Reduction of heat losses

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Design: Solar Heating of Buildings and Domestic Hot Water

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 $1.00/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%).