaker.”

Tim opened the breaker cover and said, “The breaker is tripped. I think I will reset it.”

Bob said, “No, let’s not reset the breaker until we do a little looking around. I think we need to get the ohmmeter and see if there may be a ground. The breaker is a very reliable device. It should never trip during its lifetime unless there is a problem.”

Bob then said, “This is a three phase voltage supply. Use the voltage scale and check the voltage phase to phase on all three phases and phase to ground on all three phases (Figure 1). You can turn the breaker to the ‘off’ position to be sure it is off. We don’t want to handle the voltage or use an ohmmeter until we are sure that all power is off of the line side of the system.”

Tim said, “I hear something buzzing.”

Bob explained, “The low voltage power supply is in the air handler and is still supplying power to the compressor contactor. Remember, the indoor fan was running. The compressor contactor is still energized but there is no power to the line side of the contactor because the breaker is off, so the compressor and the outdoor fan are not running. Let’s check for a circuit to ground.”

Tim fastened an ohmmeter lead to a copper line and touched the other lead to line 1 and said, “It is reading 100 ohms.”

Bob said, “That tells us that there is a ground somewhere in the system, but not where. It could be the fan motor, the compressor, or somewhere in the wiring.”

Tim asked, “How do we find out where?”

Bob responded, “By isolating the circuits, one at a time. The compressor is the easiest to find and disconnect. Take off the terminal cover and tag and remove the three terminal wires.”

Tim did that and rechecked the ground that he had detected before and said, “The ground is no longer there. The compressor must be grounded.”

Bob noted, “If you had reset the breaker, and that motor is partially burned, the breaker reset would have put more stress on the system. We may not have a bad burn; we don’t know yet. We will have to check further. We will assume it is a bad burn and recover the refrigerant using filters as a precaution.”

They got set up and recovered the refrigerant and pulled an oil sample to check for acid content (Figure 2).

After the oil test, Tim said, “It is only slightly acid. What do we do with a system like that?”

Bob said, “Since there is only mild acid showing, we will be able to clean this system up using only a liquid line acid removing filter. If a suction line filter were needed, that would mean a lot more work.”

They went to the supply house and picked up a replacement compressor and the fittings to complete the job. They then began removing the old compressor. The system had a slight pressure of dry nitrogen that was used to bring the system out of a vacuum after recovery. When they cut the suction line, Tim said, “You can smell the motor burn in that nitrogen, but it is not bad.”

Bob said, “If it had been a really bad motor burn, that nitrogen would have really had a bad smell. This is a mild burn and much easier to overcome.”

After replacing the compressor and pressure checking the connections, they were ready to weigh in a new charge, using the suggested weights for the evaporator, lines, and the condensing unit (Figure 3).

Bob said, “Turn the refrigerant cylinder upside down so we can let liquid into the system.”

Tim did that and asked, “Why upside down?”

Bob explained, “By turning the cylinder to charge with liquid, we will let refrigerant into the system in the liquid line and the liquid will move towards the condenser and the evaporator. We can’t overcharge the system this way. When we are close to the correct charge, by the scales, we can change over and charge the remainder into the suction line with vapor while the compressor is running. If we tried to charge the complete charge in using vapor, we would have to heat the refrigerant cylinder to keep the pressure from dropping too much.”

Tim said, “You really have the procedures down right. Now what?”

Bob said, “Start the system up and let it run for a few minutes to see what we see. It is quitting time, but I want to circulate some refrigerant through the acid removing filter driers before we leave. After it has run for a few minutes, we will shut it down and fix it where the crankcase heat is still on but it won’t start. We will come back tomorrow and run the system and look for what may have caused this compressor to burn.”

(To be continued next month.)

Publication date: 05/21/2012