heat loss hvac cnst 305 environmental systems 1 dr. berryman
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Heat Loss
HVAC
CNST 305 Environmental Systems 1Dr. Berryman
Heating Load Componentsroofroof
exteriorexteriorwallwall
glassglassconductionconduction
floorfloor
Infiltration Infiltration and and VentilationVentilation
partitionpartitionwallwall
Design Temperatures Inside
Outside
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
Conduction Heat Losses
BH = U A T
Rates of Heat Transfer
Air Spaces
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
Heat Loss Calculation
EXAMPLE Using wall section:
Winter Design: 10oF DB Indoor Design: 75oF 100’ long; 10’ high
Determine Heat Loss
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
Floors – Slab on Grade
Mild Cold
Climatic Conditions
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
Pier & Beam (crawl space) Same as
wall/roof Use Design OA
temperatures Calculate R-
values through floor
Use OA Design Temperature
Other Heat Losses Wall Roof Floor Windows Doors Infiltration Ventilation
BTUH Total
Sample Heat Load Form
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
Next Time Heat Transmission Condensation Vapor Barriers