MIL-HDBK-1003/13A
avoid having more than one high point. Pressure relief must be provided at
some point in each flow circuit. Check valves can be added to prevent
thermally induced gravity circulation. A flow-check valve (used in the
hydronic heating industry) will also accomplish the same purpose. Mixing
valves should be used to protect DHW systems from delivering water hotter
than specified (usually 120 deg. - 140 deg. F). Consideration should be
given to energy conservation by lowering DHW temperature whenever possible.
Often 105 deg.-115 deg. F will suffice if water is used only for showers and
washing hands.
2.10 Other considerations.
2.10.1 Architectural. Solar collector arrangements should be studied to
facilitate blending collector panels into the architecture of new or existing
buildings. Shade trees must be so located as not to cast shadows on the
collector. Other structures such as chimneys which can cast shadows should
be carefully located to avoid shading of the collector. Experience of
Florida installers indicates that if collectors are placed directly on the
roof, the life of asphalt shingles under the collector may be reduced by up
to 50%. This suggests that a small space should be left between the
collector and the roof, or the collector should be built into the roof. In
the latter case, the design must provide for simple glass replacement. A
space between collector and roof allows for snow to fall through rather than
accumulate. Collector mounting details, roof penetrations, flashing details,
pipe supports, thermal expansion provisions, etc., are best explained in
DM-3, the DOE Design Practice Manual (ref), NRCA Roofing Manual (ref), or NBS
Tech Note 1134 (ref).
2.10.2 Reduction of heat losses. Reduction of heat losses is usually one
of the most important steps in the design of a solar space heating system.
It almost always costs less to super-insulate a building to reduce losses
than to provide additional solar collector area to provide the extra heat.
Installing 12 or more inches of insulation in the attic, insulating existing
walls by injecting nonflammable foam (one manufacturer claims 30% reduction
in total heat loss at cost
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.00/ft2 floor area), multiple glazing, and
weatherstripping should all be evaluated for cost effectiveness versus a
larger solar system. NAVFAC policy currently does not permit the use of
injected foam of urethane formaldehyde. If the solar-augmented system is
found to be cost competitive with a conventional system on a life cycle cost
basis, then the cost effective amount of insulation will be the same for both
the solar and conventional systems. Thus the solar system should not be
charged for the cost of insulating the house.
2.10.3 Maintenance and accessibility. Systems should be designed for
minimum maintenance. Maintenance of glass will be minimized if vandalism can
be reduced. Collectors of flat-roofed buildings may be shielded from the
ground by a skirt around the roof perimeter. Locating the collector in the
backyard area of residences rather than on a street-facing roof reduces
probability of vandalism. Double strength glass for top surface can be used
in hail areas, and also provides protection from small stones. Still more
protection is offered by a screen of 0.5-inch mesh stretched several inches
above the collectors, but with some loss in collector efficiency (15%).
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