One common customer complaint about their central HVAC systems is that the a/c system is running but not providing any cool air to the controlled space. One of the most common reasons for this situation is that the a/c condenser or evaporator coil has frozen over. This term or phrase literally means that ice has formed on the heat exchanger, preventing the transfer of heat since the ice is acting as an insulator. This article provides an actual case study that illustrates an a/c freeze-up event in a residential unit.

In order to capture various HVAC statistics, a number of monitoring systems were installed in residential central HVAC units in the state of Texas beginning in 2016. A typical HVAC monitoring instrument is illustrated in Figure 1. These units measure and record inlet temperature, outlet temperature, and blower fan current with the following attributes.

• The inlet temperature to the HVAC heat exchanger indicates the air temperature of the space that is being cooled when the system has reached steady state.
• The outlet temperature from the HVAC heat exchanger indicates the discharge air temperature that is being supplied to the residence.
• The blower fan current is used to detect when the blower fan is ON or OFF.
• While many experts recommend the measurement of the air humidity (inlet and discharge), we have found that when the controlled space is controlled, the variation in humidity levels is insignificant.

In August 2018 the first complete a/c freeze-up event was captured and is summarized in Figure 2.

• Figure 2 provides both delta T (BLUE - LEFT AXIS) which is the difference between inlet temperature and discharge temperature (units = degrees Fahrenheit).
• Figure 2 also provides the a/c ON percentage for each day where 0 percent = completely off and 100 percent = ON for 24 hours (RED - RIGHT AXIS).

The time history provided in Figure 2 illustrates the a/c freeze-up event in its entirety, and is summarized with the following phases:

• On Aug. 14 there is a statistically significant event that leads to a decrease in delta T across the heat exchanger. The reason for this loss in performance was unknown at the time. It did lead to a delta T decrease that is measurable, but would not be noticed by the end customer.
• The system continues to run in a degraded mode for a period of 10 days until Aug. 24 when the ON percentage increased from 20 to 25 percent to almost 70 percent.
• For the following five days (Aug. 25-29), the a/c system was running at 100 percent continuously, and the system dropped significantly. The delta T across the heat exchanger was effectively zero degrees.
• The amount of electricity consumed by the unit during the a/c freeze-up event (five days) was equivalent to the amount of electricity the unit would use during a normal operating month.

Publication date: 2/18/2019

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