This section is from the book "The Gardener's Monthly And Horticulturist V18", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
Having had considerable experience with heating greenhouses with hot water, I wish to give a few suggestions.
I have learned that water pipes should be so arranged as to facilitate the rapid motion of the water, and any change in the size of pipe tends to diminish the flow of water. A large stand pipe, or expansion tank, where one pipe enters into and another leaves it, retards the motion; or where a large pipe is contracted or enlarged also retards the motion.
All steam generated in the boiler should be used to move the water, but the pipes should be so arranged that the water could not boil out of the exhaustion tank. On the question of boilers, as new designs are being introduced every year, we should be very cautious, and consider well the essential points, before we adopt them. The vital points about a good boiler are: 1st, to be capable of burning all kinds of fuel; 2d, have plenty of heating surface; 3d, the boiler should be so constructed as to give a rapid motion to the water, thereby carrying off the heat from the furnace, so as to save all its heat generated from the fuel.
Rapid circulation is the very essential thing at present, as many cultivators of plants build greenhouses from one hundred to four hundred feet long, and many times other additions to greenhouses attached, so the water has to pass through five hundred to eight hundred feet of pipe before it can return to the boiler to take on a fresh supply of heat. We should consider that all heat is generated in the furnace, and the moment the water leaves the boiler, it begins to radiate its heat, and when it moves slowly, and has parted with the high temperature, it is of but little value in repelling the sharp arrows of frost on a cold winter's night, with the thermometer marking below zero; therefore, I would say, that the boilers are very few that can heat the water and give it a momentum through more than five hundred feet of pipe, boiling until it comes back to the boiler to take on a fresh supply of heat. So a boiler that heats one thousand to three thousand feet of pipe should have separate attachments and very rapid circulation, or that portion of the greenhouse over one hundred feet away from the boiler will not be evenly heated with that near by.