HVAC Technician Academy
Learn how HVAC technicians use dry-bulb temperature, wet-bulb temperature, relative humidity, dew point, sensible heat, latent heat, enthalpy, airflow, and delivered capacity to diagnose comfort and performance problems.
HVAC psychrometrics is the study of the properties of moist air and how heating, cooling, humidification, dehumidification, and airflow change those properties. Technicians use psychrometric measurements to determine whether equipment is actually removing the expected sensible heat and moisture from the air.
A home can be 74°F and still feel uncomfortable.
Why?
Because comfort depends on more than dry-bulb temperature.
Humidity, airflow, mean radiant temperature, air distribution, and equipment runtime all influence how the space feels.
Psychrometrics gives technicians a way to measure what the air is actually doing instead of relying only on thermostat temperature.
The ordinary air temperature measured by a standard thermometer.
A temperature influenced by both sensible temperature and moisture content.
The percentage of moisture in the air relative to how much it can hold at that temperature.
The temperature at which moisture begins condensing from the air.
A measure of total heat content in moist air, including sensible and latent energy.
Describes how much of total cooling capacity is being used for sensible cooling versus moisture removal.
Dry-bulb temperature tells the technician how warm or cool the air is.
Wet-bulb temperature gives additional information about the moisture content of the air.
Two homes can both be 78°F dry bulb while having very different wet-bulb temperatures and humidity levels.
That means the same thermostat temperature can represent very different cooling loads.
Relative humidity changes when air temperature changes even if the actual amount of moisture in the air remains the same.
That is why relative humidity should not be interpreted without considering temperature.
High indoor humidity complaints may involve:
Study: High Humidity While AC Is Running
Dew point is especially useful because it represents the actual moisture condition of the air.
When a surface temperature falls below the dew point of surrounding air, condensation can form.
Dew-point thinking helps technicians understand:
Sensible heat changes temperature.
Latent heat changes moisture content.
Sensible load: cooling the air from a higher temperature to a lower temperature.
Latent load: removing water vapor from the air.
An AC system must often do both jobs at the same time.
A system can satisfy thermostat temperature while still performing poorly on humidity control.
Sensible Heat Ratio, often abbreviated SHR, describes the relationship between sensible cooling and total cooling.
A system with a higher sensible ratio is using more of its capacity for temperature reduction.
A system with a lower sensible ratio is using a greater share of capacity for moisture removal.
Airflow, coil temperature, indoor humidity, equipment design, and operating conditions all affect this relationship.
Airflow affects how the evaporator coil divides its capacity between temperature reduction and moisture removal.
In general, changing airflow changes:
A return-to-supply temperature difference is useful, but it does not directly tell you total system capacity.
Temperature split changes with:
A high split can occur because airflow is too low.
A lower split can occur because airflow is high or latent load is substantial.
Always interpret temperature split with airflow and humidity.
One useful field relationship for estimating sensible heat transfer is:
Where:
This relationship estimates sensible output, not total cooling capacity.
If airflow is guessed instead of measured or derived accurately, the capacity calculation is only as good as the airflow assumption.
Assume measured airflow is:
1,600 CFM
Return air:
78°F
Supply air:
58°F
Temperature difference:
20°F
Calculation:
This does not mean total system capacity is 34,560 BTU/hr. Moisture removal represents additional latent capacity that is not captured by the sensible-only calculation.
To understand total cooling performance, technicians need to account for both sensible temperature change and latent moisture removal.
Enthalpy measurements can be used to evaluate total heat change across the system.
This is where psychrometric measurements become much more powerful than temperature split alone.
Enthalpy represents total heat content in moist air.
If technicians know return-air and supply-air enthalpy, the difference can be used with measured airflow to estimate total delivered capacity.
This method captures both:
Accurate measurements matter. Small errors in temperature, humidity, or airflow can significantly affect the final result.
An oversized system may cool the space very quickly.
That can sound like a benefit.
But if the thermostat is satisfied before the system runs long enough to remove sufficient moisture, indoor humidity can remain high.
Common clues include:
Study: What Size AC Do I Need?
Diagnostic Case
Homeowner complaint: the house feels sticky.
Thermostat: 74°F.
Relative humidity: 68%.
New system: 5 tons.
Previous system: 4 tons.
Cooling cycles are short.
Supply temperature is cold.
Airflow is high.
The equipment satisfies the sensible load very quickly but does not remain on long enough to provide strong moisture removal.
Lesson: Achieving thermostat setpoint does not prove that the HVAC system is controlling the latent load.
1. Confirm the complaint. Is the problem temperature, humidity, room comfort, runtime, or capacity?
2. Measure return-air dry bulb. Establish entering-air temperature.
3. Measure return-air humidity or wet bulb. Determine entering moisture condition.
4. Measure supply-air conditions. Record dry bulb and humidity/wet bulb where appropriate.
5. Verify airflow. Capacity calculations are meaningless without reasonable airflow data.
6. Evaluate sensible performance. Compare CFM and dry-bulb temperature change.
7. Evaluate latent performance. Compare moisture change through the system.
8. Compare equipment operation with building load. Look at runtime, staging, infiltration, and sizing.
9. Verify after correction. Recheck temperature, humidity, airflow, runtime, and delivered performance.
| Condition | Possible Direction | Next Check |
|---|---|---|
| Temperature good, humidity high | Short runtime, airflow, oversizing, infiltration | Runtime, wet bulb, sizing, duct leakage |
| Very high temperature split | Possible low airflow | Static pressure and CFM |
| Low temperature split + high humidity load | Large latent load may be present | Wet bulb, airflow, enthalpy change |
| System runs continuously but comfort poor | Capacity, load, duct delivery, equipment performance | Delivered BTUs and building load |
| Duct sweating | Surface below surrounding-air dew point | Dew point, insulation, leakage, attic humidity |
A thermostat tells you space temperature. Psychrometrics tells you what happened to the air. Measure both temperature and moisture when diagnosing comfort or capacity.
Connect measured CFM with sensible and latent performance.
Refrigeration Diagnostics →Understand how load affects pressures, superheat, and capacity.
Installation & Commissioning →Verify delivered system capacity after startup.
Dry bulb is ordinary air temperature. Wet bulb reflects both temperature and moisture and is useful for understanding the actual cooling load on the evaporator.
Yes. A system can satisfy the sensible temperature load while failing to remove enough moisture, especially if runtime is short or airflow and equipment sizing are not appropriate.
Dew point is the temperature at which moisture begins to condense from the air. It is especially useful for diagnosing duct sweating and moisture problems.
No. Temperature split must be interpreted with airflow, humidity, refrigerant conditions, equipment stage, and load. A large split can even occur when airflow is too low.
Enthalpy represents total heat content in moist air. Comparing return and supply enthalpy can help estimate total cooling performance when combined with accurate airflow.
Psychrometric and capacity calculations depend on accurate temperature, humidity, and airflow measurements. Use properly maintained instruments, appropriate test locations, manufacturer procedures, and stable system operation before drawing conclusions.
Temperature, humidity, airflow, equipment sizing, duct leakage, and system runtime all affect comfort. Proper diagnostics measure how the entire HVAC system is actually performing.
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