Engineering Manual o.. - HVAC.Amickracing

INDIVIDUAL ROOM CONTROL APPLICATIONSCONTROLContr ol of unit hea ter s may be modula ting b ut is usuall y twoposition. Lo w-limit contr ol (sensing the w ater or condensa tetemper atur e) is usuall y pr ovided for night or summer shutdo wnif the hea ting system is of f, and pr events the f ans fr om blowingcold air if the hea ting system f ails.Two-Position Contr olModulating ControlModulating control (Fig. 17) throttles the heating medium inpr opor tion to c hang es in space temper atur e. The fan oper atescontin uously to pr event air sta gnation. On a dr op in temper atur e,the ther mosta t sends a signal to r eposition the v alve and the f anforces air acr oss the coil to r aise space temper atur e. When thevalve is completel y closed, the coil cools do wn and the lo w-limit contr ol shuts of f the fan. In some cases, the fan r unscontin uously or under time contr ol without a lo w limit switch.Figur e 15 shows two-position contr ol of a steam or hot w aterunit. When space temper atur e falls below the ther mosta tsetpoint, the ther mosta t star ts the f an. The fan forces air acr ossthe coil to w ar m the space . When space temper atur e rises tothe setpoint, the ther mosta t contacts open and the f an tur ns off.Low-limit contr ols ar e typicall y installed to pr event the f an fr omoperating until there is heat in the coil.STEAM ORHOT WATERSUPPLYFANVALVETHERMOSTATSTEAM ORHOT WATERSUPPLYUNIT HEATERCOILFANUNIT HEATERCOILROOMTHERMOSTATLOW LIMITHOT WATER ORCONDENSATERETURNSTEAM TRAP(IF STEAM SUPPLY)C3025LOW LIMITHOT WATER ORCONDENSATERETURNSTEAM TRAP(IF STEAM SUPPLY)C3026Fig. 15. Unit Heater Two-PositionControl–Steam or Hot Water Heat.Unit heaters with hot water or steam valves can also beoperated from two-position thermostats. The thermostat canoperate the valve and the fan or just the valve and let the lowlimit cycle the fan.Figur e 16 shows tha t contr ol of an electr ic-heat unit hea ter issimilar to steam or hot w ater tw o-position contr ol except tha tthe ther mosta t cycles the fan and the r elay or contactor ener gizesthe electr ic heating elements w hen the f an is r unning .RELAYFANTHERMOSTATFig. 17. Unit Heater with Modulating Control.Pneuma tic contr ol of the valve and electr ic switching of thefan motor b y the low-limit contr ol ar e the most economicalmeans of accomplishing modula ting contr ol. The low-limitcontrol is usually a strap-on type mounted on the return pipe ofeither w ater or steam systems. In a steam system, the contr ollersensing element should be mounted between the unit and thesteam trap.DOWN-BLOW UNIT HEATERThe down-blow unit hea ter (F ig. 18) circulates war m air tha tnor mally str atifies near the ceiling . Contr ol for this a pplica tionrequir es two ther mosta ts: one in the r oom (occupied ar ea) andthe other near the ceiling . The r oom ther mosta t contr ols thesuppl y valve. The fan cycles with the v alve. As the ceilingtemper atur e rises above ceiling ther mosta t setpoint, the ceilingther mosta t overrides the f an contr ol and star ts the f an. The fanruns until the ceiling ther mosta t is satisfied. This applica tionrecycles war m air and r educes the amount of hea ting mediumcirculated through the coil.RESISTANCEHEATING ELEMENTSUNIT HEATERTO POWERSUPPLYC3027Fig. 16. Unit Heater Two-Position Control–Electric Heat.410ENGINEERING MANUAL OF AUTOMATIC CONTROL