September, 2010
Inspection News and Views from the American Society of Home Inspectors

The Word: Temperature Drop


Once again, The Word invites you to travel into the dark realm of terms that often are misused or misunderstood in home inspection reports. The Word hopes you will find this trip informative and maybe a little entertaining.

The Word's term today is temperature drop, as in the temperature drop across the evaporator coil of an air conditioner. The Word finds this term interesting because its implications often are poorly explained to clients.

Should you measure temperature drop?

In The Word's opinion, temperature drop measurement is not required by ASHI Standards of Practice (SOP). Section 9.1.B.1 requires us to operate installed air conditioning equipment; however, Section 13.1.B.1 tells us that our inspections are not technically exhaustive and that measurements are not required.

Your local standard of care may, however, make temperature drop measurement wise. If most other inspectors in your area measure temperature drop, then you probably should, too. You might be held to your local standard of care even if that standard exceeds ASHI SOP. The Word believes that if you usually measure temperature drop and if you are unable to obtain a measurement during an inspection, then you are wise to report that you were not able to obtain a temperature drop measurement.

How should you measure it?

Begin by running the air conditioning for at least 15 minutes before measuring the temperature drop. It takes at least this long for the system to stabilize.

The best way to get an accurate temperature drop measurement is to insert a probe-type thermometer as close as possible to each side of the evaporator coil. The ideal place is in the supply and return plenums. A visual inspection of the plenums often will yield pre-existing holes that you may use. The Word does not like the idea of drilling a hole into a plenum, but you may decide to do so.

The next best measurement place is in a duct as close as possible to each plenum. The Word believes that the hole in flexible duct left by your thermometer will have no significant impact on the duct's integrity.

Measuring temperature drop at supply and return registers can introduce significant measurement error, particularly when using an infrared thermometer. Given the limitations of temperature drop as a diagnostic tool, The Word believes that the measurement error is too great to make measuring at registers useful.

Effects of temperature and humidity

Memo to technical purists: The following is a short discussion of a complex topic that can require entire chapters in books. It is not intended to be complete.

Air temperature and humidity have a significant effect on temperature drop. Increased air temperature and humidity will decrease the temperature drop that you measure, as the following discussion explains.

Almost all air contains some water vapor (water in a gaseous state). The common term for water vapor in air is humidity. Warm air can hold more water vapor than cold air. When an air conditioner removes heat from the air, some of the air conditioner's energy is used to condense the water in the air from a gaseous state to a liquid state. The condensation process uses energy that the air conditioner might otherwise use to remove additional heat from the air.

When you measure temperature drop across the evaporator coil, you are measuring the heat energy removed from the air. This is called the sensible load. You are not measuring the energy required to condense water vapor from the air. This is called the latent load. The energy required to process the latent load will reduce the temperature drop that you measure. Thus, in a humid climate like Florida, you expect a lower "normal" temperature drop compared to a dry climate like Arizona. This is because an air conditioner in Florida uses more energy in water vapor condensation (latent load) and has less energy available to remove sensible heat from the air.

Other factors affecting temperature drop

Many factors can affect measured temperature drop. These factors can interact to increase the temperature drop, decrease it, or even cancel each other out to produce what appears to be a "normal" temperature drop in a system that is actually malfunctioning. A low coolant charge usually decreases the temperature drop. Anything that reduces air flow across the evaporator coil can increase the temperature drop. Common culprits include a dirty evaporator coil, a dirty filter, and an undersized or constricted return duct. Improperly sized, constricted and/or leaky supply ducts can affect the temperature drop in either direction. An improperly set fan speed can affect the temperature drop in either direction.

An air conditioner presenting a "normal" temperature drop can be malfunctioning. For example, the run capacitor can be bad and the system may keep working for a while and at a higher current draw. Eventually, a bad run capacitor may damage the compressor.

What is a "normal" temperature drop?

A "normal" temperature drop is difficult to determine because so many factors can affect it. Home inspector lore holds that dry-climate temperature drops in the 18° to 24° F range and wet climate temperature drops in the 14° to 20° F are "normal."

The bottom line

Temperature drop, as we usually measure it, is, at best, an imprecise tool. It is not always an accurate indication of an air conditioning system's condition. An "abnormal" temperature drop does not always indicate a malfunctioning air conditioning system and a "normal" temperature drop does not always indicate a properly functioning system.

The Word believes that you should report "abnormal" temperature drops as an indication that further evaluation of the air conditioner by a qualified HVAC contractor may be wise, not as an indication of system condition. Reporting in this manner alerts your client to a possible problem, but does not state an unsupportable conclusion about the system's condition.

Memo to the plumbing Gods and other authorities: The Word does not reside on Mt. Olympus (just at its base) and welcomes other viewpoints. Send your lightning bolts or e-mails to The thoughts contained herein are those of The Word. They are not ASHI standards or policies.

The Word thanks Eric Lander and Eric Weiss from Trane for their help with this column.