Sizing Systems Using Coefficients of Heat Transmission
Calculating heat loss from a structure requires a thorough knowledge of the thermal properties of many materials and combinations of materials used in its construction. The term thermal property is used here to mean the overall coefficient of heat transmission (i. e., the rate of heat flow through a material). Each construction material (or combination of materials) will have its own coefficient of heat transmission. Before continuing any further, it would be advisable to review the sections of Chapter 3, “Insulating and Ventilating Structures,” that specifically pertain to the problem of heat loss, particularly Principles of Heat Transmission.
ASHRAE and other authorities suggest the following basic steps (in the sequence given) for calculating heat loss:
1. Decide upon the desired inside air temperature for the structure.
2. Obtain the winter outside design temperature for the location of the structure from published lists or a local weather bureau.
3. Determine the design temperature difference (the difference between the temperatures found in Steps 1 and 2).
4. Identify on the heat loss worksheet each room or space in the structure.
5. List every structural section in each identified room or space that has an outer surface exposed to the outdoors or to an unheated or partially heated space.
6. Determine the coefficient of heat transmission (U-value) for each structural section (e. g., walls, glass, ceiling).
7. Calculate the infiltration heat loss for each identified room or space.
8. Calculate the total area for each exposed surface (i. e., total outside wall area, total floor area).
9. Calculate the area for each door and window in the walls and add these figures together to obtain the total area for these openings.
10. Subtract the total area for doors and windows from the gross outside wall area (obtained in Step 8) to determine the total net outside wall area. Enter the amount on the heat loss worksheet.
11. Multiply the total net wall area determined in Step 10 by the U-value (Step 2) by the design temperature difference (Step 3) to obtain the total heat loss for walls (expressed in Btu/h).
12. Make the same calculations for the other surface areas (floors, ceilings, etc.) determined in Step 8.
13. Add the heat loss figures calculated for each surface category and the infiltration heat loss to obtain the total heat loss for the identified room or space.
14. Repeat the procedure outlined in Steps 1 to 11 for each identified room or space in the structure.
15. Add the various totals to obtain the total heat loss from the structure (expressed in Btu/h).
These 15 basic steps for calculating heat loss are provided as a useful means of reference for the more detailed description of the procedure contained in the paragraphs that follow.