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878 CHAPTER 39 1954 Guide , 3. Controlling the length of lime steam flows into the radiators by admitting steam intermittently and varying the length of the on and.off periods. Two types of con trols are used. (lj A clock control providing on and off settings of various lengths, which can be changed in accordance with outside temperatures. In most cases these changes are made automatically by means of a thermostatic bulb, placed out doors. (2) A control, having an outdoor bulb and a bulb attached to the radiator, which varies the length and frequency of the on intervals in such a way that the radi ator temperature is varied according to the outside temperature. In some cases heat supply is controlled by combinations of the three methods. Before installing any type of modem temperature control equipment, it is necessary to see-that the heating system is iput in good operating condi tion.- ..In general,' the heating system in a'building is not given the atten tion that other 'inechanical equipment is given, because it will continue to function, after a, fashion, even though changes in piping, location of radiation, and settlement have taken place. Because of this depreciation of the system, , operation ,becomes-more .costly, and parts of the building may have, to be greatly overheated in-order to prevent underheating in other parts; Vents, traps;! vacuum pumps, and valves should be given a careful inspection and replaced or repaired, if required; The piping should be of adequate size and graded properly. The return,piping should be inspected, and any pockets or lifts removed and properly vented. These inspections and repairs are not costly; and may prevent a much greater outlay in future years. HOT WATER HEATING SYSTEMS Some-of the more generally accepted methods of controlling hot water heating systems are . 1. Controlling the flow of water supplied to the system while maintaining a con stant water temperature in the boiler or converter storage tank by: a. Starting and stopping the circulating pump by means of a.room thermostat. b. Maintaining a constant pump circulation and stopping the water circulation to a zone or several zones by the use of positive acting valves under the control of room thermosats. 2., Varying the temperature of the water supplied to the system in accordance With1 the variations of the'outside weather conditions, while maintaining constant water circulation to the system, and ; M a. Regulating the volume of water to each room or zone by gradual acting valves under the control of room thermostats. . A b. Stopping the flow of water to each room or zone by positive acting valves under .......the control of room thermostats. c. Arranging the piping and regulating valves, such as three-way valves, under the control of room thermostats to further modify the water temperature: in- .: accordance with the heating requirements of the room or zone. ' '> . Whatever the mechanical features of the regulator may be, its fun-;[ tion.is to dictate the flow 'impulse, or rate of flow, in such a way as to maintain the desired indoor temperature in a zone, regardless of the fluctu ations in the outdoor temperature. . UNIT HEATERS The standard method is to stop and start the fan by means of a twor., position room thermostat. A limit control may be provided to prevent operation of the fan when no heat is being supplied by the connecting^ services. Sometimes a separate manual switch may be connected so as, to permit operation of the fan for ventilating purposes at times when npj heating is desired. . A proportioning room thermostat may control a modulating valve m', the connecting service pipes, if continuous fan operation is maintained. `"J1, Automatic Control 879 AIR SUPPLY VENTILATING UNITS There are several different acceptable control cycles. The choice may be governed by compulsory ventilation codes. Usually there is a room thermostat that governs a heat supply valve, an air supply damper and a thermostat in the entering air stream for each unit! The room thermostat controls heat supply to such auxiliary , heat transmitters as may be in the room. In Control cycle No. 1 a fixed minimum volume of air is drawn from out of doors at times when the room temperature is at the design levell When the room is too cool, the damper is closed to outdoor air and is open for.recirculated air from the room, and the heat supply valve is open. As the room temperature approaches the.desired de gree, the damper moves,to admit more air from out of doors and reduces the volume recirculated. A further increase in room temperature causes throttling of the heat supply valve. Ultimately if the heat supply valve"closes completely, the outside air damper opens wide and recirculation of any part of the air ceases. During the com plete cycle, the thermostat in the air. stream prevents entry of air so cool as to pro mote chilling. . '- ; Control cycle No. 2 sometimes is employed and is the same a3 cycle No. 1 except that the outside intake damper opens a minimum amount or is closed without an in termediate position; the recirculating damper being adjusted accordingly, The minimum quantity may be adjusted as any desired percentage of the capacity of the device. --- . . Control cycle No. S has a stream-line thermostat in the mixed air stream to adjust the dampers so that if the room air is too cool, the outside air damper is closed and the valve on the heat supply pipe is open. As the room air temperature approaches that desired, the room thermostat takes control of. the valve and the stream-line thermostat opens the damper in the outside air.intake while moving the recirculating damper to reduce the amount of recirculation. Auxiliary heating devices in the rooms that have unit ventilating' machines should be controlled in sequence with the control cycle employed for these machines. Day-nighl thermostats often are used with unit ventilating machines to permit reduced night temperature, with corresponding economy. For the night setting, the fan-motors may be idle, while the heaters are cir culating warm air by gravity. It is also possible to provide intermittent operation of the fan motors while operating under control of the night or reduced-teinperature thermo stat. '.1 ' In many large schools it is possible to arrange for separate control cir cuits rather than for separate steam mains so that offices and toilet rooms may be controlled at a temperature considerably wanner than that of other rooms. UNIT COOLERS Although most unit coolers, can be adapted to any control cycle, con tinuous fan operation is recommended to avoid stratification and wide fluctuations in space temperature. Should the unit be completely selfcontained, control of the direct expansion refrigeration unit may be ob tained from the temperature of the recirculated air and from suction pres sure. In the case of multiple unit systems supplied with refrigerant or chilled water from a central source, a valve in the supply to each cooling coil may be controlled thermostatically from space temperature. REFRIGERATION AND DEHUM EDIFICATION EQUIPMENT . Typical control equipment and its functions are described in the followlng paragraphs. W"ell Water. Where well water is used directly in air washers or cooling coils,