The homeowners of a medium-sized house complain of reduced airflow coming from their registers during the summer. Their air conditioner is a 4-ton (48,000 Btuh), R-22 split system with the A-coil in the plenum of the furnace located in the basement. The evaporator has an orifice for a metering device, and the condensing unit is located on the east end of the house.

It has been an unseasonably hot summer, and even though the homeowners try to keep their house at a constant 72°F this time of year, indoor temperatures are reaching 80°. In fact, the homeowners say that temperatures and humidity have been rising steadily in the house over the last two weeks.

A service technician arrives, introduces himself and his company, and then talks with the two homeowners, trying to get as much information and history about the insufficient cooling problem as possible. He then goes outside to the condensing unit and installs high- and low-side pressure gauges. He instantly notices that the suction pressure is 50 psig (26°), even though it should be about 70 psig (41°), given the outdoor temperature and humidity conditions that day.

The head pressure should be in the 255 to 265 psig range, but the technician finds it to be low for the 90° day, at 190 psig. He also notices the compressor is sweating heavily from top to bottom. He touches the crankcase area — the bottom to the compressor — and finds that it is extremely cold. This probably means that the compressor has been suffering from liquid floodback at some point during its run cycle. Floodback is when liquid refrigerant enters the crankcase of the compressor during the running cycle.

The technician installs a temperature probe on the suction line about 6 inches from where it enters the compressor, and the temperature reads 27°. He then subtracts the saturated evaporating temperature of 26° from the compressor inlet temperature 27° and finds that there is only 1° of compressor superheat:

  27°F (compressor temperature)
- 26°F (saturated evaporating temperature)
=  1°F compressor superheat

The low compressor superheat reading usually signals floodback of liquid refrigerant to the compressor crankcase. Floodback will cause oil foaming, which can cause oil to be pumped out of the discharge valve and into the system. Discharge valve damage can also occur from the oil foam/refrigerant mixture. Again, low compressor superheat reinforces that there is a floodback problem during the running cycle. Floodback can ruin a compressor by diluting its oil with liquid refrigerant. This has a tendency to ruin the lubricity of the oil and score bearing surfaces in the compressor.



The technician measures the airflow at the registers and agrees with the homeowners that there is a reduced airflow problem. He then takes an amperage reading of the fan motor and finds it to be 4 A, which 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.

The technician then decides to check the air filter that’s located in the return air cabinet before the evaporator or A-coil. He notices that it is completely filled with dust and lint; however, even with the air filter pulled, there is still a restricted airflow problem, and the fan motor continues to pull low current.

He then decides to have a look at the A-coil itself. He shuts off power to the unit, removes the plenum, and finds that the A-coil is completely covered with a blanket of ice and frost. He melts the iced coil with a high-wattage blow drier, and after putting the plenum back on the unit and installing a new air filter, he starts the air conditioner.

With the proper airflow established, the suction pressure is normal at 70 psig, and the fan motor is now drawing normal current of about 7 A. The technician tells the homeowner that a dirty air filter caused restricted airflow to the A-coil, and he explains the importance of keeping the air filter clean.

This restriction in the airflow caused a low suction pressure because of a reduced heat load entering the evaporator coil. This resulted in a slower vaporization rate of refrigerant in the evaporator. The low suction pressure caused the refrigerant flowing through the evaporator to be below freezing (26°), which finally froze the evaporator coil solid.

The restricted airflow also unloaded the fan motor, causing 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 trickled down the suction line to the compressor’s crankcase, causing floodback. This is why there was only 1° of compressor superheat when measured and the crankcase area was cold to the touch.

The low sensible 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 would hardly be any heat rejected into the condenser section of the system. This kept condensing (head) pressures down.

When technicians are faced with a system that simultaneously has low suction and head pressures, they often add refrigerant. This is not the correct thing to do! While an undercharge of refrigerant will cause low head and low suction pressures, it is not the only issue that will cause both pressures to be low. An undercharge of refrigerant will also result in low subcooling readings on the high side, while a dirty air filter for the evaporator will not produce low subcooling readings.

Something as simple as a dirty air filter was responsible for freezing the coil and causing low head and suction pressures in this scenario. In this case, the low airflow was the major clue to the problem, and it wouldn’t have been noticed if the technician did not converse with the homeowners before troubleshooting. Hopefully, the service technician would have eventually taken a subcooling reading if the low airflow problem was not noticed.

Publication date: 3/4/2019

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