=Hot-water Radiator Valves.=--Valves for hot-water radiators differ materially from those used on steam radiators. Figs. 43 and 43a show the outside appearance and the mechanical arrangement of the parts of the Ohio hot-water valve. The part _A_ in Fig. 43_a_ is a hollow brass cylinder attached to the valve-stem, one side of which has been removed. When it is desired to shut off the supply of heat the handle of the valve is given one-quarter turn and the part _A_ covers the opening to the inlet pipe. The supply of water being shut off, the radiator gradually cools. When the valve is closed a small amount of water is admitted to the radiator through a 1/8-inch hole in the piece _A_ to prevent the possibility of freezing. =Air Vents.=--In the use of the systems of hot-water heating described, every radiator must be supplied with an air vent of some kind to take away the trapped air which accumulates through use. Any kind of a valve will serve as a vent for hand regulation and generally such a cock as is shown in Fig. 10 is employed. =Automatic Hot-water Air Vents.=--It is sometimes desired to use automatic air vents on hot-water radiators. For such work a vent is used that remains closed as long as water is present and will open when the water is displaced by the accumulating air, but will again close when the air is discharged. In such vents the valve is controlled by a float, the buoyancy of the float when surrounded by water serving to keep the valve closed. These vents are not so positive in their action as automatic air vents for steam. The change in temperature which controls the steam vent does not take place with hot water. The automatic hot-water vents are not perfectly reliable. They may work with entire satisfaction for a long time and then fail from very slight cause. The failure of a hot-water vent is generally discovered by finding a pool of water on the floor or a wet spot on the ceiling or wall of the floor below. [Illustration: FIG. 44.--Automatic air vent for hot-water radiators.] One type of the automatic hot-water vent that has proven quite successful is shown in Fig. 44. The threaded lug is screwed into the radiator at the proper point. As the water enters the radiator the air is discharged through the vent, escaping at the opening _C_. When the water has risen to a sufficient height it enters the openings _G_ and _H_ until enough is present to raise the float _A_. The pointed stem attached closed the hole _C_ with sufficient force to make an air-tight joint. The float _A_ is a very light copper cylinder. Its buoyancy supplies the force to close the vent and its weight opens the vent when the water is displaced by air. It will be readily seen that very slight cause might prevent the performance of its duty. CHAPTER III THE HOT-AIR FURNACE