Every grower should know why the water from the boiler rises and flows upwards through the flow pipes and comes back again to the boiler by the return pipes. It may be possible to explain this by means of the accompanying diagram (fig. 179). Let A represent a boiler with flow pipe at B, return pipe at c, and fire at D. When heat is applied, some of the water in the boiler absorbs heat and therefore expands and becomes lighter, and requires more space.

The colder water in the return pipe c, being heavier than the heated water, rushes in at the bottom of the boiler, and thus pushes the warmer water upwards and forces it through the flow pipe to fill the space caused by the flow of water from c to the boiler. The greater the heat applied the quicker the circulation. Now, as the heated water travels along the flow pipe b it is gradually losing its heat, and its colder particles begin to sink to the bottom. It cannot, however, return the way it came, because it is being pushed forward by the hotter water coming from the top of the boiler, which in its turn is forced up by the colder water entering the boiler by the return pipe c at the base. Thus while the water in the pipes is gradually losing its heat, that in the boiler is constantly rising in temperature, and rushes to occupy the space that is being constantly vacated by the colder water.

Fig. 179. - Diagram showing the Circulation of Hot Water in Greenhouse Pipes.

If by any chance the water in the flow pipes and in the return pipes and boiler was of the same temperature, circulation would cease altogether, as when the fire goes out and the water cools. There must therefore be a difference in the balance between the hot and cold water to maintain a regular circulation. In other words, one column of water must be heavier than the other. This is secured by having the flow and return pipes at different heights, and the boiler at a lower level than either. When pipes are being set there is always a very slight rise in the flow pipe from the boiler to the end of the house, and a corresponding fall in the return pipe to the boiler. In this way a difference is secured in the two columns of water. And this difference is accentuated by having water in the supply cistern, which is placed several feet higher than the highest point of the flow pipe and is connected with a pipe to the return pipe. This supply cistern should always be kept tilled with water, and as there is a pressure of about 1/2 lb. to every square inch of its surface for every foot in height, it will be realized at once what force is being exerted to drive the cold water into the boiler at the base, and the warm water out at the top.

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Thus, if a supply cistern is 10 ft. above the base of the boiler, there will be 5 lb. pressure to every square inch; and at a height of 30 ft. the pressure would be 15 lb. on every square inch. Care must be taken not to cause too great a strain on the boiler and pipes by having the supply cistern too high. So long as the cistern is placed a foot or two above the highest point of the pipes, a good circulation will be secured with a minimum strain on the apparatus.

To show the enormous strain upon a boiler according to the height of the supply cistern the following remarks from Mr. W. Jones's work on Heating by Hot Water may be quoted: "Take a plain saddle boiler with 3-in. water space, and measuring 60 in. long by 21 in. wide by 21 in. high inside arch, the area or surface of which would be 7368 sq. in. Suppose the head of water to be 30 ft. above the centre of the boiler, 7368 x 1302 will give 95,931 lb., or nearly 43 tons pressure inside the boiler, whereas the actual weight of water in the boiler would not exceed 3 cwt. If you increase the head of water to a height of 60 ft. the pressure will be about 86 tons. If you lower it to 15 ft. it will be about 21 1/2 tons, although the weight of water may remain the same in each case".

The system of hot-water heating for glasshouses is known as the "low-pressure" system, to distinguish it from the high-pressure system by which water is brought to boiling-point. Good growers never like their pipes to get so hot that they cannot bear the hand on them. When this is the case it indicates either bad and wasteful stoking or that the boiler is too powerful for the quantity of piping attached. Great heat in the pipes is injurious to plant life. It makes the atmosphere too dry, and when water is applied the house is filled with steam from the hot pipes for a time. A genial heat in the pipes is therefore most desirable.

It sometimes happens, however, more especially in very cold weather, that the fires must be "driven" to maintain the requisite temperature. Then the water is heated so much that it flows over from the supply cistern by sheer expansion. When the water cools it naturally takes up less space than before, the supply cistern becomes empty, and air enters the pipes to fill the vacuum caused by the lost water. The air must be got out of the pipes, otherwise the water could not enter in again. This is secured by means of an air pipe F, fixed at the highest point of the flow, and in many cases carried outside the house. Sometimes stopcocks are placed at the top of the flow pipes, and are examined regularly to allow the air to escape. In any case these air pipes are necessary, because, owing to the natural leakage of water by evaporation, air enters the pipes. If not expelled or allowed to escape, not only would the circulation of the water be impeded or stopped, but with great pressure the pipes or even the boiler might burst.