In heating glass-houses, it may be considered that the radiating surface always consists of cast-iron pipe, sometimes of 3 inches in size, but more usually 4 inches.

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It is also the two-pipe system of work that always appears, for it would be difficult to adopt the one-pipe or overhead principle.

Although the cast pipe, as a radiating surface, is justly condemned and now quite abandoned in residence and other works carried out in good interiors, it possesses certain distinctly good features in horticultural work, and at present there is no prospect of anything else being substituted. Glass is a rapid heat loser, and if in, say, a 15 feet greenhouse a radiator was placed at one end, there are doubts if it would afford a proper warmth at the other end, and in any case there would be a marked difference in temperature. With pipes running the whole length this ill result is quite absent. Most importantly, however, the large pipe excels in its work by holding a comparatively large volume of water. It will be realised that a large bulk of water, when it has attained a given heat, will be slower in showing a change of temperature than a smaller bulk would. This means that neither careless overstoking nor neglect will cause so serious a change of temperature in the glass-house heated by large pipes as would be the case with the smaller quantity of water held by small pipes or radiators ; and as undesired changes of temperature are particularly prejudicial to the horticulturist's work, he will always favour anything that prevents or reduces them. Briefly put, the large pipes, by holding a good bulk of water, are slow to heat and slow to cool, the former not being objectionable in this work, while the latter is a decidedly good feature.

One of the most useful introductions for glass-house heating is the "Loughborough" boiler and its set of pipes. This type of boiler now appears in all makers' catalogues, as being not only specially suitable for nearly all small jobs, but also being easy of erection by comparatively unskilled labour. Fig. 53 illustrates this. It will be seen that no boiler pit is required, the boiler being fixed in the thickness of the greenhouse wall at one end (either end). The back of the boiler, to which the heating pipes are connected, projects inside the house ; while the front of the boiler, carrying the feeding and cleaning doors and the flue nozzle, comes outside. Thus the boiler can have all necessary attention without opening the door of the house, and the cost of fixing is greatly reduced. The heating pipes extend along the front wall inside the house and terminate in an upright box cistern. This latter appliance is ingenious as fulfilling several purposes. Without this there would require to be a siphon-end to join the pipes. This would have to be drilled and fitted with an air pipe, and then a cold-supply cistern and pipe would have to be provided and fitted. The box cistern, commonly called an expansion-box, serves all these purposes by joining the pipes, being an air vent to them (it having a loose lid), and being also a supply cistern. The joints of these complete outfits are usually of a simple kind, with a rubber ring which is compressed by screwing up two bolts. It would be quite possible to make this apparatus heat two houses by inserting an H-piece and valve to control the circulation to the second house.

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Fig. 53.

Except in heating one or two houses it cannot be hoped to get good results by running the pipes without regard to the separate regulation of the temperature of each house. There must be a pair of main pipes, with branches to each house according to its requirements, the branches having controlling valves, known as throttle valves, in them. An idea of this is given in Fig. 54.

This is an imaginary yet quite ordinary arrangement of four houses, but it should be stated here that the heating engineer never has to settle the arrangement of pipes, and not always is he required to say what quantity should be used. The gardener or grower decides such points, and his word is final. It is always arranged, when possible, that the hottest house, called a "stove," or the tropical house, come nearest to the boiler, and the average gardener considers that too much pipe cannot be put in here. The next house may be an early vinery, with a little more pipe than the next house, which is for later grapes. The last may be a "cool house" for half-hardy plants.

Fig. 54

Fig. 54.

The mains are commonly carried through the houses as shown, but quite frequently they are carried in a range of pits (for melon growing, etc.) along the fronts of the houses, the branches going from here into the houses. Near the boiler it will be seen that the first branch is carried at a low level part of the way ; this is to get it past a doorway that must exist there. All high points have air vents, air pipes being invariably used for these. These pipes must be carried with a rise at all points, as air will not escape through water in a downward direction. No parts of the air pipes which contain water may be carried outside the house. The cold supply is arranged by placing a small cistern at any point above the line of the highest pipes, and from the cistern a supply pipe is brought down and enters the main return close to the boiler. As a rule, the cistern is placed as nearly over the boiler as possible, inside the first house. It is stated above that a small cistern is used ; and this is the general practice, little thought being given to providing a cistern bearing any relation to the quantity of water in the apparatus. It has been explained that water expands to a known extent when heated, and with works inside brick buildings the cold-water cistern has to be of such a size as will accommodate the increase due to expansion. In a glass-house the gardener expects an overflow at his small cistern when first heating up, and the trouble is ended then, as the water is not allowed to cool down day or night until the warm weather comes. Expansion pipes are not always put, as a number of air pipes serve the same purpose.