The boiler is perhaps the most important feature in any installation, as upon it depends principally whether the system will be economical in the use of fuel, or very expensive.

The number of types and forms of hot-water boilers is very large, and almost every maker has some special design which he naturally considers superior to all others. In Great Britain, boilers are almost entirely made of wrought-iron or mild steel plates, welded into a solid vessel, but in the United States they are very frequently made of cast-iron. This material, however, is not by any means equal to the others, as it is liable to crack when heated, and thus perhaps let loose a flood of boiling water in the basement of the building.

In boilers which are to be used solely for purposes of heating, it is not so necessary to provide ample facilities for removing incrustation as in boilers for the supply of hot-water for domestic purposes. A system of hot-water heating consumes practically no water, it being only requisite to allow a supply of a few pints per week to replace the losses produced by evaporation in the expansion tank. The same water circulates constantly, and, having once deposited its impurities, can came no further trouble with incrustation, while, in the case of a boiler used for hot-water service, the deposit goes on gradually accumulating in the whole of the interior of the apparatus

It is always desirable, on the score of economy of fuel, to set the boiler in brickwork, as the loss of heat by radiation is far less than when the metal exterior of the boiler is exposed to the air. Of course it costs more to build a brick setting than to merely place the boiler on the floor, and for small installations, where first cost is the most important item, it may be advisable to employ a boiler without setting.

Boilers heated by gas are made for use in connection with low-pressure hotwater apparatus, and will prove useful for small installations. A good boiler of this kind is shown in Fig.

502. It has the disadvantage of being made of east-iron. The cross tubes are cast with the inner body. This boiler is made in two sizes, the smaller being 19 inches high and l3 inches from back to front, while the larger is 31 inches high and 17 inches from back to front.

The former is said to heat, as a maximum, 40 feet of 4-inch pipe, and the latter 100 feet.

The connections are 1 inch and l inches respectively.

Where the amount of pipe to be heated is small, an independent boiler of the form illustrated in Fig. 503 may be used; this is specially designed bo that a large amount of fuel may be fed into it at one time, and the inner part is made conical to prevent the fuel from sticking. Such a boiler would have one 2-inch flow-pipe and one 2-inch return, and would be made of wrought-iron plates welded together. If 4 feet high and 15 inches in diameter, a boiler of this kind will heat about 130 square feet of radiating surface,1 while one 6 feet high and 24 inches in diameter is estimated to heat about 450 square feet. For a larger installation, such a boiler as the "Marlor", shown in Fig. 504, may be used; it is made of 3/8-inch mild steel plates, in various sizes up to 6 feet high and 4 feet in diameter, this size being supposed to heat 2500 square feet of pipe-surface.

Fig 502.   View and Section of Kletcher, Russell, & Co's cross tube Gas Boiler.

Fig 502. - View and Section of Kletcher, Russell, & Co's cross-tube Gas Boiler.

Fig. 508. View and section of Independent Conical BoaVz.

Fig. 508.-View and section of Independent Conical BoaVz.

1 By "radiating surface" is meant the uncovered external surface of the pipes, radiators, coils, etc connected with the boiler. Those parts of the system which are not required to radiate heat should be carefully protected with some non-conducting covering.

The smallest size measures 3 feet in height and 2 feet in diameter, and is supposed to heat 450 square feet of pipe-surface. The water, it will be noticed, is carried below the level of section.

sectionPlan. Fig. 504   Plan and scetion of the  Marior  Boiler.

Plan.

Fig. 504 - Plan and scetion of the "Marior" Boiler.

the ash-pan, so that there is a quiet place in which sediment may easily collect, and this can be Hushed out at intervals. Such a boiler is, however, somewhat expensive when compared with the types already referred to.

For larger works, it is best to use a type of boiler in brick setting, as much greater economy can thus be obtained.