The Professor: How Airflow Woes Can Affect Refrigerant Flow
Low suction and head pressures might just be a dirty filter, not a refrigerant undercharge
One of the most often overlooked maintenance items for both the heating and cooling season is the air filter on a furnace/air conditioner. The air filter can be out of sight and out of mind and can cause a complete loss of cooling when it is plugged with dirt, lint, and dust.
Let’s say a technician receives a call from dispatch about homeowners who are complaining of poor cooling in their residence. The residence is a 2,000-square-foot ranch located in a subdivision in Traverse City, Michigan. The homeowners are an elderly couple and rely on air conditioning for health reasons. It has been an unseasonably hot summer, and temperatures in the house are reaching 83°F. In fact, the homeowners report temperatures inside the house have been rising steadily in the last two weeks. They try to keep the house at 72° throughout the entire summer. They are also complaining of high humidity inside the house as well as reduced airflow coming from the registers.
Upon arrival at the house, the technician finds the air conditioner is a 3-ton (36,000 Btuh), R-22, split-type air conditioning system with the A-coil in the plenum of the furnace (see Image 1). The furnace is a downflow model with the A-coil cabinet located at the bottom of the furnace. Supply air is delivered to ductwork in a crawlspace below the residence. The evaporator has an orifice for a metering device, and the condensing unit is located on the east end of the house.
After introducing himself and his company, the technician converses with the homeowners for about 10 minutes, trying to get as much information and history about the problem as possible. He then goes outside to the condensing unit and installs a high- and low-side pressure gauge on the Schrader valves provided by the manufacturer. This pressure access to the low and high sides of the system provides the service technician with both the evaporating and condensing pressures and temperatures, thanks to his handy pressure/temperature chart.
The technician instantly notices the suction pressure is reading 50 psig (26°). For the outdoor temperature and humidity conditions that day, the normal suction pressure should be about 70 psig (41°). The head pressure also is low at 190 psig for the 90° day. The head pressure should be in the 255-265 psig range. The technician notices the compressor sweating heavily from top to bottom. He then touches the crankcase area to the compressor and finds it is extremely cold. This means the compressor has been suffering from liquid flood back during the day at some point during its run cycle. Flood back is liquid refrigerant entering the crankcase of the compressor during the running cycle. The technician then installs a temperature probe on the suction line about 6 inches from where it enters the compressor. The temperature reads 28°. The technician then subtracts the saturated evaporating temperature of 26° from the compressor inlet temperature 28° and finds out that there is only 2° of compressor superheat, as shown in this equation:
(compressor in temp.) 28°F - (saturated evaporating temp.) 26°F = compressor superheat (20°F)
This reinforces there is a flood back problem during the running cycle. Flood back can ruin a compressor by diluting the compressor’s oil with liquid refrigerant. This has a tendency to ruin the lubricity of the oil and score bearing surfaces in the compressor. Flood back also causes oil foaming. This pressurizes the crankcase and causes the oil to be pumped out the discharge valve and into the system. Discharge valve damage can also occur from the oil-foam/refrigerant-rich mixture.
The technician then checks and verifies the homeowner’s complaints about reduced airflow. He takes a current reading of the fan motor and finds it to be 4.2 A. This is far from the nameplate current of 8 A. This tells the technician that the fan motor is only partially loaded and is not moving the proper amount of air it is designed to move.
The technician then decides to check the air filter located in the return air cabinet before the evaporator or A-coil (see Image 2). This photo shows a downflow furnace with the A-coil and cabinet located at the bottom of the furnace. The service technician notices that the filter is completely filled with dust and lint. However, even with the air filter pulled out and a new one installed, there still is a restricted airflow problem with the fan motor continuing to pull low current.
The technician then decides to have a look at the A-coil itself. He shuts off power to the unit and removes the front cover of the A-coil cabinet. The technician finds the A-coil is completely covered with a blanket of ice and frost. The technician melts the ice with a large-wattage blow drier. After putting the front cover back on the A-coil cabinet, the technician starts the air conditioner. The proper airflow has been established and the suction pressure is normal at 70 psig. The fan motor is now drawing normal current of about 7 A.
The technician explains to the homeowner that a dirty air filter has caused a restricted airflow to the A-coil. This restriction in the airflow has caused a low suction pressure because of a reduced heat load entering the evaporator coil. This caused a slower vaporization rate of refrigerant in the evaporator. The low suction pressure made the refrigerant flowing through the evaporator below freezing (26°). This finally froze the evaporator coil solid with ice. The restricted airflow also unloaded the fan motor, which caused it to draw low current.
Once the evaporator coil froze solid, the refrigerant saw very little heat and humidity load. This caused a low vaporization rate and some of the liquid refrigerant (R-22) trickled down the suction line to the compressor’s crankcase, which caused flood back. This is why there was only 2° of compressor superheat and the crankcase area was cold to the touch. The low heat and humidity load on the evaporator also caused the head pressure to be low. This happened because if there was very low heat being absorbed in the evaporator section, there will hardly be any heat to be rejected into the condenser section of the system. This will keep condensing (head) pressures down.
Many technicians will try to add refrigerant when they experience low suction and head pressures simultaneously. This is not always the answer. It is true, an undercharge of refrigerant will cause low head and suction pressures, but that is not the only thing that will cause both pressures to be low. An undercharge will have low condenser liquid subcooling readings on the high side, where a dirty air filter for the evaporator will not produce low condenser liquid subcooling readings.
In this case, something as simple as a dirty air filter was the culprit in freezing the coil and causing low head and suction pressures. The low airflow was the major clue to the problem, and it wouldn’t have been noticed if the technician had not conversed with the homeowners before troubleshooting, although one hopes the service technician would have eventually taken a subcooling reading if the low airflow problem was not noticed.
Publication date: 5/9/2016