195. Water at ordinary temperatures, if exposed to the air, is always charged with a certain amount of air and other gases, which it seems to hold in solution. When water so charged is increased in temperature, the gases are gradually evolved from the liquid and rise in small bubbles to the surface until the water has reached the boiling point, when all the air will be liberated and steam will form. Now, it will readily be seen that when a fire is first started in a hot-water boiler, air will be liberated from the water and will rise to the highest points of the heating apparatus, where it will accumulate and form air locks, if it cannot escape to the atmosphere.

This matter must be carefully looked after in constructing hot-water heating apparatus, because the motive force is so small that it may be easily neutralized and the circulation stopped by an air lock of comparatively small size. Air collects in all high places, such as the tops of radiators, the upper ends of vertical pipes, etc., and these points should always be provided with air vents.

196. All horizontal supply or flow pipes should be inclined upwards upon a uniform grade, so that the air will readily flow into the risers. The air in the pipes will then pass up into the radiators or into the expansion tank. If this cannot be done, an automatic air vent of sufficient capacity must be attached to the piping at the highest point. A bubble or small air lock in a circuit will, in many cases, completely stop the circulation.

The manner of running and connecting pipes for hot-water service is substantially the same as for steam heating. The expansion of the pipes by heat must be provided for by using spring pieces, expansion joints, etc. in the same manner.

197. All horizontal branches from the flow main should be connected into the top of the main, or at least should be taken off by means of eccentric fittings, which will bring the top of the branch flush with, or a little above, the top of the main, so that all air bubbles may pass freely forwards and not accumulate in the main.

Fig. 70.

198. In the one-pipe system the connections for a radiator are made substantially as shown in Fig. 70, b being the flow connection, c the return, and the main current moving through a in the direction of the arrow. The object here is chiefly to take the supply of hot water from the top of the main and return the cooler water into the bottom.