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