ME 425 – Heating Load Calculation
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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
� Duct Loss Building Pickup
ME 425 – Heating Load Calculation
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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
ME 425 – Heating Load Calculation
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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
ME 425 – Heating Load Calculation
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Air Film PropertiesFrom ’05F25.2
Air Gap PropertiesFrom ’05F25.4
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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
Insulation R-Values when Installed in a Confined Cavity
2 x 12 11-1/4 37 -- -- -- -- -- -- -- -- -- --
2 x 10 9-1/4 32 30 -- -- -- -- -- -- -- -- --
2 x 8 7-1/4 27 26 -- -- -- -- -- -- -- -- --
2 x 6 5-1/2 -- 21 20 21 18 -- -- -- -- -- --
2 x 4 3-1/2 -- -- 14 -- 13 15 13 11 -- -- --
2 x 3 2-1/2 -- -- -- -- -- -- 9.8 -- -- -- --
2 x 2 1-1/2" -- -- -- -- -- -- 6.3 6.0 5.7 5.0 --
2 x 1 3/4" -- -- -- -- -- -- -- -- -- 3.2 3.0
ME 425 – Heating Load Calculation
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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 0.400.17 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.
Isotherms
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Correcting for Metal Studs
ASHRAE Standard 90.1 Correction Factors
Size of Insulation Correction
Members Framing R-Value Factor, Fc
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
Inside Air Film 0.68
Total R =
Uo = 1/7.31 =
0.56
5.50
0.40 0.17
7.31
0.137 Btu
Hr-SF-deg F
ME 425 – Heating Load Calculation
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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.
ME 425 – Heating Load Calculation
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ASHRAE Window U-factors
ASHRAE Window U-factors
ME 425 – Heating Load Calculation
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Door U-Factors
Door U-factors can be found in Table 4, ‘05F31.11.
Floors on Grade
qslab
= F2 x Perimeter x ∆∆∆∆T
ME 425 – Heating Load Calculation
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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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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 & doorsweatherstripped
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-11batt 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.
ME 425 – Heating Load Calculation
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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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