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```					ME 425 – Heating Load Calculation

ME 425
Air Conditioning System Design
Keith E. Elder, P.E.

The heating load calculation begins with the
determination of heat loss through a variety
of building envelope components and
situations.
Walls            Roofs
Windows          Doors
Basement Walls   Basement Floors
Infiltration     Ventilation
Duct Loss        Building Pickup

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ME 425 – Heating Load Calculation

The Heat Loss Equation

q = Uo x A ( Ti - To )

Where:
Uo = the overall heat transfer
coefficient, Btu/hr/ft2/°F
A = the surface area, ft2
Ti = the indoor design temperature, °F
To = the outdoor design temperature, °F

Heat Loss Equation Assumptions

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
Heat flow path “ignores” temperature variations
due to construction anomalies

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ME 425 – Heating Load Calculation

Solving the Heat Loss Equation

Area Takeoffs (from plans with est. zoning)
Design Temperatures (‘05F28 CD & Std. 55)
Assembly U-factors, Uo:
Uo   = 1/ RTotal
RTotal   =   R1 + R2 + Rn + ...

R1, R2 and Rn represent 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.

Construction Material Properties

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ME 425 – Heating Load Calculation

Air Film Properties
From ’05F25.2

Air Gap Properties
From ’05F25.4

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ME 425 – Heating Load Calculation

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                0.17
4" Brick                        0.40
R-11 Batt Insulation           11.00
5/8" Gypboard                   0.56
Inside Air Film                 0.68

Total R =                      12.81

Uo = 1/R

= 1/12.81

= 0.078            Btu
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    Actual
Lumber Depth of                  Insulation R-Values when Installed in a Confined Cavity
Sizes,  Cavity,
Inches   Inches
2 x 12    11-1/4    37     --      --      --     --       --     --      --       --      --    --
2 x 10     9-1/4    32     30      --      --     --       --     --      --       --      --    --
2x8        7-1/4    27     26      --      --     --       --     --      --       --      --    --
2x6        5-1/2    --     21      20      21     18       --     --      --       --      --    --
2x4        3-1/2    --     --      14      --     13       15    13      11        --      --    --
2x3        2-1/2    --     --      --      --     --       --    9.8      --       --      --    --
2x2       1-1/2"    --     --      --      --     --       --    6.3     6.0      5.7     5.0    --
2x1        3/4"     --     --      --      --     --       --     --      --       --     3.2   3.0

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ME 425 – Heating Load Calculation

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          Through
Insulation     16" OC Stud
Outside Air Film          0.17            0.17
4" Brick                  0.40            0.40
R-11 Batt Insulation      11.0
3-1/2" Fir Stud                            3.50
5/8" Gypboard             0.56             0.56
Inside Air Film           0.68             0.68

Total R =                12.81             5.31

Uo       =               0.078            0.188

Uavg     = 0.20 x 0.188 + 0.80 x 0.078

=    0.100      Btu/(hr-S.F.-F)

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.

Isotherms

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ME 425 – Heating Load Calculation

Correcting for Metal Studs
ASHRAE Standard 90.1 Correction Factors

Size of                   Insulation Correction
Members   Framing          R-Value Factor, Fc
2x4     16 in. O.C.       R-11      0.50
2x4     24 in. O.C.       R-11      0.60
2x6     16 in. O.C.       R-19      0.40
2x6     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                 0.17
4" Brick                         0.40
R-11 + Mtl. Stud (11.0 x 0.50) = 5.50
5/8" Gypboard                    0.56
Inside Air Film                  0.68

Total R =                       7.31

Uo   = 1/7.31 =                0.137      Btu
Hr-SF-deg F

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ME 425 – Heating Load Calculation

ASHRAE Window Performance
Because of
conductive
spacers,
frames & new
coatings
available,
window U-
factors cannot
be calculated
accurately.

ASHRAE Window Performance
Reasonable estimates can be found in
Table 4, ‘05F31.8.

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ME 425 – Heating Load Calculation

ASHRAE Window U-factors

ASHRAE Window U-factors

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ME 425 – Heating Load Calculation

Door U-Factors
Door U-factors
can be found in
Table 4,
‘05F31.11.

qslab = F2 x Perimeter x ∆T

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ME 425 – Heating Load Calculation

Floors on Grade - F2 Factors

Infiltration
The uncontrolled inward air leakage
through cracks and interstices in any
building element and around windows
and doors of a building, caused by the
pressure effects of wind and/or the
effect of differences in the
outdoor/indoor air density.

qinfiltration = 0.018Q∆T
Q = air flow due to infiltration, ft3/hr

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ME 425 – Heating Load Calculation

Infiltration Estimates
From the 1972 Fundamentals Manual:
Number of Air
Changes Taking
Kind of Room or Building                         Place per Hour
Rooms with no windows or exterior doors               1/2
Rooms with windows/exterior doors on one side         1
Rooms with windows/exterior doors on two sides        1-1/2
Rooms with windows/exterior doors on three sides      2
Entrance Halls                                        2

Note: Values may be reduced by 1/3 if windows & doors
weatherstripped

What is the heat loss for a Seattle office space 12
feet deep by 50 feet long, with 10 foot ceilings and
floor-to-floor spacing of 12 feet? The office zone is
located on the third floor of a five story building. The
wall is constructed of 6 inch concrete, with R-11
batt insulation installed between metal studs 16
inch on-center, and 5/8 inch gypsum board. The
window is insulated 1/4 inch glass with 1/2 inch air
space and composes 40 percent of the gross wall
area. The window framing is metal and has no
thermal break.

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ME 425 – Heating Load Calculation

Building & Space Sketch

50’                             12’

10’

12’

Section Sketch
Outside Air Film
6 Inch Concrete
R-11 Batt Insulation
between mtl studs 16” O.C.
5/8 Inch Sheet Rock

Inside Air Film

Total R =
Uwall   =

13

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