MIL-HDBK-1003/19
4.3.6 Infiltration. Many older buildings have infiltration rates as
high as 1.5 air changes per hour (ACH). A reduction to 1.0 ACH may be
achieved by employing a plastic vapor barrier; taking care to seal all
joints and foam any cracks will generally further reduce the infiltration
rate to 0.5 ACH. It is strongly recommended that the infiltration rate be
limited to 0.5 ACH for both new construction and retrofits whenever
possible. Since extremely low rates may be hazardous to the occupants'
health due to the accumulation of indoor pollutants, further reductions in
infiltration heat loss should be attempted only through the use of window
heat recovery units. Extensive use of these units can yield effective
infiltration rates as low as 0.187 and under certain circumstances, the
additional expense involved may be justifiable.
4.3.7 Solar collection area. The solar collection areas recommended in
this section are intended to be used as starting point values for the design
analysis procedure discussed in 4.4 and 5.1; they are based on the following
assumptions:
a.
The recommendations presented in the preceeding sections on
insulation levels and infiltration rates are followed.
b.
The levelized heating fuel cost is .55/MMBtu.
c.
The heating efficiency is unity.
d.
The payback period is ten years.
The last three assumptions imply that the ratio of annual energy saved to
capital invested (E/C) is 5.4 MMBtu/K$. Furthermore, the system
productivity (PR) which is defined as the amount of energy saved annually
per square foot of collector, is given by the product of E/C and the system
dependent cost per square foot of solar collection aperture. For small
variations of the fuel costs from the assumed value of .55/MMBtu, the
aperture size may be adjusted at one-third the rate of fuel cost variation.
In other words, a 9 percent increase in fuel cost should be compensated for
by a 3 percent increase in aperture size.
Nine representative passive solar systems are included in the sizing
rules presented in this section. The nine systems and their associated
costs per ft2 of aperture are described in table II. Thermal storage mass
is characterized by the thickness in inches (THICK), and by the ratio of the
mass surface area to the area of the collection aperture (Am/Ac). For
sunspaces, the area of the collection aperture is taken to be the area
projected on a vertical plane. For all systems, the thermal storage
material is high density concrete.
Contour maps of recommended aperture size expressed as percent of floor
area are presented in sequence for each of the systems in table II in
figures 14 through 22. (Note: Large apertures occur where high solar
availability coincides with a large heat load. Small apertures occur where
the solar availability is low or the heat load is small.) These aperture
sizes, used in conjunction with the previously recommended insulation and
infiltration levels, will yield an E/C of 5.4 and a payback period of ten
years for the
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