How Air Moves Heat and Moisture Through a System

Why This Page Matters

Air conditioning does not create cold.

It moves heat.

It removes moisture.

It does this by moving air across a heat exchanger.

If airflow is wrong, everything becomes distorted:

• Temperature readings
  
  

• Refrigerant measurements
  
  

• Humidity control
  
  

• Delivered capacity
  
  

• Motor load
  
  

Airflow is the backbone of HVAC system integrity.

Psychrometrics explains what is happening inside that moving air.

You cannot measure system performance without understanding both.

Part 1: How Airflow Works

Airflow is the movement of air created by a blower motor.

That air must move through:

• The return duct
  
  

• The filter
  
  

• The evaporator coil
  
  

• The supply duct
  
  

Every restriction adds resistance.

Resistance creates pressure.

Pressure changes motor load.

Static Pressure

Static pressure is resistance to airflow inside the duct system.

Think of it like friction inside a pipe.

More restriction → higher static pressure.

Common restrictions:

• Dirty filters
  
  

• Undersized ductwork
  
  

• Closed dampers
  
  

• Dirty coils
  
  

• Crushed flex duct
  
  

Static pressure is measured in inches of water column (in. w.c.).

Total External Static Pressure (TESP)

TESP is the combined pressure on:

• The return side
  
  

• The supply side
  
  

Measured with panels secured and filter installed.

TESP tells you how hard the blower is working.

High static pressure means:

• The motor works harder
  
  

• Amp draw changes
  
  

• Airflow often decreases (PSC systems especially)
  
  

You cannot guess airflow.

You must measure pressure.

CFM (Cubic Feet per Minute)

CFM is the volume of air moving through the system.

Airflow determines:

• Heat transfer rate
  
  

• Coil performance
  
  

• Temperature split
  
  

• Moisture removal
  
  

Too little airflow:

• Coil gets too cold
  
  

• Ice risk increases
  
  

• Latent removal may increase but sensible drops
  
  

Too much airflow:

• Coil too warm
  
  

• Poor moisture removal
  
  

• High supply temperature
  
  

Airflow must be within design range.

Cause and Effect Chain (Airflow Side)

Restriction → Static Pressure → Motor Load → Airflow Volume → Heat Transfer Rate

Everything is connected.

Part 2: What Psychrometrics Is

Psychrometrics is the study of air and moisture.

Air is not just temperature.

Air contains:

• Sensible heat (temperature you feel)
  
  

• Latent heat (moisture in the air)
  
  

Temperature alone does not describe air.

Humidity changes everything.

Sensible Heat

Sensible heat changes temperature.

If air goes from 75°F to 55°F, that is sensible cooling.

You can measure it with a thermometer.

Latent Heat

Latent heat removes moisture.

When water vapor turns into liquid on the evaporator coil, heat is removed from the air.

You cannot see latent heat with temperature alone.

It requires understanding humidity.

Relative Humidity (RH)

Relative Humidity tells you how full the air is with moisture compared to what it could hold at that temperature.

Warm air can hold more moisture than cold air.

When air cools:

• Its moisture capacity drops.
  
  

• Water condenses if the coil is cold enough.
  
  

This is why airflow and coil temperature matter together.

Enthalpy (Simple Explanation)

Enthalpy is total heat in the air.

It includes:

• Sensible heat
  
  

• Latent heat
  
  

Two air samples can have the same temperature but different total heat because moisture content differs.

That is why psychrometrics exists.

Airflow + Psychrometrics Together

Airflow controls how much air moves across the coil.

Psychrometrics explains what happens to that air.

Low airflow:

• More moisture removal
  
  

• Lower coil temperature
  
  

• Risk of freezing
  
  

High airflow:

• Less moisture removal
  
  

• Higher coil temperature
  
  

• Higher sensible ratio
  
  

The system must balance both.

Where This Shows Up in the 14 Steps

Airflow and psychrometrics are required for:

Step 3

Static pressure measurement
Motor configuration verification
Delta T capture
Temperature rise verification

Step 13

Refrigeration cycle evaluation
Superheat and subcooling interpretation
Delivered performance understanding

Without airflow discipline, refrigerant readings become misleading.

Without psychrometrics, temperature readings become incomplete.

Common Misunderstandings (That We Do Not Accept)

• “Delta T tells you everything.”
  (False. It ignores humidity and airflow volume.)
  
  

• “If air is cold, the system is fine.”
  (False. Moisture removal and airflow volume matter.)
  
  

• “High static just means strong airflow.”
  (False. High static often means restriction.)
  
  

Measurement replaces guessing.

Key Takeaways

• Airflow moves heat.
  
  

• Static pressure reveals restriction.
  
  

• Motor load reflects resistance.
  
  

• CFM determines heat transfer rate.
  
  

• Air contains both temperature and moisture.
  
  

• Psychrometrics explains total heat in air.
  
  

• Airflow and humidity must be evaluated together.
  
  

• Temperature alone is incomplete.
  
  

Before Moving Forward

Next, you must understand:

How Refrigeration Works

Because airflow moves heat.

Refrigeration changes its state.

And the interaction between the two determines system integrity.