Heat Transfer Fundamentals for Trades Reference

Why this matters

Heat moves three ways: conduction (through solids), convection (with fluid movement), and radiation (electromagnetic waves). Every HVAC, plumbing, and roofing problem involves one or more of these modes. Knowing how each works - and which one dominates in a given situation - gives you the diagnostic lens to figure out where heat is going (or not going) and why.

Conduction - heat through solids

Mechanism: vibrating molecules transfer kinetic energy to adjacent molecules. No bulk material movement.

Driving force: temperature difference (ΔT).

Rate equation (Fourier's Law):

Q = (k × A × ΔT) / L

Where:

• Q = heat rate (BTU/hr)
• k = thermal conductivity (BTU·in/hr·ft²·°F)
• A = cross-section area (ft²)
• ΔT = temperature difference across the material (°F)
• L = thickness (inches)

R-value (thermal resistance) = L / k. Higher R = better insulator.

Common materials by conductivity (BTU·in/hr·ft²·°F):

R per inch = 1/k. Fiberglass batt = 1/0.27 = R-3.7 per inch.

Practical examples:

• Cold copper line in a hot attic: condensation forms on the cold surface because heat conducts away rapidly through copper.
• Steel stud in an insulated wall: thermal bridge - heat travels through the high-k stud bypassing the low-k insulation.
• Aluminum coil fins: pick a high-k material so heat conducts efficiently from the refrigerant tube to the air-contact surface.

Convection - heat with fluid movement

Mechanism: fluid (air, water, refrigerant) carries heat by bulk movement.

Driving force: temperature difference AND fluid motion.

Rate equation:

Q = h × A × ΔT

Where:

• Q = heat rate
• h = convection coefficient (BTU/hr·ft²·°F)
• A = surface area
• ΔT = surface-to-fluid temperature difference

Convection coefficient (h) values:

Two types of convection:

Natural (free) convection: fluid moves due to density differences. Warm air rises, cold air sinks. Slow heat transfer.

Forced convection: fluid moved by external force (fan, pump). Much faster.

This is why HVAC equipment uses fans and pumps - forced convection from air across a coil is 5-10× faster than natural convection alone. Same evaporator coil with no blower transfers a fraction of its rated capacity.

Practical examples:

• Air handler blower: increases h on indoor coil, increasing heat transfer rate.
• Hot water radiator without thermosiphon: relies on natural convection alone; works but slow.
• Cooling tower fan: increases h on condenser water side.

Radiation - heat by electromagnetic waves

Mechanism: all bodies emit electromagnetic radiation based on temperature.

Driving force: temperature difference (in absolute terms, T in °R or K).

Rate equation (Stefan-Boltzmann):

Q = ε × σ × A × (T₁⁴ − T₂⁴)

Where:

• ε = emissivity (0 to 1; 1 = perfect black-body emitter)
• σ = Stefan-Boltzmann constant
• A = surface area
• T = absolute temperature

Key features:

• No medium required (works through vacuum)
• T⁴ dependence: small temperature changes have large radiation effects at high temps
• Emissivity varies widely:
  • Polished metal: 0.05-0.10 (low emitter)
  • Painted surface: 0.85-0.95 (high emitter)
  • Black-body (theoretical perfect): 1.0
  • Glass (visible): mostly transparent; (infrared): mostly opaque

Practical examples:

• Solar gain through windows: sunlight passes through glass (transparent to visible) and warms interior surfaces (which then emit IR back, but glass is opaque to IR - greenhouse effect).
• Radiant heating: hot surface (boiler tubes, radiant panel) heats objects in the room without heating the air much.
• Radiant barriers in attics: low-emissivity surface (foil-faced sheathing or radiant-barrier paint) reduces radiative heat gain from hot roof to attic insulation.
• Cold-night ground frost: ground radiates heat to clear sky (effectively 0 K) and cools below ambient air temperature, allowing frost even when air temp is above freezing.

The three modes work together

Real systems involve all three modes simultaneously:

Wall heat loss in winter:

• Radiation: warm interior surfaces emit IR; cold exterior surfaces absorb solar IR (in daytime)
• Convection: warm indoor air carries heat to the inside wall surface; outside, wind drives convective loss to outdoor air
• Conduction: heat passes through the wall material from interior to exterior

The total heat loss is the sum, and the dominant mode varies by wall location and condition.

Air conditioning coil:

• Convection: blower-driven indoor air across the cold coil fins (high h, primary mode)
• Conduction: heat through fin material into refrigerant tubes
• Convection: refrigerant boiling inside tubes (very high h on liquid side)
• Radiation: minor; the coil radiates a small amount but mostly convective

Roof heat gain in summer:

• Radiation: hot sun (effective ~5500 K source) heats the roof surface
• Conduction: through shingles, sheathing, into attic
• Convection: hot attic air mixes with outdoor air via vents OR pushes into living space via leaks
• Radiation again: hot roof underside radiates to attic floor (insulation surface)

Insulation defeats all three modes

A well-insulated wall has:

• Low conductivity (high R-value): low thermal-mass material slows conduction
• Air-sealed: no air movement (forced or natural convection) through gaps
• Radiant barrier (sometimes): reflects radiation, doesn't absorb and re-emit

Real insulation products combine these - closed-cell foam stops convection (no air movement through), has low k, AND has some radiant-blocking properties from the foam structure.

Common heat-transfer misconceptions

References

• ASHRAE Handbook - Fundamentals (heat transfer chapter)
• Cengel & Ghajar, Heat and Mass Transfer
• Building Science Corporation (buildingscience.com) - applied building heat transfer
• NIST material property tables
• ASTM thermal property test methods (ASTM C518, C177)