221, Greenhouses may be heated satisfactorily with either steam or hot water, but the latter is generally preferred, because of the simplicity of the apparatus, and its ability, when properly arranged, to store up large quantities of heat. The apparatus is made to contain an amount of water that is very large in proportion to the cooling surfaces of the greenhouse, thus constituting a reservoir of heat. If the fires burn low or go out, the stored heat is given out gradually and serves to keep the temperature from falling too rapidly, thus protecting the plants from damage until the fires are attended to.

Heating Greenhouses 193

Fig. 81.

222. Figs. 81 and 82 show a greenhouse which is supposed to be located on the side of a hill. It is constructed with two or three large parallel benches or platforms, A, B, C, which run the whole length of the building, for the purpose of holding plants that grow in pots; or, they may-contain a bed of earth or sand in which seeds and cuttings are propagated. The outside or wall benches should have a space about 2 or 3 inches wide made between the bench and the wall, so that the warm air rising from the heating pipes will pass upwards in a sheet, as it were, in front of the windows, and thus neutralize the downward current of cold air which would otherwise exist at that point.

Heating Greenhouses 194Heating Greenhouses 195

Fig. 82.

It is necessary to control the temperature at each bench, in order to grow plants to the best advantage; therefore, each set of pipes must be provided with suitable controlling valves.

The pipes commonly used for hot-water service in greenhouses are made of cast iron and are slightly less than 4 inches in diameter, 1 lineal foot of pipe being equal to 1 square foot of heating surface.

223. The following ratios of heating surface to glass surface have been found in practice to give good results, and may be used in designing hot-water heating systems for greenhouses. The external temperature is supposed to be 0° F., the exposure of the greenhouse moderate, and the construction good.

Table 21. Proportion Of Glass To Heating Surface


Hot Water.

For 45° inside temperature, divide glass



For 50° inside temperature, divide glass



For 55° inside temperature, divide glass



For 60° inside temperature, divide glass



For 65° inside temperature, divide glass



For 70° inside temperature, divide glass



224. Arrangement Of Pipes

Arrangement Of Pipes. The pipes are usually laid in long parallel lines, under the benches, as shown in Figs. 81 and 82, with one or two flow pipes a resting on top of two or three return pipes b. They are supported at intervals by brick piers, at a sufficient height above the floor to secure a good supply of air to the inside of the group of pipes. They are all laid upon an upward grade from the boiler to the farther end of the line.

The head available in greenhouse apparatus is seldom more than 6 feet, and is usually much less, As the buildings are frequently from 300 to 400 feet long, it is evident that the grading of the pipes must be carefully done. In order to secure as much head as possible, the boiler should be set in a pit or cellar.

225. The expansion tank is usually placed at the end of the line of pipe most remote from the boiler. Each line or group of pipes may be provided with an expansion tank, or one large tank may be used for the whole system. Both the flow and return pipes are connected to the tank, which thus serves as a return connection and as a vent for air. The top is closed by a loosely fitting cover.

In the plan view, Fig. 82, the pipes shown under the side tables are provided with separate tanks t, while the middle line has none. As it may be inconvenient to place a tank at the end of the middle table, it is therefore omitted in the drawing. These pipes are relieved of air by means of a 1/4-inch or 1/2-inch pipe, which is tapped into the highest point, and is extended upwards above the level of the top of the expansion tanks. The boiler w is set in a pit, as shown in the side elevation.

Rust joints are used instead of lead-calked joints for cast-iron heating pipes. The cement is made by mixing 100 parts, by weight, of iron filings or borings, pounded fine, with from 1 to 2 parts of sal ammoniac, enough water being added to make the mixture into a thick mortar.

The bottom of the socket is closed by calking in a strand of oakum in the usual manner, and then the remaining space is filled with the cement and lightly calked. The sal ammoniac attacks the iron and rapidly converts it into rust, which hardens into a dense, tough mass, and clings to' the iron pipe with great tenacity.