This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
Market growers and nurserymen who have to erect large greenhouses for their crops do so strictly on business principles. The ornamental structures seen in private gardens and public establishments do not appeal in the least to men who have to grow plants for a living, and who erect glass structures not because they like to, but because they must. Not only must money bo spent in the erection of glasshouses and frames, but a suitable temperature must be maintained in them by means of artificial heating. In the old days, before hot-water pipes came into use, glasshouses were heated by means of "flues", and in very old gardens some of these still exist. Flues consist of a passage from the furnace up and down one side, or all round, a house, and enclosed by tiles or bricks in such a way that sulphurous fumes shall not leak into the house and destroy the plants. The heat and smoke are carried along these flues, and find an exit in the chimney. The heat obtained from the flue surfaces was much or little according to the way the furnaces were fired, and excellent results were obtainable by this method of heating.
The flue system, however, is now obsolete, and no one would dream of heating a modern glass structure by it. Hot-water pipes and boilers have come to stay, and taking everything into consideration, they not only supply all the heat required, but they can be regulated by means of valves to raise or lower the temperature. Not only that, but hot-water pipes can be arranged wherever the grower wishes - either along the floor, around the walls, under or over the stages or benches, and along the roof itself if necessary. Indeed in many modern glasshouses in large market nurseries 4-in. pipes are run along the entire length of a house overhead. It is claimed for this method of heating that the air throughout the house is kept at an equable temperature, and in the event of severe frosts no danger is to be apprehended to plants inside near the glass. The installation of these pipes overhead naturally entails extra expense, but that is counterbalanced by the great advantages derived. The pipes are supported at intervals by Uprights, so that there is no strain upon the roof or sash bars. Although 4-in. pipes are generally used, 3-in. and 2-in. are also employed under special circumstances.