Document OJJLNMQ0nGdoZQjZRgxGobqqp

HEATING VENTILATING AIR CONDITIONING CUIDE 1944 may involve spray-head equipment to supply humidity. Such instal lations distribute heated and humidified air during the heating cycle, and during the summer or cooling cycle may be used effectively as conditioners if equipped with refrigeration means. Regulation of the humidity during the heating cycle is normally accom plished by opening and closing a solenoid water valve supplying water to the spray-heads, the sblenoid valve being under control of a room type humidity control. In the average installation the fan is permitted to run only during such intervals as the thermostat is calling for heat or at the command of a limit control to prevent the overheating of the bonnet of a warm air furnace. The limit control should also prevent the operation of the fan at the command of the thermostat until the circulating air tem perature has increased to a predetermined point. For the cooling equipment provided in such installations, control during the cooling cycle will be an adaptation of the control principles described for central fan systems selected for the type of cooling equip ment utilized. CONTROL OF AUTOMATIC FUEL APPLIANCES It is essential that automatic controls be used with oil burners, gas burners, and stokers in order to. maintain even temperatures and provide safe and. economical operation of the heating plant. Oil Burner Controls In the normal oil' burner installation as encountered in residential and small commercial installations, the. burner operation is frequently regu lated by electric controls and primarily governed by a room thermostat. It is essential that a limiting control be incorporated in the control system to prevent the temperature of the heating medium from, exceeding any predetermined safe maximum. The type of limit control selected will depend on. the type, of the heating system. In a warm air furnace instal lation, a limit control would be used, reacting to the temperature of the heated air in the bonnet of the furnace; in a hot water system a control reacting to the temperature of the water in the boiler; and in a steam system a control reacting to the pressure of the steam in the boiler. . In addition to the normal control of the burner from the room thermostat mid limit control, it is necessary; that a combustion safety device be used to prevent operation of the burner under hazardous conditions. The oil fire is automatically ignited by means of gas, electric spark or incandescent element and the combustion safety control acting through a sequence device .permits the'burner operation only when the fire is prop erly established as-the burner starts up. A further function.of the com bustion safety- control is to react: to any major disturbance in the flame during the running operation, shutting down the burner and preventing . the discharge of unbumed fuel if for any reason tiie flame is extinguished. Gas Burner Controls In the case of the domestic buirner, full automatic operation is the normal requirement and the burner is started and stopped at the com mand of a room thermostat which, in turn, opens and closes a control valve in the gas supply line. Modulating controls and controls providing 646 CHAPTER 34. AUTOMATIC CONTRQL a high and low fire are also available for gas burners. For purposes of preventing abnormally high temperatures in the bonnet of gas-fired' furnaces or in the temperature of the water in gas-fired hot water heating boilers or excessive pressures in gas-fired steam boilers, temperature and pressure limit controls are used. Ignition is normally secured through the use of a gas pilot.flame and a safety device is provided, utilizing the heat of the pilot flame in such a manner that if the pilot light is extinguished for any reason, the main gas valve cannot be opened. For satisfactory and economical operation, all automatically-fired gas burners should be equipped with pressure regulators on the gas supply line. Stoker Controls Domestic stokers are normally placed under command of a room thermostat for primary operation subject also to the command of a limit control to prevent their operation when conditions in the boiler or furnace exceed predetermined safe maximums. Utilizing coal as fuel, automatic ignition is not provided and the stokers, once ignited, maintain their fire, merely changing the rate of combustion by changing the draft and the rate at which the coal is fed. Thus, at the command of the room ther mostat the stoker motor is started, driving a forced draft fan and fuel feeding mechanism. The rate of combustion is thus increased and this operation continues until the thermostat has been satisfied when the motor is stopped and the fuel in the combustion chamber continues to bum at a slow rate with.reduced draft. Automatic controls may be used to operate the stoker sufficiently to maintain a fire in mild weather and also to prevent feeding of fuel if the fire is extinguished. ZONE CONTROL Zone control for winter heating of buildings in which a single room thermostat will not provide satisfactory conditions and where the refine ment of control from individual room thermostats is not a requisite, may be secured in various ways. According to its size, use and exposure, the building may be divided into zones where the general requirements will be relatively constant. The number of zones is determined by (l) size of building, (2) number and character of exposures, (3) variation in occupancy and other inside, conditions, and (4) cost of additional zones. For large buildings it is desirable to have at least one zone for each exposure and for high buildings, each exposure may . be sub-divided into upper and lower zones in order to properly provide.for the stack effect. In buildings of this type it is advisable to provide a separate main with local thermostats for the street floor heat sources, especially those adjacent to entrances and exits. For small buildings, or where cost or other conditions limit the number of zones, a frequent compromise is to combine the North and West exposures in one zone and the South and East exposures.in a second zone. For steam heating systems, the radiator output may be proportionately reduced as the outdoor, temperature rises by several methods. Some of those in common use are: 1. Throttling steam pressure to allow flow through orifices in proportion to the needs for heating. 647