Btu Buddy 166: Burned Heat Pump Compressor
Bob and Tim are on a service call on a really cold and blustery day. The temperature is 27°F, the wind is blowing, and it’s snowing and sleeting. When the customer called, she said that their home is getting cool inside, and their heat pump is running all the time with the temperature dropping inside the house. They arrived at the front door, talked to the homeowner, and decided to look around. They went to the closet where the indoor unit for the heat pump was located and determined that the heat pump itself was not running, only the fan and strip heat. The unit had three 20 KW strip heat units.
“Check the amperage on all three strip heat units and see if they are all heating,” Bob said. Tim used his ammeter to check all three strip heat units and found that only two of them were drawing current. “Let’s see why this strip heat unit is not heating,” said Bob.
They discovered that one of the units needed to be reset, so they pushed the reset button, and that unit started heating. Bob said, “That’s a sign that there may not be enough air flow across the system — check the air filter”.
Tim said, “The filter was stopped up.”
They changed the filter and then went to the outdoor unit to see if they could figure out what was going on out there. When they got outside, they discovered the outdoor unit wasn’t running. Tim looked at the breaker, and the breaker was tripped. He then asked Bob if he should reset the breaker and try to start the unit.
Bob said, “I am not a fan of resetting breakers without doing some prior checking. Leave the breaker off, and check for voltage on the line side of the contactor to make sure that the breaker is thoroughly tripped.”
Tim checked the voltage on the line side of the contactor and there was no voltage from phase-to-phase or phase-to-ground. He then switched the meter to the Ohm scale and checked the load side of the breaker. He found a short from both phase-to-phase and phase-to-ground.
“We need to figure out what is shorted. It should either be the compressor or the fan motor, so disconnect the fan motor leads first, and check it,” Bob said
Tim checked the fan motor, and it seemed to be good phase-to-phase and phase-to-ground. He then checked the compressor by removing the compressor leads and checking them phase-to-phase and found a short. He also checked phase-to-ground and found a short. “Here is our problem,” Tim said.
They went in and talked to the homeowner and explained that the compressor was burned and needed to be replaced. The unit was about six years old, so they explained to the homeowner that changing the compressor was probably the best option. Bob then went on to say, “The weather is too bad outside to change the compressor under these conditions. It needs to be dry and warmer to do this kind of work outside. There seems to be a warm front coming later on in the week, and I would suggest that we do the work then. We want to do a little further checking on the unit to see how bad the burn is, and we will come back Thursday with the parts”.
They went out to the unit and Bob explained to Tim, “We need to leave this unit with the crankcase heater on to keep the compressor warm and keep the refrigerant out of the oil. If we start trying to recover refrigerant out of this cold unit, most of refrigerant charge will be in the form of liquid in the crankcase. If we let the crankcase heater do its work overnight, we will come back and most of refrigerant will be in the liquid and vapor state out in the system and easier to recover.”
Tim disconnected the wire to the coil side on the contactor so the contactor wouldn’t close and start the compressor or fan. Then they closed the disconnect to the unit while they were gone.
Bob said, “Let’s let a small amount of refrigerant out of the charging port and see what it smells like. Maybe we can get an idea of whether the compressor is burned bad or grounded.” They removed the cover to the valve port, touched the valve stem with a little screwdriver, and smelled the vapor.
Both of them agreed that the vapor smelled really bad. “It smells really burnt. We need to prepare for a bad motor burn — what should we do?” Tim asked.
“We must be prepared to filter all of the refrigerant that goes into the compressor or goes through the metering devices. There are three places in the system where trash can cause trouble: the compressor, the four-way valve, and the metering devices. We must protect them as best we can with filtration. This means we will have to install a suction line filter in the permanent suction line between the four-way valve and the compressor (Figure 1) and a two-way filter dryer (Figure 2) in the liquid line. When we cut the old compressor out, we will get a picture of what the suction line and discharge line in the compressor look like. They may have a lot of motor burn in either the suction line or the discharge line. If there is a lot of burn material in the suction line, it’s typically a sign that the motor burned while starting up. If there is a lot of burn material in the discharge line, it’s a sign that the motor burned while the compressor was running, pushing carbon and motor burn out into the discharge line. We will have to determine that at the time we remove the old compressor,” Bob said.
They went in and explained everything to the owner and said that they would be back Thursday with the parts.
They got all their parts together and were on the job Thursday morning. The outdoor temperature was about 40°, and it was dry. They went to work and recovered the refrigerant. While the refrigerant was recovering, they removed enough panels to access the compressor and compartment. After the recovery, they pressured the unit back up to atmospheric pressure with dry nitrogen and cut the compressor loose as close as they could to the compressor fitting. They set the compressor out and took a cloth and examined the inside of the suction line and discharge line. When wiping the cloth up inside the line, they discovered the discharge line had a lot of soot and carbon in it. The suction line was clean.
“It appears that the compressor burned while the unit was running, and I don’t believe we need to go through the piping exercise of installing a suction line filter. The discharge line will push any carbon particles into the indoor coil because it was running in heating. When we install a liquid line filter dryer, it will catch the particles coming towards the outdoor metering device and clean the refrigerant up before it reaches the device,” Bob said.
They used dry nitrogen to purge through the lines and blowout anything they could while the pipes were disconnected. Then they set the new compressor and connected it to the piping and installed a liquid line acid removing filter dryer. They were ready to pressure[AJA1] the unit up and pressure check their connections. They performed a system pressure check with a small amount of R-22 and nitrogen. When the system was deemed leak-free, they connected their vacuum pump and started running it. While the vacuum pump was evacuating the system, they checked the contactor and found that the contacts were in very bad shape, so they replaced the contactor. They also replaced the run capacitor and the start capacitor for good measure.
They had a good deep vacuum on the system and were ready to start up. They valved the vacuum pump off and allowed liquid refrigerant into the liquid line port until the liquid stopped flowing from the tank. They were ready to operate the unit, and Tim got the meter to check voltage and amperage.
They started the unit, but it was still a little bit low on charge — the charge called for 4 lbs. 3 oz. They had the refrigerant tank on their electronic scale and let in the rest of the refrigerant charge into the suction charging port. The unit ran and was heating normally, with the gauge readings about like they should be. They put all panels back on, using all available screws to keep the panels tight, and went inside to explain to the homeowner what they had done and that the unit was ready to go.
Tim said, “This took several hours and several hundred dollars’ worth of parts but was far more cost-effective than changing the system. This system is new enough to be considered efficient by today’s standards. I think we did a good job for the customer, the unit, and the system. It should last for many years of good operation.”
“You are right, that was a job well done. We can be proud of doing good work — it should last for many years and give good service,” Bob said.
Publication date: 1/23/2017