This is the third and final installment in the systematic air conditioning troubleshooting series. Part 1 dealt with low and high condenser entering air temperatures, an inefficient compressor, and noncondensables in the system. Part 2 dealt with liquid and suction line restrictions, overcharges and undercharges, and liquid subcooling in the condenser. Part 3 will cover how to diagnose air-side and refrigerant problems.
DIAGNOSING AIRFLOW PROBLEMS
In air conditioning system diagnostics, a service technician must realize that system problems fall mainly into two categories: air-side problems and refrigerant problems. In addition, air-side problems fall into two major categories: either too much or too little air blowing across the evaporator coil.
Assuming proper setup and commissioning of the air conditioning system and the fact that the system has been running fine in the past, too much air probably will not be a problem, since air handling systems do not suddenly increase their air volume flow (cfm) without some outside intervention. When the fan is not directly driven by the motor, the motor will have a sheave, or adjustable pulley, with a connecting belt between it and the fan to drive it. If the driven sheave on the blower motor became loose, the driven belt would ride lower on the driven sheave, thus decreasing the fans speed, not increasing it.
It is also possible to rule out the fan’s speed being too high, since most air conditioning units call for higher fan speeds in order to move the higher densities of colder air associated with air conditioning. This is why the majority of airflow problems will fall into the low airflow category rather than too much airflow.
However, remember that many times, air conditioning systems are not set up properly in the first place. There can be bad duct design, which can include either oversized or undersized ducts — or simply leaky ducts.
To determine if it is an airflow or a refrigerant problem, first record the air temperatures in and out of the evaporator coil and determine if it is higher or lower than it should be. Airflow that is too low will result in greater temperature differences across the coil, rather than too much airflow. This greater temperature difference is from the air being in contact with the coil longer, thus decreasing its temperature coming out of the coil. By comparing the measured temperature difference to the manufacturer’s required temperature differences, a technician can establish whether there is an airflow problem or a refrigerant flow problem.
MEASURE TEMPERATURE DIFFERENCE
What should the temperature difference across the evaporator coil be? To determine, a technician must obtain the wet bulb temperature (wbt) and dry bulb temperature (dbt) of the air entering the coil. A psychrometer is the only instrument needed for these measurements, and it has both wet and dry bulb thermometers in one package.
Today, psychrometers are often digital; however, a thermistor or thermocouple with a wet piece of cotton wrapped around it can give the wbt accurately enough for air conditioning work.
Once both the wbt and dbt of the entering air are measured, the relative humidity of the entering air can be obtained from a chart or table from the manufacturer (see Figure 1). In Figure 1, for a constant air entering or return air dbt, the temperature difference across the coil increases with decreasing relative humidity. The reason for this larger temperature difference with lower relative humidity is the decreased moisture (latent) load that the air conditioning coil has to condense. If the coil doesn’t have to condense as much moisture out of the air, it can perform more sensible cooling because the coil’s temperature will be lower.
Sensible cooling is what is determined when measuring the temperature difference across a cooling coil with a dry bulb temperature. If the temperature difference is greater than the required temperature difference across the coil (see Figure 1), then it is an airflow problem. The problem would be low airflow, causing the air to stay in contact with the coil much too long, giving a greater temperature difference across the coil.
However, if the temperature difference across the coil is less than the required temperature difference, it would be a refrigerant flow problem instead of an airflow problem. This is because air conditioning systems hardly ever increase in airflow without some kind of human intervention.
Listed below are some causes that may cause a decreased airflow in an air conditioning system:
- Restriction in the duct system;
- Dirty evaporator coil;
- Dirty or missing fan blades;
- Direct drive blower with wrong speed tap on the motor;
- Variable-frequency drive malfunctioning and running the fan motor too slow;
- Dirty air filters;
- Faulty duct design;
- Fan pulleys loose;
- Fan belts slipping; and/or
- Blower motor running slow (burning out).
By understanding the causes behind air-side and refrigerant problems, technicians will be better able to quickly diagnose issues in an air conditioning system.
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