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Vital Signs
Project: Siegel House Case Study
Background: The Design of the Siegel House
This page describes the design of the Siegel House. The first section, Basic
Elements, summarizes the major design elements of the house and briefly describes the
climate in Davis, California. The second section, Passive Solar
Features, explains the passive solar design implemented in the house.
Basic Elements
 View of the
two-story living room in the Siegel House,
looking southeast into the sunspace
The Siegel House was designed by architect Michael Corbett as part of the Village Homes subdivision in Davis, California, and was built in
1978. The house was built with no central heating or air-conditioning. The intent was to
let a passive solar design keep the house comfortable, with a small wood-stove providing
heating during the stretches of cool, cloudy days that are common in Davis winters.
Davis has a demanding climate for passive solar design. It has both a distinct heating and
cooling season--2844 heating degree days (65 F), and 1171 cooling degree days (65 F).
Temperatures there in the winter often dip below 35 degrees fahrenheit, and summer
temperatures commonly rise to above 95 degrees fahrenheit.
Plans of the Siegel House first and second floors
This house is a modest 2000 square feet, with 2 baths, a first floor master bedroom, and
two bedrooms upstairs. The house is built as slab-on-grade construction, with no basement.
A large sunspace occupies most of the south wall of the house. And inside, a two-story
living room provides the main gathering place for the residents.
Section drawing of the Siegel House
Passive Solar Features
The Siegel House is designed primarily around a passive solar design scheme that uses a
thermosiphon loop. In a thermosiphon loop, the buoyancy of warm air is used to circulate
heat from a collector area to a storage area. The Siegel House uses a sunspace collector,
an air-loop, and rock-storage as the main elements in its thermosiphon loop design.
 Section diagram of the thermosiphon
loop in the Siegel House (10 k gif)
The three main components in the thermosiphon loop are shown in the diagram at left.
(Click on the diagram for a more detailed view.) Air heated during the day in the sunspace
rises to the top of the sunspace and travels up through the ducts to the top of the
rock-filled tubes. This air is pushed down the tubes by more hot air coming behind it from
the sunspace. The air then cools as it passes through the rocks. It begins to fall faster
as it becomes cooler, in turn pulling some of the hot air above with it.
Another way to describe this system is that the motion in the loop is driven by pressure
differences created by warmer air in the sunspace and cooler air in the rock storage.
Warmer air from the sunspace has a higher pressure than the colder air in the rock
storage, and air will flow to eliminate this difference.
It is important to note, though, that this circulation occurs only when the pressure
difference is greater than the resistance in the loop created by the rocks and any bends.
Another important fact to note is that flow in this loop will reverse at night when the
sunspace is cooler than the rock storage.
The three loop elements are described in more detail below.
Sunspace
 Interior view of Siegel House sunspace showing the
south wall (with glazing) and west wall
The sunspace collects solar energy through southern, eastern, and overhead double-pane,
aluminum framed glazing. On the south wall, there is 140 sq. ft. of glazing, on the east
wall there is 60 sq. ft. of glazing, and on the roof, slanted at 18 degrees from
horizontal, there is 112 sq. feet of glazing. There are no shading devices in place for
any of the vertical glass. Light through the skylights can be moderated with translucent
white blinds that operate from inside the sunspace. On the original plans, a white
masonite cover was detailed for summer use on the exterior of the skylights, but no cover
like this was in use at the house when we visited in February.
Ductwork
 The grille on one of the three ducts
in the sunspace floor
Ducts in the ceiling and floor of the living room provide the path by which air moves from
the sunspace to rock storage and back. Hot air from the sunspace is carried to rock tubes
where the heat from the air is stored. As energy is transferred from the air to the rocks,
the cooled air "falls" and is carried via the ducts in the floor back to the
sunspace. No fans are included to help create airflow through the ducts.
"Rock Tubes"
 Steel tubes filled
with river rock
that act as thermal storage in the Siegel House
A series of seven two-story steel tubes filled with river rock act as the thermal storage
element in the loop. The tubes, filled with fist sized rocks, are 14 feet high by 3 feet
in diameter, and are painted white. The tubes are surrounded on the west, north, and east
sides by 2 by 4 framed walls. They sit on the slab of the first floor and extend upward
along the wall of the two-story living room.
The tubes are easily the most dominant feature in the space. They create a very dramatic
wall, but to many of us they looked a little dated.
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