HVAC Technician Academy
Learn how temperature, moisture, airflow, enthalpy, sensible heat, latent heat, and dew point work together to determine real HVAC comfort and system performance.
Because temperature alone does not tell the full story. Psychrometrics helps technicians understand humidity, moisture removal, coil performance, sensible and latent capacity, condensation, comfort complaints, and whether the system is actually changing the condition of the air as intended.
A room at 75°F with 45% relative humidity can feel very different from the same room at 75°F and 70% relative humidity.
HVAC systems do two major jobs:
Step 1: Learn dry-bulb temperature
Step 2: Learn wet-bulb temperature
Step 3: Understand relative humidity
Step 4: Understand dew point
Step 5: Learn humidity ratio and enthalpy
Step 6: Separate sensible and latent cooling
Step 7: Understand sensible heat ratio
Step 8: Combine psychrometrics with CFM to evaluate delivered capacity
Learn the core air-property measurements used to diagnose comfort and moisture performance.
HVAC Sensible & Total Capacity →Combine airflow, Delta-T, enthalpy change, and humidity to estimate real delivered BTUs.
CFM From Blower Tables →Establish airflow before using psychrometric measurements for capacity calculations.
Dry-bulb temperature is the ordinary air temperature measured by a standard temperature sensor.
It is useful for:
Wet-bulb temperature reflects the cooling effect of evaporation and therefore responds to both temperature and moisture condition.
It is commonly used in:
Relative humidity describes the amount of water vapor in the air relative to saturation at the same temperature.
This means RH can change even when the actual amount of moisture in the air stays the same.
Dew point is the temperature at which air reaches saturation and water vapor begins to condense.
It is especially useful when diagnosing:
If a surface falls below the surrounding air's dew point, condensation can form.
Humidity ratio describes the mass of water vapor compared with the mass of dry air.
Unlike relative humidity, it provides a more direct description of actual moisture content.
It may be expressed as:
Enthalpy represents the total heat content of moist air relative to a reference condition.
It includes both:
Enthalpy change across a cooling coil can help estimate total cooling capacity when airflow is known.
Sensible cooling lowers dry-bulb temperature.
A common field approximation is:
This uses standard-air assumptions and should be treated as an estimate.
Latent cooling removes moisture from the air.
Moisture condenses on the evaporator coil when the coil surface is below the dew point of the entering air.
A system can satisfy thermostat temperature while still performing poorly on latent removal.
A lower SHR means a greater share of total cooling is being used for latent moisture removal.
A higher SHR means a greater share is sensible cooling.
Correct SHR depends on equipment design and actual application.
A properly operating cooling coil typically:
Supply-air relative humidity may still be high because colder air can have high RH even after moisture has been removed.
Within the equipment's approved operating range, airflow affects coil temperature and the balance between sensible and latent performance.
Higher airflow may favor sensible capacity.
Lower airflow can increase latent removal in some conditions, but excessive reduction can cause:
Follow manufacturer airflow requirements.
Temperature split depends on:
A 20°F split may be reasonable in one condition and misleading in another.
Temperature split without airflow and humidity information is incomplete.
A common field approximation is:
Where Δh is the return-to-supply enthalpy change.
Accurate CFM is critical. If airflow is guessed, calculated capacity is also a guess.
Possible causes include:
Continue: High Humidity With AC Running
Some homes have moisture loads that require dedicated dehumidification even when the cooling equipment is operating correctly.
Continue: Whole-Home Dehumidification
Technician Case File
Indoor temperature: 75°F.
Relative humidity: 65%.
Supply temperature: 56°F.
Delta-T: 19°F.
Homeowner complaint: house feels sticky.
Further diagnosis: excessive airflow and short equipment runtimes are limiting moisture removal.
Lesson: A normal-looking temperature split does not prove good latent performance.
House A: 75°F / 50% RH.
House B: 75°F / 70% RH.
Dry-bulb temperature: identical.
House B has a significantly greater moisture and latent load. Dry bulb alone cannot describe the difference.
Equipment: reaches setpoint quickly.
Runtime: short.
Indoor RH: remains high.
Investigation moves toward equipment sizing, staging, airflow setup, infiltration, and moisture load instead of assuming low refrigerant.
1. Stabilize system operation.
2. Measure return dry bulb.
3. Measure return RH or wet bulb.
4. Determine dew point and moisture condition.
5. Measure representative supply conditions.
6. Determine actual airflow.
7. Evaluate sensible temperature change.
8. Evaluate humidity-ratio or enthalpy change when useful.
9. Compare delivered performance with manufacturer data and load conditions.
10. Investigate infiltration, ventilation, duct leakage, sizing, and airflow when comfort remains poor.
11. Retest after corrections.
Apply psychrometrics to a real indoor-comfort complaint.
Whole-Home Dehumidifier →Understand dedicated latent-load control.
AC Runs All Day →Connect runtime, sensible load, latent load, and outdoor conditions.
Upstairs Hotter Than Downstairs →Connect load, airflow, distribution, and comfort diagnostics.
Psychrometric diagnosis depends on accurate, stabilized measurements. Use calibrated temperature and humidity instruments, choose representative sampling locations, avoid radiant heat and duct leakage effects, and combine air measurements with verified airflow when calculating system capacity.
Equipment sizing, airflow, runtime, infiltration, duct leakage, coil performance, and latent load all affect indoor humidity. Diagnose temperature and moisture together.
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