In Fig. 57 the motor is connected with a hot-air furnace. The cord _D_ is attached to the front arm of the motor and connects with the direct-draft damper _F_. The cord _C_ connects the rear arm of the motor with the check-draft damper at _E_. In the position of the dampers shown, the direct-draft damper is closed and the air is entering the chimney through the check draft _E_. While this damper is open there is very little induced draft to supply the fire with air that might leak through the crevices around the ash-pit door, but the gases from the furnace are completely carried away to the chimney by the air entering at _E_. [Illustration: FIG. 57.--Thermostat motor connected with the dampers of a hot-air furnace.] In Figs. 3, 6, 8, 34, etc., the same motor is connected with the furnaces of various other systems of heating. The object is the same in all; when less heat is required, the air supply is cut off and the furnace fire subsides; when more heat is demanded the air is again admitted to produce greater combustion. The check draft is an important feature as it checks the flow of air through the furnace regardless of the position of the direct-draft damper. Even should the direct draft be left open, the check draft when open would destroy in a great measure the supply of air entering the furnace. CHAPTER V MANAGEMENT OF HEATING PLANTS The following instructions on the care and management of steam and hot-water heating plants is printed with permission of the American Radiator Co. They were prepared as a guide to the successful operation of the Ideal heating plants but apply with equal force to other plants of a similar character. =General Advice.=--No set rules can be given for caring for every boiler alike--chimney flues are not alike--some have strong draft, some are average and some are weak. There is much more difference in the heat-making qualities of coal than is commonly known, and it is important that the right size coal for the draft be used. These rules apply to most all fuels. A little trying of this way or that way of leaving the dampers (when regulators are not used) often discovers the better way. It is well to vary from the rules a little if any of them do not seem to bring about the best results. With good, average chimney flue draft and the right kind of fuel, these rules will govern the large majority of cases. =The Economy of Good Draft.=--In many cases a boiler with sluggish draft will burn more coal than a boiler with good draft. In the first case the fuel may be said to “rot”--in lacking air supply the gases pass off unburned. The “nagging” which a boiler has to take under these conditions increases the waste of fuel. A boiler under sharp, strong draft maintains a clear intense fire and burns the gases--getting the larger amount of heat from the coal. =General Firing Rules.=-- 1. Put but little coal on a low fire.