Heat Transfer Fundamentals
Sensible heat, latent heat, BTUs, tons of cooling, and the laws of thermodynamics that govern every HVAC system.
- Define sensible heat and latent heat and explain how each affects HVAC system performance
- Convert between BTUs, tons of cooling, and watts
- Use the sensible heat formula Q = 1.08 x CFM x delta-T for airflow calculations
- Read basic psychrometric chart properties including dry bulb, wet bulb, and relative humidity
- Explain how latent heat removal relates to moisture control and comfort
Lesson 1
Sensible Heat vs Latent Heat
Why Heat Transfer Matters
Every HVAC system exists for one purpose - to move heat from where it is not wanted to where it is either needed or can be rejected. Understanding the two types of heat is the foundation of everything you will learn about air conditioning, heating, and refrigeration.
Heat always flows from a warmer object to a cooler object. This is the second law of thermodynamics and it governs every refrigeration cycle, every furnace, and every heat pump you will ever work on.
Sensible Heat
Sensible heat is heat that changes the temperature of a substance without changing its phase. You can sense it - you can feel it and measure it with a thermometer. When air passes over a warm heat exchanger and its temperature rises from 70 degrees F to 120 degrees F, that 50 degree increase is sensible heat.
Sensible Heat
Changes temperature
Measured with a thermometer
No phase change occurs
Example: Air heated from 70 to 120 degrees F
Latent Heat
Changes phase (state)
Cannot be measured with a thermometer
Temperature stays constant during change
Example: Water boiling at 212 degrees F
Latent Heat
Latent heat is heat involved in a phase change without a temperature change. When water boils at 212 degrees F, it absorbs enormous amounts of energy to change from liquid to vapor - but the temperature stays at 212 degrees F during the entire process. This hidden heat is what makes refrigeration possible.
In HVAC, latent heat is critical because it relates directly to moisture removal. When warm, humid air passes over a cold evaporator coil, water vapor in the air condenses on the coil surface. The energy released during this condensation is latent heat. This is how your air conditioning system dehumidifies.
NATE Exam Tip
The exam will test whether you know the difference: sensible heat changes temperature, latent heat changes phase. If a question asks about moisture removal, the answer involves latent heat.
Heat Transfer Methods
Heat moves by three methods in HVAC systems:
| Method | How It Works | HVAC Example |
|---|---|---|
| Conduction | Direct contact between materials | Heat through copper tubing walls |
| Convection | Movement of fluid or air | Warm air rising from a furnace |
| Radiation | Electromagnetic waves through space | Sun heating a rooftop condensing unit |
Sensible heat changes temperature and is measured with a thermometer. Latent heat changes phase (liquid to vapor or vapor to liquid) without changing temperature. Both types of heat are present in every cooling system - sensible heat lowers air temperature while latent heat removes moisture.