In the foregoing chapter on homemade greenhouses very brief reference was made to the various methods of heating. It will be well to understand a little more in detail how to heat glass structures, as temperature is, next to moisture, the most important factor of success. If steam or hot water is used in the dwelling house and a greenhouse of the lean-to type is used, the problem becomes a very simple one, as additional pipes can be run through the greenhouse. But as this advantage is not always ready to hand, we will consider the heating of an isolated house, and the principles involved may be adapted to individual needs. There are three systems of heating: flues (hot air), hot water, and steam—the latter we need not take up as it is economical only for larger structures than the amateur is likely to have.

[Illustration: Fig. 7—The best arrangement for heating a greenhouse by hot air, is to run a brick or cement flue from the furnace around under the benches and into the chimney over the fire. AA—storages space; B—furnace; C—chimney; DDD—benches; E—furnace door.]

Heating by hot air carried through brick or tile flues is the simplest and cheapest method for very small houses. The best way of constructing such a system is illustrated in the diagram adjoining, which shows the flue returning into the chimney (after traveling the length of the house and back). This method does away with the greatest trouble with flue heating—a poor draft; for immediately the fire is started, the air in the chimney becomes heated, and rising, draws the hot air from the furnace around through the flue with a forced draft. This forced draft accomplishes three other good things: it does away with the escape of noxious gases into the greenhouses, lessens the accumulation of moisture and dust from wood smoke, and distributes the heat much more evenly throughout the house. The furnace may be built of solid brick, with doors and grates and an arched dome, and the flue should be of brick for at least one-third the length from the furnace into the house; for the rest of the way cement or vitrified drain pipe will be cheaper and better. The flue should have a gradual upward slope for its whole length and will vary in size with the house to be heated, from five to eight or nine inches in diameter, the latter being sufficient for a house 60 by 21 feet. The flue should be raised a little from the ground, and at no point should any woodwork be nearer than six inches to it. Very small houses, especially if not started up until January, may be heated by an ordinary wood stove with the stove pipe run the length of the house, but such an arrangement will give off a very drying and uneven heat, and require a lot of attention, to say nothing of its danger.

[Illustration: Fig. 8—Hot water is undoubtedly the most satisfactory method of heating the small greenhouse. The diagram shows a 1-1/2-inch supply pipe leading out from the boiler, with 1-inch returns under the benches, making a satisfactory system for the lean-to type described in detail in the previous chapter.]

[Illustration: Fig. 9—For the larger greenhouse of the isolated double-slope type, 21 x 50 feet in size, a 2-inch supply pipe, with five 1-1/2-inch returns under the outer benches, will secure a temperature of 55 degrees.]

By far the most satisfactory way will be to use hot water. If the size of the house will not justify the purchase of a small heater—a second-hand one may often be had at a very reasonable figure—a substitute may be had by inserting a hot-water coil in a stove or in the house furnace. In one of the diagrams is shown an arrangement of pipes for heating a house 21 x 50 feet, and in another piping for lean-to described in the preceding chapter. With the small pipe sufficient for such a house as that illustrated in the latter diagram, the work can be done by anyone at all acquainted with the use of pipe tools; if possible, the pipes should be given a slight downward slope, say one inch in ten feet, from as near the heater as practical. For all this work second-hand piping, newly threaded, will answer very well, and it may be bought for about four cents per foot for one-inch pipe; six cents for one and one-half inch, and eight cents for two-inch. In putting the stove or heater in place, it should be sunk below the level upon which the pipes will run, and attention should also be given to the matter of caring for the fire, removing ashes, etc., making the management of these things as convenient as possible.