There will, in that case, be a circulation of the air in the room only, and the out let-ventilator, at the ceiling or the floor-level as the case may be, must be kept closed. As no fresh air is allowed to enter from outside, the atmosphere of the room will rapidly become "stuffy", and therefore judgment is needed in opening and closing the external grating.

Such radiator-cases are not consi dered suitable in all circumstances, but there are other means of arriving at the same end: for instance, Fig. 531 represents a ventilating radiator quite different from those just described; each separate loop is screwed into the base, and no india-rubber is used in making the joints. In the non-ventilating type the fitting ends with tin-hollow base into which the pipes are screwed, but in the ventilating type this is fixed upon a special box provided with a large number of small holes in the front of the case near the floor. The usual inlet is provided in the external wall, and protected by a grid. In the base casting a special valve is fixed, consisting of two hinged plates, coupled together by bolts. When this compound valve is pulled forward the holes in the case are closed, and the external air has a free course up between the two rows of tubes, and out through the holes in the top grating; when the valve is pushed in, the external air cannot enter, but the air in the room is free to circulate through the inside of the radiator, as in the case of the radiator previously described. In rare cases cotton cloth or muslin is fitted in the inlet passage, in order to partially clean the air entering the room, but in the great majority of instances this is not done. All arrangements of this kind require attention at regular intervals for cleaning, otherwise they become mere receptacles for dirt, and the air passing into the room may be rendered more impure than the external air.

It is always desirable to have the connections of the pipes to the radiators made in such a manner as to permit of the removal of the radiator for cleaning:, as in the last instance it is by no means easy to render the lower box perfectly clean The dark stain, which will appear above the radiators if they are placed against a wall will clearly prove what a quantity of dust is carried up with the current of heated air.

Fig. 531   Sectional View of Rosser & Rusell's Ventilating Radiator.

Fig. 531 - Sectional View of Rosser & Rusell's Ventilating Radiator.

Stop-valves are very important items in a heating-installation, and it is very poor policy to buy cheap valves, as these are a never-ending source of annoyance. For hot-water work there can be nothing better than a full-way cock, either of the "Peet" type, sown in sction in tig. 532, or one of the other types described late. The "Peet" valve consists of two separate disc separated by a wedge-shaped part controlled by a screw. When closed. the two discs shut tightly upon their seatings, and are held there by the pressure of the hand-screw. Such valves are not so suitable for steam, but are well adapted for hot water; the great point which requires attention in a hot-water heating-installation, is that no resistance which can possibly be avoided be offered to the passage of the current.

Another type of stop-valve is shown in tig. 533, and is specially adapted to form an elbow for a radiator connection. These valves should be of gun-metal of good quality, and the plug may consist merely of a conical plug, fitting into a hole turned accurately to receive it; the plug is entirely withdrawn when the valve is fully open, and therefore leaves a full water-way.1

I laving described in some detail the various pieces of apparatus used in hot-water heating, 1 shall now consider the various ways in which the apparatus may be arranged. The heat, whether obtained from the combustion of coal or coke, gas or oil, must be applied to the lowest part of the system, for the simple reason that heated water will naturally be forced upward by colder water. From the usual tallies we learn that, when water is heated from 32° to 212° Fahr., it will expand about one-twentieth of its original volume; such an amount of expansion must obviously be allowed for in any form of apparatus, and if this is not done, the containing parts will be liable to burst. In a low pressure system the pipes must be open to the air at one or more points. Fig. 534 shows the simplest possible system for heating pipes by low-pressure hot water. T represents an open metal tank filled with water, with a pipe connected to it as shown. So long as the tank and pipe are full of water at the same temperature, there is no tendency to circulate, but let a lighted lamp

Fig. 532. Section of Full way  Peet Valve .

Fig. 532.-Section of Full-way "Peet Valve".

Fig 533   Radiator with Elbow stop valve.

Fig 533 - Radiator with Elbow stop-valve.

1 Safety-valves are important adjuncts of hot water apparatus. Numerous explosions, many of which have been attended with loss of life, have occurred in conaequanoe of the omission of these safeguards. The principal cause of explosion is the blocking of the pipes with ice in frosty weather, bat stoppage by incrustation may also occur. The simple dead weight safety -valve is among the best, but as the subject of safety-valves has already been treated in section IV, pages 253 to 257. nothing further need be said here-ED.

Fig. 534.   Slmplest System of Low preaaure Heating

Fig. 534. - Slmplest System of Low-preaaure Heating.

Fig. 535   Simple Low preasure Hot water Apparatus with Air pipe.

Fig. 535 - Simple Low-preasure Hot-water Apparatus with Air pipe.