York ZT078-102 Installation Manual

Also see for ZT SERIES: Technical guide

6021980-UIM-A-1120Johnson Controls Ducted Systems 61Cooling Operation ErrorsEach cooling system is monitored for operation outside of theintended parameters. Errors are handled as described below. Allsystem errors override minimum run times for compressors.Note: The following components are needed to access thecontrol points in the Smart Equipment™ control.1. Local LCD on Unit Control Board.OR2. Mobile Access Portal (MAP) Gateway (Portable).• Source 1 P/N S1-JC-MAP1810-OP• MAP Gateway Quick Start Guide P/N 24-10737-16• MAP Gateway Instruction P/N 24-10737-8High-pressure limit switchDuring cooling operation, if a high-pressure limit switch opens,the UCB de-energizes the compressor, initiates the ASCD (anti-short cycle delay), and stops the condenser fans. If the call forcooling is still present at the conclusion of the ASCD, the UCBre-energizes the halted compressor.If a high-pressure switch opens three times within two hours ofoperation, the UCB locks out the compressor.Low-pressure limit switchThe low-pressure limit switch is not monitored during the initial30 seconds of a cooling system's operation. For the following30 seconds, the UCB monitors the low-pressure switch toensure that it closes. If the low-pressure switch fails to closeafter the 30-second monitoring phase, the UCB de-energizesthe compressor, initiates the ASCD, and stops the condenserfans.When the low-pressure switch is proven (closed during the 30-second monitor period described above), the UCB monitors thelow-pressure limit switch for any openings. If the low-pressureswitch opens for greater than 5 seconds, the UCB de-energizesthe compressor, initiates the ASCD, and stops the condenserfans.If the call for cooling is still present at the conclusion of theASCD, the UCB re-energizes the halted compressor.If a low-pressure switch opens three times within one hour ofoperation, the UCB locks out the compressor.Evaporator low limitThe evaporator low limit sensor (EC1) is located on the suctionline at the evaporator coil. During cooling operation, if theevaporator low limit sensor detects a temperature below 26°F(default), the UCB de-energizes the compressor, initiates theASCD, and stops the condenser fans. If the call for cooling isstill present at the conclusion of the ASCD, the UCB re-energizes the halted compressor.If the UCB detects the evaporator low limit sensor (EC1) fallingbelow 26°F (default) three times within two hours of operation,the UCB locks out the compressor.Low ambient coolingTo determine when to operate in low ambient mode, the UCBhas an outdoor air temperature sensor (OAT) with a lowambient setpoint at 45ºF (default). When the OAT sensorsenses a temperature below the low ambient setpoint and thethermostat is calling for cooling, the UCB operates in the lowambient mode.Low ambient mode operates the compressors in this manner:10 minutes on, 5 minutes off. The indoor blower is operatedthroughout the cycle. The 5-minute off period is necessary todefrost the indoor coil.Low ambient mode always begins with compressor operation.Compressor minimum run time may extend the minutes ofcompressor operation. The off cycle begins immediatelyfollowing the elapse of the minimum run time.When operating in low ambient mode, an evaporator low limitsensor (EC1) temperature below 26ºF de-energizes thecompressor. If the call for cooling is still present at the end ofthe ASCD and the evaporator temperature sensor (EC1)temperature is above 26°F, the unit resumes operation.Safety controlsThe unit control board monitors the following inputs for eachcooling system:1. An evaporator low limit sensor (EC1) to protect against lowevaporator temperatures due to a low airflow or a lowreturn air temperature, set at 26°F. The evaporator low limitsensor is located on the suction line at the evaporator coil.2. A high-pressure switch to protect against excessivedischarge pressures due to a blocked condenser coil or acondenser motor failure. The switch opens at 625 ± 25psig.3. A low-pressure switch to protect against loss of refrigerantcharge. the switch opens at opens at 50 ± 5 psig.The above pressure switches are hard-soldered to the unit. Therefrigeration systems are independently monitored andcontrolled. On any fault, only the associated system is affectedby any safety or preventive action.The UCB monitors the temperature limit switch of electric heatunits and the temperature limit switch and the gas valve of gasfurnace units.Compressor protectionIn addition to the external pressure switches, the compressorsalso have inherent (internal) protection. If there is an abnormaltemperature rise in a compressor, the protector opens to shutdown the compressor. The UCB incorporates features tominimize compressor wear and damage. An anti-short cycledelay (ASCD) is used to prevent operation of a compressor toosoon after its previous run. Additionally, a minimum run time isimposed any time a compressor is energized.The ASCD is initiated on unit start-up and on any compressorreset or lock-out.