heat loss hvac cnst 305 environmental systems 1 dr. berryman
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Heat Loss
HVAC
CNST 305 Environmental Systems 1Dr. Berryman
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Heating Load Componentsroofroof
exteriorexteriorwallwall
glassglassconductionconduction
floorfloor
Infiltration Infiltration and and VentilationVentilation
partitionpartitionwallwall
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Design Temperatures Inside
Outside
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Design Weather Data
St. Louis,MissouriSt. Louis,Missouri
DBDB DBDB
99.6%99.6% 99%99%
2°F[-16.7°C]
2°F[-16.7°C]
8°F[-13.4°C]
8°F[-13.4°C]
1997 ASHRAE Handbook–Fundamentals, Chapter 26
St. Louis,MissouriSt. Louis,Missouri
DB WBDB WB DB WBDB WB DB WBDB WB
0.4%0.4% 1%1% 2%2%
95°F[35°C]
95°F[35°C]
76°F[25°C]
76°F[25°C]
93°F[34°C]
93°F[34°C]
75°F[24°C]
75°F[24°C]
90°F[32°C]
90°F[32°C]
74°F[23°C]
74°F[23°C]
Summer
Winter
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Conduction Heat Losses
BH = U A T
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Rates of Heat Transfer
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Air Spaces
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Determining Rates
thermal resistance (R)
outdoor-air film
AL siding (hollow back)
8” LW concrete block
R13 insulation board
½”gypsum board
indoor-air film
Table 2-2 (Toa)
Table 2-3
Table 2-4
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Heat Loss Calculation
EXAMPLE Using wall section:
Winter Design: 10oF DB Indoor Design: 75oF 100’ long; 10’ high
Determine Heat Loss
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Degree Day Heating load is proportional to the
temperature difference between 65oF and the average daily winter outdoor temperature.
• Example: Omaha, NE
65oF - -8oF = 73 degree days
Winter Design = -8oF w/ 6612 degree days*
*From ASHRAE Handbook of Fundamentals
6612/73 = 90.6 heating days90.6 x 24hrs = 2174 heating hours/year
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Floors – Slab on Grade
Mild Cold
Climatic Conditions
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Basements Heated
basements:
Unheated basements
• Use 50oF for OA design temperature
• Calculate R-values through floor
Note: Any exposed wall above grade has to be calculated a different wall section
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Pier & Beam (crawl space) Same as
wall/roof Use Design OA
temperatures Calculate R-
values through floor
Use OA Design Temperature
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Other Heat Losses Wall Roof Floor Windows Doors Infiltration Ventilation
BTUH Total
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Sample Heat Load Form
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Assignment Wall Section
Sketch (on graph paper)• Show in one section: roof, wall, floor• Provide dimensions, material call-outs
Determine rates of heat transfer• R values of each construction• R Total• U value
Determine indoor/outdoor design temperatures
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Next Time Heat Transmission Condensation Vapor Barriers