a solar sauna, office and hot tub SOLAR.KnowledgePublications.com

a solar sauna, office and hot tub
8 mar 1999
the local unitarian church believes in worshiping with water, and they
would like to build a minister's study. and some of the people at the
local training center in human relations seem interested in a hot tub.
they might enjoy this solar-warmed wigwam combination.

here's an 8' tall x 16' square structure with an 8x8' sauna and sunspace
and 6 1'x 2' double-pane windows. with r15 insulation, the sauna has
a thermal conductance of about 12ft^2/r2 = 6 btu/h-f for the windows,
308/15 = 21 for walls and roof, and 1achx512ft^3/55 = 9 for air leaks,
so it requires about 24h(100f-30f)36 = 60k btu to stay warm over a 24
hour 30 f day or 300k btu to stay warm for 5 cloudy days in a row, which
might come from p pounds of water in the sauna cooling from 120 to 80f,
where p(120-80) = 300k, so p = 7,500 pounds, about 120 ft^3 of sealed
water containers, say a bench 8' wide x 4' tall x 4' deep, heated and
cooled by sauna air. the sauna might contain a small woodstove...

the sunspace might be a quarter-cylinder with an 8' radius and an
8x16' footprint. it might have some overhead big fins for summer
shade and hot water.

                          ------------------r15--www-------
              .          |       8'       |       8'       |
          .       .      |                |                |
                         w     163 f      w      70f    8' w 
        .  hot tub  .    w       ?        w                w
                         w     sauna      w      room      w
          .       .      |                |                |
              .          |                |                |
                         |-------www-------------www-------|
                         |        .       .       .        | 
                         |        .       .       .        |
                         |        .       .       .        |
                         |        .    sunspace   .     8' |
                         |        .       .       .        |
                         |        .       .       .        |
                         |        .       .       .        |
                          ---------------------------------  
                                        south

over an average 6 hour 30 f january solar collection day in philadelphia,
with an average amount of sun (1,000 btu/ft^2 on a south wall and 620 on
a horizontal surface), the sunspace receives about 187k btu of sun and
provides about 10k btu for the room and loses about 6h(t-30)192ft^2/r1
to the outdoors through the glazing and 24h(t-30)36 via the sauna, so
187k-10k = 1416(t-30) and t = 163 f, in this linear model.

the room temperature might be controlled during the day by opening the
window between the room and the sunspace or opening an outdoor window
if the room gets too warm. the sauna-sunspace window might be wide open
whenever the sunspace is warmer than the sauna. at night, an open sauna-
room window might warm the room, cooling the sauna by 32kbtu/7500btu/f
= 4 f to 159. this could all be done automatically. 

the windows might be made from two layers of clear polycarbonate plastic.
the insulation might be 2 layers of 1.5" r7.5 styrofoam with latex paint
on the outside and plywood or cedar flakeboard inside. the sauna wall and
roof might have an extra layer of insulation. the rooftop might be an epdm
rubber pond which supplies an average of 8 gallons a day of rainwater for
a hot tub with a $100 ozonator (heliotrope general, (800) 552-8838)) in
the room or outside to the west of the sauna. a shallow pond south of the
sunspace might add about 30% more heat in midwinter and supply water for
local fauna and beautiful rippling solar reflections in the sunspace
when it is not frozen.

nick