Heating Apparatus 237 



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. 



Thus, if a supply cistern is 10 ft. above the base of the boiler, there 

 will be 5 Ib. pressure to every square inch; and at a height of 30 ft. the 

 pressure would be 15 Ib. 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 13'02 

 will give 95,931 Ib., 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 J 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. 



Quantity of Piping- Required. The following table, taken from 

 Hood's work on hot-water heating, may be given as showing the length 



