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All HVAC work involves moving heat from one place to another. Understanding how heat transfers is fundamental to diagnosing and designing any heating or cooling system. There are three modes of heat transfer:
Heat transfers directly through solid materials by molecular vibration. Faster-moving (hotter) molecules transfer energy to slower-moving (cooler) adjacent molecules. Examples in HVAC: heat moving through a heat exchanger wall, heat loss through building walls, heat transfer from a refrigerant to a coil.
Materials have different thermal conductivity - metals conduct heat very well, insulation materials conduct poorly. This is why heat exchangers use copper or aluminum (high conductivity) and duct insulation uses fiberglass (low conductivity).
Heat transfers through fluid (liquid or gas) movement. Natural convection occurs when warmer, less dense fluid rises and cooler, denser fluid sinks, creating circulation. Forced convection uses fans or pumps to move fluid over heat transfer surfaces. Most HVAC systems use forced convection - fans move air across coils, pumps circulate water through hydronic systems.
Heat transfers through electromagnetic waves without requiring any medium. All objects with temperature above absolute zero emit radiant heat. In HVAC: radiant floor heating, infrared heaters, solar heat gain through windows, and heat loss from warm objects to cooler surroundings.
The British Thermal Unit (BTU) is the standard unit of heat measurement in the U.S. HVAC industry:
One BTU is the amount of heat required to raise one pound of water by one degree Fahrenheit.
HVAC equipment capacity is often expressed in BTUs per hour (BTU/hr or BTUH) or in tons of refrigeration:
1 ton of refrigeration = 12,000 BTU/hr
This comes from the amount of heat required to melt one ton (2,000 lbs) of ice in 24 hours: 2,000 lbs � 144 BTU/lb (latent heat of fusion of water) = 288,000 BTU/day � 24 hours = 12,000 BTU/hr.
| Capacity | BTU/hr | Common Application |
|---|---|---|
| 1 ton | 12,000 | Small apartment, room AC |
| 2 ton | 24,000 | Small home (up to 1,000 sq ft) |
| 3 ton | 36,000 | Medium home (1,000-1,500 sq ft) |
| 4 ton | 48,000 | Larger home (1,500-2,000 sq ft) |
| 5 ton | 60,000 | Large home / light commercial |
This distinction is critical for HVAC work - the exam always tests this:
Heat that causes a change in temperature with no change in state (solid, liquid, or gas remains the same). When you heat air in a furnace and the temperature rises, that's sensible heat gain. Measured with a thermometer.
Heat that causes a change in state with no change in temperature. When ice melts to water, or water evaporates to steam, the temperature stays constant while the state changes. This is where the energy goes in refrigeration systems - the refrigerant absorbs latent heat as it evaporates in the evaporator coil, cooling the air without a temperature change in the refrigerant until the phase change is complete.
| Type | Temperature Changes? | State Changes? | HVAC Example |
|---|---|---|---|
| Sensible heat | Yes | No | Air heating in furnace, air cooling by sensible coil |
| Latent heat | No | Yes | Refrigerant evaporating, moisture condensing on coil |
Latent heat of vaporization: 970 BTU/lb (energy to convert water to steam at 212�F)
Latent heat of fusion: 144 BTU/lb (energy to melt ice to water at 32�F)
Gas furnaces are the most common heating appliance in North America. Understanding the combustion process and safety controls is essential for NATE certification:
Natural gas or propane combines with oxygen to produce heat, carbon dioxide, and water vapor. Complete combustion requires sufficient air supply - typically about 10 cubic feet of air per cubic foot of natural gas. Incomplete combustion produces carbon monoxide (CO), a colorless, odorless, and highly dangerous gas.
| Component | Function | Failure Effect |
|---|---|---|
| Gas valve | Controls gas flow to burners | No heat or continuous gas flow |
| Igniter (hot surface or spark) | Ignites gas-air mixture | No ignition, lockout |
| Flame sensor (flame rod) | Proves burner flame is present | Shuts gas valve if no flame detected (safety) |
| Heat exchanger | Transfers heat to air while containing combustion gases | Cracked exchanger = CO leak hazard |
| High limit switch | Shuts furnace if plenum overheats | Furnace shuts off before reaching set temp |
| Pressure switch | Verifies induced draft motor is operating | Furnace won't fire if draft motor fails |
| Inducer motor | Creates negative pressure to vent combustion gases | Pressure switch opens, furnace locks out |
A cracked or failed heat exchanger is a life-safety hazard. Combustion gases including carbon monoxide can mix with circulated air. Signs include: visible cracks or holes, soot near joints, flame distortion when blower starts, CO detector alarms. A furnace with a confirmed cracked heat exchanger must be taken out of service immediately.
Electric resistance heating converts electrical energy directly to heat. Common types in HVAC:
Electric heating efficiency is measured as COP (Coefficient of Performance). Electric resistance heating has a COP of 1.0 - every watt of electricity input produces exactly one watt of heat output. Heat pumps achieve COPs of 2-4, making them far more efficient for heating.
Memorize: 1 ton = 12,000 BTU/hr. Know the difference between sensible heat (temperature changes, state doesn't) and latent heat (state changes, temperature doesn't). Know that CO results from incomplete combustion and that a cracked heat exchanger must be replaced before the furnace is returned to service.