Pipes for heating horticultural structures are preferably made of cast iron, painted black. Earthenware has been recommended for the purpose, but they are so much more liable to breakage and leakage, as to outweigh any original saving in the cost. For draining, earthen pipes with a bore an inch in diameter are the best.

Table of the quantity of pipe, four inches diameter, which will heat one thousand cubic feet of air per minute, any required number of degrees; the temperature of the pipe being 200° Fahrenheit.

To ascertain by the above Table the quantity of pipe which will heat one thousand cubic feet of air per minute: - find, in the first column, the temperature corresponding to that of the external air, and in one of the other columns find the temperature of the room; then in this latter column, and on the line which corresponds with the external temperature, the required number of feet of pipe will be found. See Hot water and Steam.