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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 thedetermination of heat loss through a varietyof building envelope components andsituations.
Walls Roofs
Windows Doors
Exposed Floors Slab-on-Grade Floors Basement Walls Basement Floors
Infiltration Ventilation
Duct Loss Building Pickup
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The Heat Loss Equation
q = Uo x A ( Ti - To )
Where:
Uo =the overall heat transfercoefficient, Btu/hr/ft2/F
A = the surface area, ft2
Ti = the indoor design temperature, FTo = 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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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 ofthe elements in the path of the "heat flow.
Properties of typical construction materials can be foundin Table 3 of 05F25.
Construction Material Properties
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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 wallcomposed of 4 inch brick, R-11insulation 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 bymanufacturers for predetermined depths.Corrections must be made to calculations forinstallation 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- " 6- " 5- " 6- " 3- " 3-5/8" 3- " 2- " 1- " "
Nominal
LumberSizes,
Inches
Actual
Depth ofCavity,
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
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Correcting for Wood Framing
What is the U-factor of the previousexample if the wall is framed withDouglas 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 thefirst 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
0.400.17
7.31
0.137 Btu
Hr-SF-deg F
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ASHRAE Window Performance
Because ofconductivespacers,frames & newcoatingsavailable,window U-factors cannot
be calculatedaccurately.
ASHRAE Window Performance
Reasonable estimates can be found inTable 4, 05F31.8.
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ASHRAE Window U-factors
ASHRAE Window U-factors
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Door U-Factors
Door U-factorscan be found inTable 4,05F31.11.
Floors on Grade
qslab
= F2 x Perimeter x T
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Floors on Grade - F2 Factors
Infiltration
The uncontrolled inward air leakagethrough cracks and interstices in anybuilding element and around windowsand doors of a building, caused by thepressure effects of wind and/or theeffect of differences in theoutdoor/indoor air density.
qinfiltration = 0.018QT
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 12feet deep by 50 feet long, with 10 foot ceilings andfloor-to-floor spacing of 12 feet? The office zone islocated on the third floor of a five story building. Thewall is constructed of 6 inch concrete, with R-11batt insulation installed between metal studs 16inch on-center, and 5/8 inch gypsum board. Thewindow is insulated 1/4 inch glass with 1/2 inch air
space and composes 40 percent of the gross wallarea. The window framing is metal and has nothermal break.
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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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