HVAC Technician Academy
Learn how dry-bulb temperature, wet-bulb temperature, relative humidity, dew point, and enthalpy describe the real condition of air moving through an HVAC system.
Temperature alone does not describe comfort or cooling performance. Psychrometrics helps technicians understand both sensible heat and moisture in the air so they can evaluate dehumidification, coil performance, total capacity, comfort complaints, and whether the system is actually changing the air condition as expected.
A thermometer can tell you how warm the air is.
It cannot tell you how much moisture that air contains.
Two rooms can both be 75°F and feel completely different if one is dry and the other is humid.
Dry-bulb temperature is the ordinary air temperature measured by a standard thermometer that is shielded from direct radiant heat.
It is the temperature homeowners normally see on a thermostat.
Dry bulb is important for:
Wet-bulb temperature reflects the cooling effect of evaporation.
It gives information about both air temperature and moisture content.
In traditional measurement, air passes across a wetted sensing surface.
As moisture evaporates, the sensor cools.
The difference between dry bulb and wet bulb depends strongly on the moisture condition of the air.
| Measurement | What It Represents |
|---|---|
| Dry Bulb | Actual air temperature |
| Wet Bulb | Temperature influenced by evaporation and moisture content |
Relative humidity describes how much moisture is in the air relative to how much moisture the air could hold at that temperature.
This is why relative humidity changes when air temperature changes even if the actual moisture content stays the same.
Dew point is the temperature at which the air becomes saturated and moisture begins to condense.
Dew point is extremely useful because it directly relates to moisture content.
If a surface temperature falls below the air’s dew point, condensation can form on that surface.
Suppose attic air has a high dew point.
If the exterior surface of a supply duct becomes colder than that dew point, moisture can condense on the duct insulation or jacket.
This is why duct sweating is not just a "cold duct" problem.
It is a relationship between surface temperature and surrounding-air dew point.
Enthalpy represents the total heat content of moist air.
It accounts for both:
In HVAC work, enthalpy can help describe how much total energy is removed from the air as it passes across a cooling coil.
Sensible heat changes dry-bulb temperature.
If return air enters at 78°F and supply air leaves at 58°F, the system has produced a sensible temperature change.
But that number alone does not tell you how much moisture the coil removed.
Latent cooling removes moisture from the air.
When water vapor condenses on the evaporator coil, the system is removing latent heat.
That is why a cooling system can be doing significant work even when the dry-bulb temperature split does not tell the entire story.
Sensible heat ratio, or SHR, describes the portion of total cooling capacity devoted to sensible cooling.
A lower SHR generally means a greater portion of system capacity is being used for moisture removal.
Possible causes include:
Study: High Humidity With AC Running
Moisture removal requires part of the evaporator surface to operate below the entering-air dew point.
If the coil is not cold enough relative to the entering-air moisture condition, dehumidification will be limited.
But colder is not always better.
Excessively low evaporator temperature can lead to freezing and loss of airflow.
Lower airflow across a properly operating coil can increase moisture removal in some applications, while higher airflow may increase sensible capacity and reduce dehumidification.
This does not mean airflow should be arbitrarily reduced.
Follow the manufacturer’s allowed airflow range and equipment setup.
Supply-to-return dry-bulb split depends on:
A technician should not diagnose charge or airflow based on temperature split alone.
Technician Case File
Indoor dry bulb: 74°F.
Relative humidity: 68%.
Homeowner: feels sticky and uncomfortable.
Equipment: reaches thermostat setpoint quickly and cycles off.
Further diagnosis: cooling equipment is oversized and runtime is too short for effective moisture removal.
Lesson: Temperature control does not automatically mean humidity control.
Return air: warm.
Supply air: very cold.
Temperature split: large.
Technician assumes system is performing exceptionally well.
Static and airflow testing: reveals severely low airflow.
A large temperature split was not proof of good capacity. The system was moving too little air.
Attic: hot and very humid.
Supply duct surface: cold.
Surrounding-air dew point: above the duct-jacket surface temperature.
Condensation is expected when the surface temperature falls below dew point. The diagnosis now moves toward insulation, vapor barrier, infiltration, and attic moisture conditions.
1. Measure return dry-bulb temperature.
2. Measure return humidity or wet-bulb condition.
3. Determine entering-air dew point.
4. Measure supply dry bulb and humidity where appropriate.
5. Evaluate sensible temperature change.
6. Evaluate moisture removal / latent change.
7. Measure airflow.
8. Compare performance with equipment and load conditions.
9. Investigate infiltration and duct leakage when indoor moisture load is high.
10. Diagnose comfort using both temperature and moisture data.
| Measurement | What It Helps Explain |
|---|---|
| Dry Bulb | Sensible temperature condition |
| Wet Bulb | Temperature + moisture relationship |
| Relative Humidity | Moisture relative to current air temperature |
| Dew Point | Actual condensation threshold / moisture condition |
| Enthalpy | Total heat content of moist air |
Temperature tells you only part of the story. Measure the moisture condition if you want to understand real HVAC performance.
1. What does dry-bulb temperature measure?
2. Why is wet bulb useful in HVAC diagnosis?
3. What happens when a surface falls below the surrounding-air dew point?
4. What does enthalpy represent?
5. Why is temperature split alone not enough to judge cooling performance?
1. The actual air temperature.
2. It reflects the relationship between air temperature and moisture content.
3. Moisture can condense on the surface.
4. The total heat content of moist air, including sensible and latent energy.
5. Because airflow, entering moisture, sensible load, and latent load all affect the resulting temperature change.
Return to the psychrometrics training hub.
High Indoor Humidity →Apply psychrometrics to real comfort complaints.
CFM From Blower Tables →Combine airflow data with psychrometric measurements.
Psychrometric diagnosis depends on accurate temperature and humidity measurements. Use calibrated instruments, allow sensors to stabilize, avoid radiant heat and poor sampling locations, and measure representative return and supply air conditions.
Airflow, equipment sizing, infiltration, duct leakage, coil performance, and runtime all affect moisture removal. Diagnose temperature and humidity together.
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