Heat Transfer + Thermodynamics for Field Trades

Why this matters

Every HVAC + plumbing + solar service call involves heat moving from somewhere to somewhere. Customer's water heater not heating? Heat isn't transferring from burner to water. Customer's AC not cooling? Heat isn't transferring from indoor air to refrigerant. Customer's home losing heat? Heat is transferring from inside to outside faster than equipment can replace it. The contractor who understands heat transfer fundamentals diagnoses problems by tracing the heat path; the contractor without that grounding does parts-swap diagnostics. This article gives the field-applicable basics that separate journeyman from apprentice.

The three modes of heat transfer

Heat moves through three mechanisms (covered in detail in Building Envelope Fundamentals; this article expands them):

Conduction: heat flows through a stationary material from hot side to cold side. Solid materials matter.

• Rate: Q = k × A × (T1 - T2) / L
• k = thermal conductivity of material
• A = surface area
• T1 - T2 = temperature difference
• L = thickness

Convection: heat moves from a surface to a moving fluid (air or liquid). Two types:

• Natural convection: warm fluid rises, cool fluid falls; driven by buoyancy
• Forced convection: pump/fan moves fluid; faster heat transfer than natural

Radiation: heat moves through electromagnetic waves; no medium needed. Sun warming earth; cool body losing heat to cold ceiling.

• Rate: Q = ε × σ × A × (T1⁴ - T2⁴)
• ε = emissivity (0 to 1; perfect black body = 1)
• σ = Stefan-Boltzmann constant
• T = absolute temperature

Critical point: radiation depends on temperature to the FOURTH POWER. Small temp differences = big radiation differences when hot.

Temperature scales

Fahrenheit (°F) - U.S. standard.

Celsius (°C) - global standard; thermodynamic reference.

Kelvin (K) - absolute scale; 0 K = absolute zero (no molecular motion); 273 K = 0°C; 373 K = 100°C.

Rankine (°R) - absolute Fahrenheit; 0 °R = absolute zero; 460 °R = -460°F = 0°F.

Conversions:

• °F to °C: (°F - 32) × 5/9
• °C to °F: (°C × 9/5) + 32
• °C to K: + 273.15
• °F to °R: + 459.67

Specific heat capacity

Energy to raise 1 unit of mass by 1 degree: water 1.0 BTU/lb/°F (highest of common materials), air 0.24, steel 0.12, copper 0.09. This is why HVAC + plumbing use water - 4x air's heat per pound, 10x steel's.

BTU + heat content

1 BTU = energy to raise 1 lb water by 1°F. Common values: 1 cu ft natural gas ~1,000 BTU; 1 lb propane 21,500 BTU; 1 gal #2 oil 138,500 BTU; 1 kWh electrical 3,412 BTU; 1 ton cooling 12,000 BTU/hr. Equipment is sized in BTU: 60,000 BTU/hr heating load + 80 AFUE × 100,000 BTU furnace = 80,000 BTU/hr usable. Knowing BTU + sizing is fundamental.

Phase changes + latent heat

Phase change releases/absorbs heat without temperature change. Latent heat of vaporization of water at 212°F: 970 BTU/lb. Latent heat of fusion at 32°F: 144 BTU/lb. Refrigerants behave similarly at different P + T (see Refrigeration Cycle Theory). Practical: AC cools by evaporating refrigerant (latent absorbing), steam heat efficient because 1 lb steam carries 1,000+ BTU, ice melt takes 144 BTU/lb.

Heat exchange equipment

• Shell-and-tube: multiple tubes in a shell, fluid in tubes/other in shell. Common in boilers, condensers, water heating. Compact + high capacity.
• Plate: stacked plates with hot + cold fluids in alternating channels. Very compact + efficient. Common in commercial water heating, pool heat exchangers.
• Finned tube (air coil): tube with fins for air-side transfer. HVAC indoor/outdoor coils, furnace heat exchangers, condenser coils.
• Cross-flow: air one direction, refrigerant/water perpendicular. Radiators + cooling towers.

Efficiency concepts

First law: energy is conserved. Second law: heat flows hot → cold; reversing requires work (refrigeration cycle). Carnot efficiency (theoretical max): 1 - T_cold/T_hot; refrigeration COP can never exceed Carnot for that temp range. Practical efficiency: heat pump COP 2 - 4 (3.5 excellent), furnace AFUE 80 - 96%, AC SEER 14 - 22, water heater EF 0.6 - 4.0 (heat pumps highest).

Insulation as heat-transfer impedance

R-value = inverse of thermal conductivity per inch. Heat flow Q = ΔT × A / R_total; insulation in series adds (R1 + R2 + R3); parallel uses weighted average by area. Example: 2x6 wall R-19 cavity + studs (15% of wall) at R-6.5 effective = whole-wall ~R-15 instead of R-19. This is why thermal bridging matters + exterior continuous insulation is increasingly required.

Dew point + moisture

Dew point: temperature at which vapor condenses out of air. Indoor 70°F + 50% RH → dew point ~50°F; 70°F + 30% RH → ~37°F. Practical: AC must keep coil below indoor dew point to dehumidify; cold pipe in humid room collects condensation; wall-cavity moisture = vapor reaching surface below dew point. Psychrometric chart relates dry-bulb + wet-bulb + RH + enthalpy + dew point for HVAC sizing.

Diagnosing with heat transfer in mind

"My heat pump can't heat my house": calculate house heating load (Manual J), calculate heat pump capacity at design outdoor temp; if load > capacity, undersized OR backup needed; if load < capacity, other issue (ducts, controls).

"My water heater takes forever to recover": time = (mass × specific heat × ΔT) / burner BTU; 50 gal × 8 lb/gal × 1 BTU × 70°F = 28,000 BTU; 40,000 BTU burner = 0.7 hr (matches spec). Slower? Scale, low gas pressure, wrong-size burner.

"My home overheats in summer": solar gain (Q = SHGC × area × incident sun), high-mass walls store + release at night, oversized AC doesn't dehumidify. Solution depends on dominant path.

Field calculations: wall Q = ΔT × A / R (e.g., 50°F × 30 sq ft / R-13 = 115 BTU/hr). Air leakage Q = CFM × 1.08 × ΔT (sensible). Cooling capacity: 1 ton = 12,000 BTU/hr = ~400 CFM with 20°F TD across coil.

References

• ASHRAE Fundamentals Handbook (the bible)
• ACCA Manual J + S (load calc + equipment selection)
• NIST Standard Reference Data (thermophysical properties)
• Carrier Air Conditioning Company Handbook of Air Conditioning Design
• Manuall internal: Building Envelope Fundamentals, Refrigeration Cycle Theory