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 a clear night sky whose effective radiant temperature sits well below ambient (commonly 20 to 30°F below air temperature, colder in dry air), so the surface cools below ambient and frosts even when the 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-k material slows conduction. This is a conductivity property, not a mass property; see the thermal-mass misconception below.
• 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

"Cold gets in." There is no such thing as cold flowing. There is only heat leaving. This is not pedantry; it changes where you look. A cold floor is not admitting cold, it is losing heat to something below it, and you find the fix by asking what is on the other side.

"R-value is all that matters." R-value describes conduction only. A wall packed with high-R batts that leaks air still loses heat by convection through the assembly, and an installed batt with gaps, compression, or a missing corner performs far below its label. Air sealing frequently returns more than adding insulation on top of an already-insulated assembly.

"Radiant barriers are super insulation." A reflective surface only works facing an air space, and only against radiation. Sandwich it against a solid surface and it does essentially nothing. It has no meaningful R-value on its own, and the numbers printed on some products are system numbers under specific test conditions, not material R.

"Thermal mass is the same as insulation." Mass stores and delays heat. It does not resist its passage. A masonry wall with high mass and low R will still transfer heat, just later in the day. That delay is valuable in the right climate and useless in the wrong one.

"Closing vents in unused rooms saves energy." It raises system static pressure, reduces total airflow, drops coil temperature in cooling and raises heat exchanger temperature in heating, and pushes more air through duct leaks. The heat you thought you saved leaves through a different path, and the equipment pays for it.

"Aluminum foil and copper are good insulators because they feel cool." They feel cool because they conduct heat away from your hand fast. Feel is a measure of conductivity, not temperature. Same reason a metal tool in an unheated garage feels colder than the wood bench beside it at identical temperature.

"A bigger unit will fix the comfort complaint." Oversizing shortens run time. Short cycles move less air, remove less moisture, and leave the distant rooms unserved. The complaint is usually distribution or envelope, both heat-transfer problems the equipment cannot solve by being larger.

"Heat rises." Hot air rises because it is less dense. Heat itself moves in every direction, toward cold. Conduction goes down through a floor and sideways through a wall just fine, and radiation crosses a room regardless of orientation.

"The infrared camera reads temperature through the wall." It reads surface temperature, and it reads it correctly only when you have the emissivity setting right. Shiny metal reads badly. What the camera actually shows you is a pattern to investigate, not a number to quote.

"Insulating the pipe stops the heat loss." It slows it. Insulation reduces the rate; it never stops the transfer, because there is no such thing as zero conductivity. Long runs still arrive cooler, which is why recirculation and run length matter.

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)