F. Room-by-room Heat Losses

To determine the portion of the total load to be installed in each room, the use of Table 6 is recommended, with the following procedures:

1. Insert in Column 1 the room designation.

2. Insert in Column 2 the total linear feet of exposed outside wall for each room. Exposed outside walls of closets and other areas without heating units should be added to those of the room into which they open.

3. Insert in Column 3, window and door square feet, the value determined as follows:

a. Actual number of square feet of windows glazed with double glass (Thermopane, Twindow, storm windows, or equivalent), and of wooden doors.

b. Twice the number of square feet of windows if single glazed, and of metal doors.

c. Two-thirds the number of square feet of windows if triple glazed.

d. Glass and door areas may not all meet any single situation described in a, b or c. Equivalent areas for the portion under each circumstance should then be calcu

lated and the sum of all inserted in Column 3. No part of the area should appear in more than one situation; all glass and door areas should appear in some one of the situations.

4. Add Columns 2 and 3 for each line, and insert the sum in Column 4. (Column 2 + Column 3= Column 4.)

5. Total Column 4. Divide the value shown for each room in Column 4 by that total, and enter the quotient in Column 5 for that room. (Column 4 value divided by total for Column 4 = Column 5.)

6. Multiply the value in Column 5 by the total building load (determined as described in Part 2, par. D) to determine the heating load for each room, and enter the product in Column 6.

7. Select units to provide the required capacity (Column 6) for each room. Enter the name or rating of the units selected in Columns 7 and 8.

8. Insert in Column 9 the total capacity of the unit or units in each room (Columns 7 and 8).

A. Formula

B. Experience Factor (C)

The value of the experience factor (C) to be used in Equation 4 is normally secured from the local power supplier. The power supplier determines the appropriate value from experience and judgment. Individual applications may be expected to deviate somewhat from the recommended value because of a number of variables on each installation which may affect the factor actually experienced. Some of these variables are:

1. Living habits of the occupants:

a. How frequently the outside doors are opened.

b. How many people live in the house and how much they use various appliances will affect the internal heat gains.

c. What inside temperature is maintained. This may vary from 65 to 77 F or wider. d. Whether occupants are away for extended periods with heat turned down.

e. Whether heat is turned off in rooms not

in use.

2. Orientation and design of the particular house.

exposures.

Heat loss is affected by wind Solar heat gain is affected by roof overhang, shading from trees, hills, and surroundings.

3. Actual weather conditions may vary considerably from year to year compared to the aver age used in estimates.

4. The performance of insulation and the effectiveness of weather-stripping and calking is affected by workmanship and sometimes by deterioration.

5. The type of equipment chosen for the partic ular application.

It is impractical to take all variables into account in estimating energy consumption. Local averages provide a good index to anticipated usage. One or two percent run over 10 percent high and about 5 percent run 15 percent lower than averages for a locality.

Areas without local experience should note that experience with hundreds of thousands of installa tions has shown a conservative figure of 17 for C when three-quarters of an air change per hour is figured for infiltration and exhaust, and 18.5 when half an air change per hour is figured.

The local power supplier should be consulted for the recommended value of C. In comparing results of actual applications to determine the C-factor experienced, care should be taken that the same infiltration rate is used for calculating the heat loss

in all cases.

Parts 1, 2 and 3 of this Manual presented procedures for determining heat losses of walls based on the installed resistance of the insulation (R/) used in the wall. Values were given for similarly determining the heat losses of other exposed sections. All procedures were based on insulation levels approximately as recommended in the "All-weather

Thermal Performance Recommendations" (see Part 1, par. G).

The appendixes provide procedures for more exact determination of heat losses in any construction and should be used whenever the insulation used varies materially from that recommended in the "Allweather Thermal Performance Recommendations."

The Fundamentals and Equipment Volume of the ASHRAE Guide and Data Book should be consulted for a comprehensive discussion of the principles of heat transfer (Chapter 4, pages 49 to 74, in the 1965 edition); for design heat transmission coefficients (Chapter 24, pages 417 to 454, in the 1965 edition); and for thermal insulation and water vapor barriers (Chapter 22, pages 381 to 404, in the 1965 edition).

This preamble is presented to call attention to the complexity of some construction methods and the difficulty of calculating their transmission losses. When metal layers are included in the section construction, and especially when internal metallic structure is bonded on one or both sides to a metal skin, special problems of lateral heat flow com plicate the calculation. A reliable guarded hot box test of transmission values is recommended in such cases. Where tests are impractical, calculation by procedures given in the ASHRAE Guide and Data Book should be carefully made. Most constructions are adequately handled by the procedures which follow, beginning with Appendix B.

Building heat transmission losses have been determined, based on the steady-state overall coeffi cient of heat transfer. Heat transfer through building or insulating materials can take place by conduction, convection or radiation. Ordinarily, all three are involved, with one or two predominating. The solution to problems is much simpler if the concept of thermal circuit and thermal resistance is em. ployed in connection with the overall coefficient of heat transfer. The overall coefficient of heat transfer for thermal transmittance), frequently called the U-value, may be determined by test or may be computed from known values of the thermal conductance of the various components.

By definition: U = 1/R↑

(Equation 2, page 12)

+ (Equation 5)

In many practical installations, components of the building section are arranged so that parallel heat flow paths of different conductances result. If there is no lateral heat flow between paths, each path may be considered to extend from inside to outside, and the U-value may be calculated using Equations 2 and 5. The average U-value is then:

U1s = a (U2) + b (Up) + ... + n (Un)

(Equation 6) where a, b... n are respective fractions of a typical basic area composed of several different paths whose U-values are U„, Up ... ... . Un.

If heat can flow laterally in any continuous layer so that isothermal planes result, the total average resistance RTY will be the sum of the resistance of the layers between such planes, each layer being calculated by Equation 5 or a modification of Equation 5 (using the resistance values), whichever is appropriate. This is a series combination of layers of which one or more provide parallel paths.

The calculated U-value, assuming parallel heat flow only, is considerably lower than that calculated with the assumption of combined series-parallel heat flow. The actual U-value will be some value between the two calculated values. If the construction contains any highly conducting layer in which lateral conduction is very high compared to the transmittance through the section, a value closer to the series-parallel calculation should generally be used. If there is no layer of high lateral conductance, a value closer to the parallel heat flow calculation should generally be used.

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A. Resistances (R) of Building Materials,

Films and Spaces

Tables 7 and 8 give the resistance (R) per inch of thickness for homogeneous materials, or for the thickness listed for nonhomogeneous materials, air films and air spaces. Adding all R resistances for each layer making up the section provides the total resistance (RT) of the building section.

B. Disregarding Framing

The great majority of building sections are not complicated by significant parallel paths and lateral layers of highly conductive material. The total resistance for such sections may be determined by adding the resistances of the various materials through which the heat must pass in going from inside to outside. A sketch (such as Figure 2, page 7) may be helpful in visualizing these materials. Then a list of the materials should be made, appropriate values of resistance applied to each (see Tables 7 and 8), and the sum of resistances determined. For example, if a wall consisting of 8-inch lightweight aggregate concrete block and face brick with 11⁄2 inch cavity between, filled with expanded vermiculite insulation, is to be used with paint on the block as the inside surface, the resistances would be:

It can be seen that the resistance of corrugated metal sheets can be disregarded in calculations.

C. Correction for Framing

1. Thermal Resistance Method - Insulated building sections usually have the spaces be tween inside and outside surfacing materials and structural supports partially or completely filled with insulation. Actually, there are parallel paths for heat flow between the sur. facing materials. They are (a) through the insulation (and air spaces, if any) and (b) through the structural framing materials.

In frame construction, the structural framing usually consists of nominal 2-inch X4inch lumber. For a wall with 16-inch stud spacing, there are 15-inch-wide face studs from floor to ceiling each 16 inches of wall length; there is a similar 15-inch-wide face of sole plate extending along the length at the bottom, a 15-inch-wide face of fire block be tween studs, and two 15-inch-wide faces in the top plate (one 2 inch X4 inch on top of the other). Where windows or doors occur, there is additional framing, usually involving a double lintel, double studs and perhaps a double sill, but omitting, of course, studs in the space occupied by the window or door.

If a sketch is made of the framing involved, the area covered by the structural members can be determined, and the heat loss per degree F for that part of the section deter mined. The heat loss for the windows and doors are similarly determined separately. For the balance of the section, heat flows through the insulation.

For example, for a wall (with no windows or doors) 12 feet long and 8 feet high, with 2-inch X4-inch framing on 16-inch centers, a total of about 122.6 linear feet of 1%-inch wide face is occupied by the framing. Conse quently, an area of about 16.7 square feet of framing and 79.3 square feet of insulation make up the total area of 96 square feet.