Heating A Greenhouse Using Solar Heat and Compost Heat

MOTHER's bioshelter greenhouse uses both solar heat and compost heat, housing plants as well as rabbits and chickens. The article includes information on structure, notes on construction, and floor plans.

| July/August 1986

Reprinted from MOTHER EARTH NEWS NOs. 92 and 93. 

This quality greenhouse uses both solar and compost heat. It even houses chickens and rabbits! 

Heating A Greenhouse Using Solar Heat and Compost Heat

Our new greenhouse does so many different things that it's been difficult to figure out what to call it. Naming it by its separate functions could lead to a conglomeration such as this: compost-heat- and active-solar-heat-augmented, photovoltaic, earth-bermed, plant propagation and production rabbit hutch/chicken coop/terraced growing bed/runway greenhouse system. See what we mean? Let's compromise and use a term coined by the New Alchemy Institute—bioshelter. (See the bioshelter photos and diagrams in the image gallery).

The design of the structure was conceived in an attempt to get as many quality uses as possible out of one building by integrating it with its living occupants wherever possible. The goal, however, isn't so much to see how many interactions of plant, animal, and building we can create as it is to develop the most effective ones. For example, in the back of the bioshelter is a small room where chickens and rabbits can come in out of the weather. The solar input helps keep the critters warm, while the animals themselves add their body heat to the building. More important, the structure of their home adds to the overall mass of the greenhouse. The fully bermed masonry walls help to stabilize interior temperature. All these factors (and more) work together to create a beneficial thermal environment.

Despite the attention paid to creature comfort, the bioshelter is still primarily intended for plant production. And the key to getting the most from the greenhouse beds is to keep soil temperature up—preferably in the 80 degree Fahrenheit range. (Up to a point, plants double their growth rate for each 10 degrees Fahrenheit rise in soil temperature.) Air temperature is less important as long as it's high enough to prevent leaves from freezing. Consequently, our growing beds are heavily insulated on the sides, and the 10 inches of medium in each rests on a layer of rock through which warm air can be circulated. The areas under the beds are sealed but are accessible through hatches that allow us to experiment with several different supplemental heating methods, and we've borrowed ideas from a few other research organizations to pump warmth from these chambers into the soil.

First, we've taken a lesson from Rodale Press's Residential Passive Solar Greenhouse and are picking up hot air from the ceiling and distributing it below the beds. A squirrel cage blower powered by a Solarex photovoltaic panel hooked to a battery moves the air around. The fan takes orders from a blower control thermostat that switches it on when the temperature at the peak reaches 85 degrees Fahrenheit and from a heating thermostat that turns it back off when the temperature drops to 75 degrees Fahrenheit. There's also a manual override switch we use to force air into the compost piles when necessary for maintaining decomposition.

Omer Vahle
9/6/2008 11:22:56 PM

Another good source for tempered glass is your local glass shop. Standard size patio doors, (usually 34" x 76") will work great. When your local shop does a repair or replacement, they will have te opld units to dispose of. If you contact your local shop, they likely will save a few old units for you, since they will have to pay for disposal anyway. Sweeten the deal with some fresh produce and they will probably be happy to set some aside for you. You may have to cut the broken side off, or if you are lucky, it will be a failed unit and you will get two pieces! I own a glass shop, and would rather give an old unit away than throw it away. So as long as you don't make a nuisance of yourself, most shops would probably feel the same way. Just remember to use their services when you need new glazing.

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