FACTORS EFFECTING HEAT LOSS
Actual heat loss calculations are much more involved than as described in this section. Several important factors such as "air infiltration" rate are not discussed here. However, the information provided will give you a basic understanding of the steps used to calculate heat loss.
To make a heat loss calculation, several pieces of information are needed from the home itself. Total exposed wall area, exposed foundation area, and ceiling area. Then, "U" Factors (the number of heat BTUs passing through a specific area of the home in one hour) and values for "UA" (Relative Heat Loss) and "DTD" (Design Temperature Difference) must be determined.
Calculating Wall, Foundation and Ceiling Areas
Total Exposed Wall Area: The front and back wall areas, plus the two side areas equal the total exposed wall area. For example, if a one-story house is 30 feet long and eight feet high, the front wall area (30 x 8) is 240. You will also have to consider the areas of windows, doors, sliding-glass doors, fireplace walls, wall framings (10 percent of the total wall area), and rim joist areas (the parallel arrangement of wood beams that support the floor). To determine the rim joist area, measure the perimeter of the house and multiply by the height of the rim joist.
Exposed Foundation Area: The square footage of all visible foundation surface plus the total foundation window area is the exposed foundation area.
Ceiling Area: This is the depth of the house multiplied by its width. The portion of a ceiling which is cathedral (pitched) has about 10 percent more area than a flat ceiling.
Determining "DTD," "U," and "UA"
"DTD" Values: The "DTD" Value (Design Temperature Difference) is the difference between the desired indoor temperature and the anticipated coldest winter temperature. If the coldest winter temperature in your area is minus 9 degrees Fahrenheit, and the desired indoor temperature is 68 degrees Fahrenheit, the difference between these two temperatures is the design temperature difference, or 77 degrees Fahrenheit.
"U" Factors: The number of BTUs that will pass through a square foot section of a building in one hour for each one degree Fahrenheit of temperature difference is known as the "U" Factor. Both the Air Conditioning Contractors of America (ACCA and the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) overprinted tables to determine the "U" Factor. A partial list of typical "U" Factors is given below.
| Building Section "U" Factors | |
| Standard Frame Wall (Uninsulated) | 0.23 |
| Standard Frame Wall (Insulated) | 0.07 |
| Glass Window, Single | 1.10 |
| Glass Window, Double 1/4" Air Space | 0.63 |
| Glass Window, Single Width Storm | 0.55 |
"UA" Values: To determine the "UA" Value, or relative heat loss of each building section, multiply the area of each section by the stated "U" factor.
The Heat Loss Calculation
Multiply the area of exposed building section by the corresponding "U" Factor to obtain the "UA" Value. Then multiply the "UA" Value by the "DTD" Value to obtain the BTU heat loss per hour of the building section. The sum total of the BTU heat loss values per building section is the BTU heat loss per hour for the entire house (An example is provided below).
For correct results, factor relative to air infiltration and the heat distribution system must be considered. For these reasons, a proper calculation can only be made by a professional HVAC technician. Consult with an experienced heating, venting and air conditioning contractor for the actual results.
| Building Section | SQ.FT. | x "U" Factor | = "UA" Factor | x "DTD" Value | = HEAT LOSS PER SECTION |
| Window Area | 64 | .55 | 35 | 77 | 2,695 |
| Door Area | 41 | .23 | 10 | 77 | 770 |
| Wall Area | 1120 | .07 | 78 | 77 | 6,006 |
| Framing Area | 112 | .23 | 26 | 77 | 2,002 |
| Rim Joist Area | 140 | .40 | 56 | 77 | 4,312 |
| Foundation Area | 300 | .18 | 50 | 77 | 3,850 |
| Ceiling Area | 1200 | .07 | 84 | 77 | 6,468 |
| BTU HEAT Loss for the Entire Home | 26,103 | ||||
