MIL-HDBK-1003/19
Next, determine the building time constant given by:
[tau] = 24 [multiplied by] DHC/(NLC + 24 [multiplied by] Uc
[multiplied by] Ac)
(Equation 5.14)
All parameters in this equation are available from the first three
worksheets.
Finally, the effective thermostat setpoint is obtained from the
relation:
Te = T1 - e -0.1[tau]/P (T1 - Tave)
(Equation 5.15)
Use Te in place of Tset whenever a night time setback strategy is
employed.
5.1.6 Base temperature worksheet. Worksheet 4 is provided to help the
user determine the base temperature for either a constant thermostat setting
or for a night time setback strategy as outlined in the preceeding section.
All of the equations needed are provided on the worksheet. Remember that
Qint is the internal heat generation rate in Btu/day by people, lights,
and appliances. Unless other information is available, use Qint = 20,000
Btu/day per occupant.
5.1.7 Weather parameters. Having recorded the NLC on Worksheet 2, the
system parameters (F, G, Uc, and [alpha]) on Worksheet 3, and the base
temperature on Worksheet 4, evaluate the weather parameters that are needed
for design analysis of passive solar heating systems. The required
parameters are the transmitted radiation to degree day ratio (VTn/DD) and
the city parameter (a). These quantities are tabulated for 210 cities in
the continental United States in Appendix B. Provision is made for
obtaining parameter values for single, double, or triple glazed systems
operating at base temperatures ranging from 30deg.F to 80deg.F. The solar
aperture may depart from true south by 60 degrees to the east or west and
may be tilted 60 degrees from the vertical. Use of the tables in Appendix
B is discussed in the following subsections.
5.1.7.1 Transmitted radiation to degree day ratio. First, locate the
city of interest in Appendix B. The locations are alphabetized, first by
state and second by city within each state. Next, locate the column with
the appropriate value of the base temperature Tb. Base temperatures
ranging from 30deg.F to 80deg.F are provided; interpolation may be required.
Having located the correct column, read and record the value from the row
labeled VT1/DD, VT2/DD, or VT3/DD, depending on whether the system of
interest is single, double, or tripled glazed. (Note: The minimum monthly
value of VTn/DD is tabulated in these columns and the reference month (m) is
indicated in parentheses.) If the symbol NA (not applicable) appears, it is
an indication that, for the specified base temperature, solar heating is not
required.
5.1.7.2 City parameter. The city
parameter (a) is obtained from the
same column in which VTn/DD was found;
again, interpolation may be required.
The number is read from the row marked
"PARAMETER A" under the reading
"SOUTH-VERT". The adjustment required
for off-south or tilted apertures is
discussed next.
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