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Heating Ventilating Air Conditioning Guide 1938 and designed to harmonize with the trim, they may be unsightly. Better air distribution for cooling is obtained when high side wall registers are used, and this same location is satisfactory for heating when the openings are installed at least 7 ft above the floor line, providing .the air velocity through the registers is greater than 600 fpm. Registers which are located in side walls above the baseboard or in the ceiling should be of an effective air-diffusing type. All registers should be equipped with dampers, and should be sealed against leakage around the borders or margins. Velocities through registers may be reduced by the use of registers larger than the connecting pipes. Some suggestions for equalizing veloci ties over the face area of the register by means of diffusers are illustrated in Fig. 4. Merely to use a larger register may not result in materially reduced velocities unless diffusers are used. Dampers Suitable dampers are essential to any trunk or individual duct system, as it is virtually impossible to so lay out a system that it will be absolutely in balance without the use of dampers. Special care must be used in the Chapter 20. Mechanical Warm Air Furnace Systems AUTOMATIC CONTROLS Air stratification, high bonnet temperatures, excessive flue gas tem peratures, and heat overrun or lag in the system can be largely elimi nated through proper care in the planning and installation of the control system.* The essential requirements of the control are: 1. To keep the fire burning when using solid fuel regardless of the weather. bas2e.mTeonta. void excessive bonnet temperatures with resultant radiant heat losses into the . 3. To avoid the overheating of certain rooms through gravity action during off periods of blower operation. 4^ To have a sufficient supply of heat available at all times to avoid lag when the room thermostat calls for heat. 5. To prevent cold air delivery when heat supply is insufficient. 6. To avoid heat loss through the chimney by keeping stack temperatures low. Fig. 4. Diffusers in Transition Fittings to Equalize Velocities Through Register Faces design of any system to avoid turbulence and to minimize resistance. Sharp elbows, angles, and offsets should be avoided. See Figs. 1 and 2, ' Chapter 29. Three types of dampers are commonly used in trunk and individual duct systems. Volume dampers are used 'to completely cut off or reduce' the flow through pipes. (See A and B, Fig. 5.) Splitter dampers are used where a branch is taken off from a main trunk. (See C, Fig. 5.) Squeeze dampers are used for adjusting- the volume of air flow and resistance through a given duct. (See D, Fig..5.) It is essential that a damper be provided for each main or duct branch. A positive locking device should be used with each type of damper. " Ducts , The ducts may be either round or rectangular. Rectangular ductsshould be as nearly square as possible; the width should not be greater than four times the breadth. The radii of elbows should be not less than ; one and one-half times the pipe diameter for round pipes, or th6 equiva lent round pipe size in the case of rectangular ducts. 398 Fig. 5. Three Types of Dampers Commonly Used for Trunk and Individual Duct Systems 7. To provide quick response to the thermostat, with protection against overrun. 8. To provide for humidity control: 9. To provide a means of summer control of cooling. 10. To protect against fire hazards. The following controls are desirable: 1. A thermostat located at a point where maximum fluctuation in temperature can be expected, in order to secure frequent operation of fans, drafts, and burners. This location would be near an outside wall but not upon it, in a sun room, or in a room with some unusual exposure. The thermostat, of course, should not be located where it will be affected by direct radiant heat from the sun or from a fireplace, or by direct heat from any warm air duct or register. 2. A furnacestat located in the bonnet to permit blower operation only between the temperatures of 100 F and 150 F. In certain extreme cases it may be necessary, or weather conditions may make it advisable, to adjust the high limit to a higher tempera ture than that given. Another location sometimes used for the furnacestat is in the main duct near the frame opening from the bonnet.' TR^siSSNS. Vol'To1? 15^^ Heating Systems- S- K" and A. F, Hubbard (A.S.H.V.E. 399