ME 425 – Heating Load Calculation
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
Windows
Exposed Floors
Basement Walls
Infiltration
Duct Loss
Roofs
Doors
Slab-on-Grade Floors
Basement Floors
Ventilation
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
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
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?
Outside Air Film
4" Brick
R-11 Batt Insulation
5/8" Gypboard
Inside Air Film
Ri
0.17
0.40
11.00
0.56
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
Standard Thickness
Nominal
Actual
Lumber Depth of
Cavity,
Sizes,
Inches
Inches
2 x 12
11-1/4
2 x 10
9-1/4
2x8
7-1/4
2x6
5-1/2
2x4
3-1/2
2x3
2-1/2
2x2
1-1/2"
2x1
3/4"
38
30
12" 9-½ "
22
21
19
15
13
11
8
5
6-¾ " 5-½ " 6-¼ " 3-½ " 3-5/8" 3-½ " 2-½ " 1-½ "
3
¾"
Insulation R-Values when Installed in a Confined Cavity
37
32
27
------
-30
26
21
-----
---20
14
----
---21
-----
---18
13
----
----15
----
----13
9.8
6.3
--
----11
-6.0
--
------5.7
--
------5.0
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.
Outside Air Film
4" Brick
R-11 Batt Insulation
3-1/2" Fir Stud
5/8" Gypboard
Inside Air Film
Through
Insulation
0.17
0.40
Through
16" OC Stud
0.17
0.40
11.0
0.56
0.68
3.50
0.56
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)
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.
Isotherms
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ME 425 – Heating Load Calculation
Correcting for Metal Studs
ASHRAE Standard 90.1 Correction Factors
Size of
Members
2x4
2x4
2x6
2x6
Framing
16 in. O.C.
24 in. O.C.
16 in. O.C.
24 in. O.C
Insulation Correction
R-Value Factor, Fc
R-11
0.50
R-11
0.60
R-19
0.40
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 31/2" 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
0.56
5/8" Gypboard
0.68
Inside Air Film
Total R =
Uo
= 1/7.31 =
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 Ufactors 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.
Floors on Grade
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
In-Class Heating Load Exercise
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
=
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