The air conditioning season seems to always be in full swing somewhere in the U.S. All air conditioning systems should be given a preventive maintenance tune-up before starting for the season. One important but often overlooked piece of equipment in the air conditioning system is the air filter, which protects the evaporator from dirt and lint. This article will explore how a dirty air filter can restrict airflow over the evaporator and cause very serious and expensive system problems.


Poor Cooling Complaint

Homeowners are complaining of poor cooling in their residence. They also complain of reduced airflow coming from the registers, as well as high humidity in the house. The air conditioner is a 3-ton (36,000 Btuh), R-410A, split-type, air conditioning system with the A-coil in the plenum of the furnace located in the basement (see Figure 1). The condensing unit is located on the east end of the house.

The residence is a 2,500-square-foot ranch located in a subdivision in Traverse City, Michigan, and the homeowners are an elderly couple who rely on air conditioning for health reasons. It has been an unseasonably hot summer, and temperatures in the house are reaching 80°F. According to the homeowners, temperatures inside the house have been rising steadily in the last two weeks, even though they try to keep the house at 72°F throughout the entire summer.

A service technician arrives at the location. After introducing himself and his company, the technician converses with the two homeowners in order to get as much information and history about the air conditioning problem as possible. The technician then goes outside to the condensing unit and installs both high- and low-side gauges. He instantly notices that the suction pressure is reading 90 psig (26°F). The normal suction pressure should be about 121 psig (41°F) for the outdoor temperature and humidity conditions that day. The head pressure is also low at 340 psig for the 90°F day. The head pressure should be in the 390 to 420 psig range.

The technician also notices the compressor sweating heavily from top to bottom. He then touches the crankcase area, or bottom of the compressor, and finds that it is extremely cold. This means that the compressor is suffering from refrigerant floodback, which is when liquid refrigerant enters 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, and it reads 27°F. He then subtracts the saturated evaporating temperature of 26°F from the compressor inlet temperature 27°F and finds out that there is only 1°F of compressor superheat:

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

This reinforces that there is a refrigerant floodback problem. Refrigerant floodback 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. Floodback also causes oil foaming, which can cause 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.


Airflow Problem

The technician then checks the airflow and agrees with the homeowners that there is a reduced airflow problem. He takes an amperage reading of the fan motor and finds it to be 2.5 amps. This is far from the nameplate amperage of 8 amps. 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. He notices that it is completely filled with dust and lint. However, even with the air filter pulled, there still is 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 and removes the plenum. He finds that the A-coil is almost completely covered with a blanket of ice and frost.

The technician then melts the iced coil with a large wattage blow drier, and after putting the plenum back on the unit and installing a new air filter, the technician starts the system. The proper airflow has been established, and the suction pressure is normal at 121 psig (41°F). The fan motor is now drawing normal current of about 7 amps.

The technician then explains to the homeowners that a dirty air filter has caused restricted airflow over the A-coil evaporator and talks about 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, which caused a slower vaporization rate of refrigerant in the evaporator. The low suction pressure (90 psig) made the refrigerant flowing through the evaporator below freezing (26°F), and this finally froze the evaporator coil solid with ice.

The restricted airflow also unloaded the fan motor, causing it to draw low amperage. 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-410A) trickled down the suction line to the compressor’s crankcase, causing floodback. This is why there was only 1°F 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 low head pressures simultaneously, but this is not always the answer. It is true that an undercharge of refrigerant will cause low head and low suction pressures, but that is not the only thing that will cause both pressures to be low. 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. 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 homeowner before troubleshooting.