=59. Hot-Air Pipes.= The hot-air pipes lead from the top of the jacket about the furnace to the floor above. In most houses, one pipe goes to each room. This is unnecessary if the rooms are not closed off, but if they are, they need the pipe entering the room. To economize with heat and regulate the amount of air passing up these pipes, there must be a shutter in them, near the furnace, as well as in the register. This shutter is placed near the furnace so that no heat passes into the pipe when not wanted in the room to which it leads. This saves waste in radiation from the pipe in the cellar. When a room is not in use, close this damper. [Illustration: FIG. 28. Pipeless furnace.] Since warmed air will continue to travel upward so long as it stays warmer than the air above, it is important that the pipes have a continuous rise thruout their entire length, the in some parts the rise may have to be only very slight. The shorter the pipes, the better, for there will be less loss of heat from radiation on the way to the rooms. [Illustration: FIG. 29. One-room, hot-air heater.] =60. Location of the Furnace.= A central location for the furnace is best because the pipes may be shorter, and this makes possible a greater elevation per foot of each pipe, so that the air can flow thru it faster. A central location also permits a uniform distribution of pipes about the furnace, which, in turn, produces a more even flow of air to all the rooms. The air from the hot register rises to the top of the room, or, if the way is open, to the top of the house. Here it spreads over the upper area. As it cools or is displaced by still hotter air, it falls. When it reaches the floor, it flows down the cold-air shaft in the floor. If the cold-air shaft is not in the floor, there may be a layer of colder air there so the room will not be comfortable. =61. Air.= There is a constant change of air in all houses, due to opening of doors and the fact that walls are not air-tight. This may not be enough for comfort. If a room is not heating well, it has been found that opening the window to change the air in the room, even when the outside air is very cold, helps in the circulation of air in the room, and so with the warming of it. It is difficult to warm a room filled with stagnant air. =62. Pipeless Furnaces.= The pipeless furnace works on the same principle as the one with pipes (Fig. 28). One large opening above the furnace lets the heat in to some central room, and from here it circulates into all other rooms not closed off from the central room. The cold-air shaft may be around the opening for heated air. Stoves encased in a metal jacket that operate like hot-air furnaces (Fig. 29) are used in heating one-room schoolhouses and other small public buildings. CHAPTER VIII HOT-WATER SYSTEM OF HEATING