Our
household of 2 adults and three children obtained all our
household hot water from a composting greenhouse we constructed
in Portland, Oregon in 1994. It provided hot water at a
temperature of 90-130 degrees (Fahrenheit) continuously until it
was dismantled 18 months later. We used the space to grow several
species of mushrooms and to house plants from our garden during
winter.
The greenhouse
design was similar to inexpensive "tube" greenhouses.
Outer dimensions were 16x30 feet. The foundation walls consisted
of 3 courses of rye grass straw bales pinned together with 1/2
inch steel rebar. Bale size was 2 feet x 2 feet x 4 feet, giving
two-foot thick walls along the base. Therefore inner dimensions
were 12 feet wide by 26 feet long. Bales were stacked like
bricks, as is typical of straw
bale construction. A layer of 3 mil plastic film surrounded
the bottom bales, separating the straw from a layer of wood chips
on which the bales rested and the compost which filled the
greenhouse about three feet deep inside (except for a 5 feet by
12 feet entry at one end). The roof consisted of 6 mil
ultraviolet resistant plastic film supported on 20 foot arches of
rebar spaced every 2 feet along the length of the structure.
These arches were held rigidly into a 2 feet x 2 feet matrix with
horizontal rebar spaced every 2 feet running the length of the
structure. The straw bales on the sides and end walls were also
covered with the same plastic film as the roof with a door framed
out of lumber at one end. A single sheet of 32 feet wide by 32
feet long plastic covered the roof.
Two
PVC 3/4 inch water lines ran underground from the house to the
greenhouse. The cold water supply originated at the washer hookup
cold line. Hot water returned from the greenhouse in an insulated
line after circulating in the hot compost and entered the house
plumbing at the washer hot water hookup. Therefore no
modifications to the original house plumbing system were
required. While the greenhouse heater was operative, the original
hot water heater was turned off and its intake valve closed. Heat
exchange occurred in the compost in which was embedded one
hundred feet of coiled 1.5 inch internal diameter plastic hose.
Compost mass totaled 3 feet deep, 12 feet wide, and 21 feet long,
or approximately 28 cubic yards. It required replenishing several
times during its lifetime because of continual slow
decomposition.
The
total amount of hot water contained in the hose inside the
compost (comprising a cylinder 100 feet long by 1.5 inch
diameter) was 9.17 gallons. This (when mixed with appropriate
cold water) was an adequate volume to take 3 quick showers
without running out of hot water.
The
compost biomass consisted of wood chips and other ground tree
material run through a chipping machine. This material is
delivered to our site free of charge from many tree service
companies. We supplemented this primarily high-carbon matter with
high-nitrogen matter from household waste such as garden debris,
kitchen compost, and manures. Eventually, when the greenhouse
was dismantled to reclaim our back yard as a garden area, we had
enough finish compost to cover our entire yard 8 inches deep.
Needless to say, we have a fabulous garden from this new soil
fertility.
Return to Experiments in Sustainable Urban Living / Feedback welcome / revised May 16, 2005