In system diagnostics, a service technician must realize that A/C system problems fall mainly into two major categories: air-side problems and refrigerant cycle problems. This article will cover problems on the air side of the system. Next month's article will cover refrigerant cycle problems.

Air Systems

Air system problems also fall into two major categories:

1. Too much air.

2. Too little air.

If we assume that the entire unit was originally set up and operated properly, too much air probably will not be a problem; air-handling systems do not just suddenly increase their air volume flow (cfm) without some outside intervention or electronics failure. If, for example, the driven sheave on the blower motor became loose, the drive belt would ride lower on the sheave, thus decreasing the fan's speed, not increasing it.

If the fan drive motor's speed is controlled by an electronic variable-frequency drive (VFD), faulty electronics or sensing may speed up the motor. However, usually a digital readout of the motor's speed on the VFD or computer screen will be recognized in a short time. Many times, parameters can be set on VFD controllers to recognize when speed problems or set parameters have been exceeded.

We can also rule out the fan's speed being too high, since most A/C units call for higher fan speeds to operate properly in order to move the higher densities of colder air associated with air conditioning. This is why the majority of airflow problems will be not enough airflow rather than too much.

However, one must remember that sometimes air conditioning systems are not set up properly in the first place. There can be bad duct design, which encompass either oversized or undersized ducts, or simply leaky ducts.

Airflow Determinations

To determine if you are dealing with an airflow or refrigerant flow 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 by checking against a psychrometric chart.

These temperatures in and out of the evaporator coil should be measured with a dry-bulb thermometer. For example, if the return air entering the coil is 80 degrees F and the air leaving the coil is 60 degrees, the temperature difference or temperature drop would be 20 degrees.

Too low of an airflow will give you greater temperature differences across the coil than too much airflow. This greater temperature difference is caused 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 a manufacturer's required temperature differences, a technician can establish whether there is an airflow problem or a refrigerant flow problem.

But, what should the temperature difference or temperature drop across the evaporator coil be?

Temperature Differences

To determine the required temperature difference across the coil, a technician must obtain the wet-bulb temperature (WBT) and dry-bulb temperature (DBT) of the entering air to the coil.

A psychrometer is the only instrument needed for these measurements. In fact, 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 using the wet-bulb depression (Table 1). The wet-bulb depression is simply the dry-bulb temperature minus the wet-bulb temperature. The example shows a wet-bulb depression of 10 degrees (80 degrees – 70 degrees).

Table 1. Psychrometric table that gives the percent of relative humidity from the dry-bulb temperature and wet-bulb depression (the difference between the dry-bulb and the wet-bulb temperature). In the example, the DBT is 80 degrees and the WBT is 70 degrees.

Now, line up the wet-bulb depression with the dry-bulb temperature and the chart gives 61 percent relative humidity (rh). So, now we have air entering the evaporator coil (return air) with an 80 degrees DBT and 60 percent rh. The actual temperature drop or temperature difference across the coil is 20 degrees (80 degrees – 60 degrees).

For an 80 degree return air temperature and 60 percent rh, the required temperature drop across the coil is 17.5 degrees. (See Figure 1.) Remember, the actual temperature difference across the coil was 20 degrees.

The 17.5 degrees is pretty close to the required temperature difference, considering the accuracy of the thermometers or thermistors used in the measuring of the temperatures. However, it could mean there is not enough airflow.

Figure 1. Temperature difference or drop across A/C coils as a function of return air dry-bulb temperature and relative humidity.
For a constant air entering DBT, the temperature difference across the coil increases with decreasing rh. The reason for larger temperature differences with lower rh is the decreased moisture (latent) load that the A/C coil has to condense when experiencing a lower rh. If the coil doesn't have to condense as much moisture out of the air, it can achieve more sensible cooling - the coil's temperature will be lower without the large latent moisture load.

Sensible cooling is exactly what we are measuring when we measure the temperature difference across a cooling coil with a DBT.

Now, if the actual temperature difference is greater than the required temperature difference across the coil, then we are dealing with an airflow problem. The problem would be too low of an 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, we would be dealing with a refrigerant flow problem instead of an airflow problem. This is because A/C systems hardly ever increase in airflow without some kind of human intervention or electronic malfunction.

Circumstances that may result in decreased airflow in an A/C system include dirty air filters, faulty duct design, loose fan pulleys, slipping fan belts, dirty evaporator coil, and dirty or missing fan blades.

John Tomczyk is a professor of HVACR at Ferris State University, Big Rapids, Mich., and the author of Troubleshooting and Servicing Modern Air Conditioning & Refrigeration Systems, published by ESCO Press. To order, call 800-726-9696. Tomczyk can be reached by e-mail at

Publication date: 12/01/2003