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American Society of Heating and Ventilating Engineers Guide, 1936 3i fc OJ g go ?* <' ||2 <| ^aga S<e Ubo oHaCU^gph<n <Ji fe o* 5 I3s5h P! *,3 X Q - Og uo 2H za 5< 5 2l<S<5 <QoO p g< 25g8 mK S | Kg'" (2 J SJ d-5 2 tu w-B. aHa 1 2O U o*<S o ow gg <g H< yaUW X c& 6 Uh Chapter 23--Mechanical Warm Air Furnace Systems 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. 2. To avoid excessive bonnet temperatures with resultant radiant heat losses into the basement. 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. 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. 3. A protective limit control located in the bonnet to shut down the system inde pendently of the thermostat if the bonnet temperature exceeds 225 F. 4. On oil and gas burner installations, a control is usually included which will shut down the system if the fire goes out or if there is a failure of the ignition system. 5. A humidistat to regulate the moisture supplied to the rooms. 6. On automatic stoker installations, a control is usually included which will start the operation regardless of thermostat settings whenever the bonnet temperature indicates that the fire is dying. While it is usually all right to start and stop the fan in residential con trol work and in. auditoriums and other places where many people may gather, the fan should as a rule be allowed to run continuously and the control should be cared for in other ways. SELECTING THE FURNACE The following formula may be used to compute the grate area of a residence furnace, assuming a ratio of heating surface to grate area of 20 to 1: G = FX CX E (1^ 411 1