MIL-HDBK-1003/19

where the subscript i has been dropped because only a single system is

present. As a general rule, the corrections for azimuth do not become

significant until the departure from due south approaches +/-30 degrees.

If two systems having either different numbers of glazings or different

orientations are employed it will be necessary to determine the weather

parameters for the second system using the blanks provided. Then the

weather parameters for the two systems are area weighted using the formula

provided on the worksheet.

5.3.7 Auxiliary heat requirements. The auxiliary heat requirements of

the building are calculated using Worksheet 6 which is reproduced for the

example calculation. The scaled solar load ratio (SLR*) is computed from

parameters previously recorded on other worksheets and found to be 0.64.

Using this value and the city parameter (a) from Worksheet 5, the annual

heat to load ratio is read from the nomograph in figure 23 as 0.37.

Finally, using the formula at the bottom of the worksheet, calculate an

annual auxiliary heat requirement of 32.6 MMBtu for the four-plex unit.

Dividing this figure by the floor space of 6800 ft2 and the annual heating

degree days of 2778 yields an auxiliary heating factor of 1.73 Btu/ft2-DD.

5.3.8 Distribution of the solar aperture. In general, the total solar

aperture of a multi-family unit should be distributed in a manner that

provides greater solar gains to the sections of the unit that experience the

greater loads. We can accomplish this by performing the calculations

presented herein once for each unique thermal zone within a unit. The

worksheets are set up to allow this procedure by entering appropriate values

for the heated floorspace and using the specialized definition of total

perimeter (Pt) that excludes partitions between distinct thermal zones.

However, in many cases the much simpler procedure described below is

adequate.

On Worksheet 2 we determined that the four-plex unit has a total NLC of

28,248 Btu/DD. Each of the four sections, therefore has, on the average, a

NLC of 7,062 Btu/DD, or one fourth of the total value. The average NLC

value must be adjusted to account for the different loss characteristics of

the two unique thermal zones that exist in the four-plex units. The two

outer sections will have a larger loss coefficient than the two interior

sections which have two shared or common side walls. It is assumed that a

negligible amount of heat is transferred through these common walls because

only small temperature differences are likely to exist from one side to the

other. The exterior side walls on the end sections, however, lose heat to

ambient conditions that may be quite cold.

We can easily calculate the loss characteristics of the end walls using

the equations on Worksheet 2. The end wall area i:s

Aw = 18 x 23 = 414 ft2

.

Note that we have assumed that there are no windows on the end walls.

The

load coefficient of the wall is therefore:

LCw = 24 Aw/RWALL = 24 x 414/21 = 473 Btu/DD

80

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