Mechanical Systems 1 - Heat Transfer Notes

Building Envelope

• Separates interior and exterior environments.
• Consists of floors, roofs/ceilings, walls, windows, and doors.

Thermal Envelope

• May differ from the building envelope.
• Encloses the thermal insulation barrier.

Thermal Performance

• Heat transfer occurs through convection, radiation, and conduction.

Heat Transfer Coefficients

• Temperature difference is essential for heat transfer.
• Rate of heat transfer depends on resistance (R-value).
• U-factor quantifies the rate of heat transfer, accounting for resistance.

Thermal Resistance (R-Value)

• Ability of a material to resist heat flow.
• $ΣR = R1 + R2 + R3$
• Units: °F-ft²-hr/Btu

Thermal Conductivity (k)

• Rate at which heat flows through a material per inch of thickness.
• $R = x/k$
• x = thickness of material
• Units: Btu-in/hr-ft²-°F

Thermal Transmittance (U-Factor)

• Rate at which heat flows through a material or building section.
• $U = 1/ΣR$
• Units: Btu/hr-ft²-°F

R-Values (Building Materials)

• Reference ASHRAE Principles book, Table 5-5.
• R = inches / k (when conductivity k is given)

R-Values (Air Space)

• Reference ASHRAE Principles book, Table 5-4.

Effective Emittance

• Reference ASHRAE Principles book, Table 5-3.
• Typically use 0.82

R-Values (Surface Film)

• Reference ASHRAE Principles book, Table 5-2.
• Indoor air film: $R_i = 0.68$
• Outdoor air film: $R_o = 0.17$

Total Assembly Coefficients

• $R<em>t = R</em>i + R + R_o$
  • $R_t$ = Total Resistance
  • $R_i$ = Inside air film coefficient
  • R = Sum of material and air gap R-values ($ΣR$)
  • $R_o$ = Outside air film coefficient

Thermal Bridging

• Path of least resistance through an exterior assembly.
• Metal is an excellent conductor.
• Use continuous insulation (CI) to combat thermal bridging.

Nominal vs Effective R-Value

• Nominal R-value:
  • Accounts for insulation layer only.
• Effective R-value:
  • Accounts for insulation, thermal bridging, framing, interior finish, sheathing, and cladding.

ASHRAE 90.1 Climate Zones

• Defines climate zones for the United States.

ASHRAE 90.1 Envelope Requirements

• Specifies building envelope requirements for different climate zones.

Thermal Gradient

• Temperature change is related to thermal resistance.
• $T<em>1 = OAT + (R</em>1/ ΣR)*ΔT$
• $T<em>2=T</em>1 + (R_2/ ΣR)*ΔT$
• $T<em>3= T</em>2 + (R_3/ ΣR)*ΔT$
• R# = thermal resistance of given material
• T# = temperature at given material
• ΣR = total thermal resistance of assembly
• ΔT = temperature difference in °F (outdoor-indoor)

U-Factors (Windows and Doors)

• Typical window U-factors are in Table 5-6.
• Glass doors (>50% glass) use Table 5-6.
• Solid/partial glass door U-factors are in Table 5-7.
• Revolving doors use Table 5-8.
• Emergency exit doors use Table 5-9.

Floor Perimeter

• Heat transfer is significant at the perimeter.
• Table 5-11 provides heat loss coefficient Fp.

Heat Flow

• Measured in BTUh (British Thermal Units per hour).
• $q = U x A x ΔT$
  • q = heat transfer rate in BTUh
  • U = heat transmission factor = 1/ΣR
  • A = area in SF
  • ΔT = temperature difference in °F