York ZT078-102 Installation Manual

Also see for ZT SERIES: Technical guide

6021980-UIM-A-112060 Johnson Controls Ducted SystemsOperationCooling operation• A Y1 call for the first stage of cooling is passed to the unitcontrol board (UCB). The UCB then determines whetherthe requested operation is available and if so, whichcomponents to energize. With a Y1 call for first stagecooling, the UCB determines if a first stage cooling outputis valid as long as all safeties and time-delays allow a C1output for cooling. The C1 relay on the UCB closes andsends 24 volts to the M1 relay. This starts compressor #1on low and also energizes terminal block TB3-1 for cond.fan #1 and cond. fan #2 on low speed. The UCBenergizes the VFD-equipped blower at the first stagespeed as set in the Smart Equipment™ control.• If a Y2 call is present, it is passed to the UCB. The UCBthen determines whether the requested operation isavailable and if so, which components to energize. With aY2 call for second stage cooling, the UCB determines if asecond stage cooling output is valid as long as all safetiesand time-delays allow a C2 output for cooling. The C2relay on the UCB closes and sends 24 volts to the M2relay. This starts compressor #2 on low speed and alsoenergizes for cond. fan #3 and cond. fan #4 on low speed.The UCB energizes the VFD-equipped blower at thesecond stage speed as set in the Smart Equipment™control.• If a Y3 call is present, it is passed to the UCB. The UCBthen determines whether the requested operation isavailable and if so, which components to energize. With aY3 call for third stage cooling, the UCB determines if athird stage cooling output is valid as long as all safetiesand time-delays allow a C3 output for cooling. The C3relay on the UCB closes and sends 24 volts to theterminal block TB3-2 which energizes compressors #1 &#2 on high speed and also cond. fan #1, cond. fan #2,cond. fan #3, and cond. fan #4 on high speed. The UCBenergizes the VFD-equipped blower at the third stagespeed as set in the Smart Equipment™ control.Free cooling operation with economizer• With a demand for first stage cooling either from athermostat or space sensor and the outside air is suitablefor free cooling, the dampers are modulated to maintainsupply air temperature to within +/- 1 degree of the freecooling SAT upper setpoint. If the output to theeconomizer actuator is at 10 VDC and the supply airtemperature cannot be controlled to within 5 degrees ofthe free cooling SAT upper setpoint, one compressor isenergized. The economizer is then modulated to maintainsupply air temperature to within +/- 1 degree of the freecooling SAT upper setpoint.• If a demand for second stage cooling occurs and theeconomizer is already modulating to maintain the supplyair temperature to the cooling SAT upper setpoint, thesetpoint is changed to the free cooling SAT lower setpoint.Any compressors that are energized remain energizedand the economizer dampers are modulated to maintainsupply air temperature to within +/- 1 degree of the coolingSAT lower setpoint. If the output to the economizeractuator is at 10 VDC and the supply air temperaturecannot be controlled to within 5 degrees of the coolingSAT lower setpoint, compressor stage 2 is energized. Theeconomizer is then modulated to maintain supply airtemperature to within +/- 1 degree of the cooling SATlower setpoint.• If the output to the economizer actuator is at minimumposition and the supply air temperature drops more than 5degrees below the current economizer SAT setpoint, thehighest stage compressor is de-energized as long as theminimum run time has elapsed.Economizer with single enthalpy sensorWhen the room thermostat calls for cooling, the low voltagecontrol circuit from R to G and Y1 is completed. The UCBenergizes the blower motor (if the fan switch on the roomthermostat is set to AUTO position) and drives the economizerdampers from fully closed to their minimum position. If theenthalpy of the outdoor air is below the setpoint of the enthalpycontroller (previously determined), Y1 energizes theeconomizer. The dampers modulate to maintain a constantsupply air temperature as monitored by the discharge air sensor.If the outdoor air enthalpy is above the setpoint, Y1 energizesthe compressor and condenser fan motor only.When the thermostat has been satisfied, it de-energizes Y1. Ifthe compressor has satisfied its minimum run time, thecompressor and condenser fan are de-energized. Otherwise, theunit operates the cooling system until the minimum run times forthe compressor are completed. After the compressor de-energizes, the blower is stopped following the elapse of the fanoff delay for cooling, and the economizer damper goes to theclosed position. If the unit continues fan operation, theeconomizer damper goes to the min. position.Economizer with dual enthalpy sensorsThe operation with the dual enthalpy sensors is identical to thesingle sensor except that a second enthalpy sensor is mountedin the return air. This return air sensor allows the economizer tochoose between outdoor air and return air, whichever has thelowest enthalpy value, to provide maximum operatingefficiency.Economizer with power exhaustA unit equipped with an economizer (single or dual enthalpy)and a power exhaust operates as specified above with oneaddition. The power exhaust motor is energized 45 secondsafter the actuator position exceeds the exhaust fan setpoint onthe economizer control. The R to G connection providesminimum position but does not provide power exhaustoperation.Motorized outdoor air dampersThis system operation is the same as the units with no outdoorair options with one exception. When the R to G circuit iscomplete, the motorized damper drives open to a position setby the thumbwheel on the damper motor. When the R to Gcircuit is opened, the damper spring returns fully closed.