Heating Load Calculation
ME 425
Air Conditioning System Design
Keith E. Elder, P.E.
Heating Load Calculation
The heating load calculation begins with the
determination of heat loss through a variety
of building envelope components and
situations.
Walls
Roofs
Windows
Doors
Exposed Floors
Slab-on-Grade Floors
Basement Walls
Basement Floors
Infiltration
Ventilation
The Heat Loss Equation
q = U
o
x A ( T
i
- T
o
)
Where:
U
o
= the overall heat transfer
coefficient, Btu/hr/ft
2
/
°
F
A = the surface area, ft
2
T
i
= the indoor design temperature, °F
T
o
= the outdoor design temperature, °F
Heat Loss Equation Assumptions
Steady State Conditions
No Thermal Storage
Heat Transfer is Instantaneous
One-Dimensional Parallel "Heat Flow"
Thermally Homogeneous Construction
Construction Anomalies can be Accounted for by
Transmission “Weighting” Corrections
Solving the Heat Loss Equation
Area Takeoffs (from plans with est. zoning)
Design Temperatures (‘05F28 CD & Std. 55)
Assembly U-factors, U
o
:
U
o
= 1/ R
Total
R
Total
=
R
1
+ R
2
+ R
n
+ ...
R
1,R
2 andR
nrepresent the thermal resistance of each of
the elements in the path of the "heat flow.”
Properties of typical construction materials can be found
in Table 3 of ‘05F25.
Air Film Properties
From ’05F25.2
Wall with no Framing Elements
What is the U-factor of a 3-1/2 inch wall
composed of 4 inch brick, R-11
insulation and 5/8 inch gypboard?
Ri Outside Air Film
4" Brick R-11 Batt Insulation 5/8" Gypboard Inside Air Film Total R = Uo = 1/R = 1/12.81 = 0.078 Btu 0.17 0.40 11.00 0.56 0.68 12.81 Hr-SF-deg F
Correcting for Cavity Depth
The R-value of batt insulation is rated by
manufacturers for predetermined depths.
Corrections must be made to calculations for
installation in non-standard cavities.
TABLE 20-A
R-Value of Fiberglass Batts Compressed within Various Depth Cavities Insulation R-Value at Standard Thickness
R-Value 38 30 22 21 19 15 13 11 8 5 3 Standard Thickness 12" 9-½ " 6-¾ " 5-½ " 6-¼ " 3-½ " 3-5/8" 3-½ " 2-½ " 1-½ " ¾ " Nominal Lumber Sizes, Inches Actual Depth of Cavity, Inches
Correcting for Wood Framing
What is the U-factor of the previous
example if the wall is framed with
Douglas Fir studs spaced 16" on-center.
Through
Insulation 16" OC Stud Outside Air Film
4" Brick R-11 Batt Insulation 3-1/2" Fir Stud 5/8" Gypboard
Inside Air Film 0.68 0.68 Total R = Uo = Uavg = 0.20 x 0.188 + 0.80 x 0.078 = 0.100 Btu/(hr-S.F.-F) Through 0.56 0.56 3.50 11.0 0.40 0.40 0.17 0.17 12.81 5.31 0.078 0.188
What About Metal Studs?
Transmission “weighting”
is not appropriate if the
conductivity of the
penetration exceeds that
of the layer being
penetrated by more than
an order of magnitude.
Correcting for Metal Studs
ASHRAE Standard 90.1 Correction Factors
Size of
Insulation Correction
Members Framing
R-Value
Factor, F
c
2 x 4
16 in. O.C. R-11
0.50
2 x 4
24 in. O.C. R-11
0.60
2 x 6
16 in. O.C. R-19
0.40
2 x 6
24 in. O.C R-19
0.45
Using Metal Stud Factors
What is the heat loss of the wall in the
first example if the wall is framed with
3-1/2" metal studs spaced 16" on-center?
Ri Outside Air Film
4" Brick
R-11 + Mtl. Stud (11.0 x 0.50) = 5/8" Gypboard
