Yes, Virginia, There is Free lunch: It's Called the Solar Greenhouse!

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The operating principle of Yanda's add-on unit during the winter growing season is that the greenhouse provides a considerable amount of heat for the home in the daytime . . . and, at night, the household heating system helps-in turn-to keep the sheltered plants from freezing.

According to Bill: "Though the home return heat to the 55o:, ... § `I=a . .-_,a .- , _ greenhouse overnight, there's still a positive gain in ,the opposite direction during the winter. In other words, in this climate you collect more warmth during the day than you spend after dark. In addition, heat retention can be increased with rocs, storage, or by stacking water-filled drums on the north wall of the unit."

So impressed am I with the simple practicality of Bill-s design that I'm going to add one of his structures onto our house in time to have it supplement our home's heat this winter. During the day, the greenhouse will warm the living room . . . and at night, an Ashley heater will keep the thermometer in the growing area from dropping below 40° F (the theoretical minimum temperature to which a growing area can be allowed to fall and still perform efficiently).

None of the Yanda greenhouses that I visited actually haul stoves installed in them, but depended instead on the home heating system to provide warmth after sundown. In some cases this worked out well enough. At the higher altitude though, the nights can get really cold and a little wood-burner is cheap insurance in December and January.

Bill, who lives at an altitude of 6,400 feet-considerably lower than my own location-left his unit open to his home (without supplemental heat) during the coldest period of the winter, and reports that "the greenhouse temperature average from November 1 to February 1 were 42.8° F (low) and 80.4° F (high). If the structure is closed off from the house during the night, the lower figure drops about 5 additional degrees,"

Most participants in Yanda's research project were more interested in heating their dwellings than in raising food and did, in fact, close off their greenhouses overnight. Even so. they managed to produce cool-weather crops such as lettuce, onions, and cabbages. A simple insulating cover-such as Steve Baer's "Night Wall", for example-on the shelter's south face and clear roof areas should improve growing conditions by raising the minimum temperatures in the coldest period to 50° F, a figure. within the optimal range for winter horticulture. Yanda plans to test this refinement of the design over the coming months.

If we put aside the unit's food-growing potential for the moment and consider it just as a solar heater, the performance of Yanda's device is impressive. Bill calculates that one square foot of southfacing greenhouse panel will meet the heating requirements of between one and three square feet of dwelling space . . . depending on conditions.

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