Carrier 48AJ Operating And Service Manual

Also see for 48AJ: Installation, Start-Up and Service Instructions Installation instructions Operation and service guide Started guide

59Outdoor Dewpoint Limit Check — If an outdoor relativehumidity sensor is installed, the control is able to calculate theoutdoor air dewpoint temperature and will compare thistemperature against the outside air dewpoint temperaturelimit configuration (Configuration→ECON→O.DEW). If theoutdoor air dewpoint temperature is greater than O.DEW, freecooling will not be allowed. Fig. 9 shows a horizontal limit linein the custom curve of the psychrometric chart. This is the out-door air dewpoint limit boundary.Custom Psychrometric Curves — Refer to the psychrometricchart and the standard Honeywell A-D curves in Fig. 8. Thecurves start from the bottom and rise vertically, angle to the leftand then fold over. This corresponds to the limits imposed bydry bulb changeover, outdoor enthalpy changeover andoutdoor dewpoint limiting respectively. Therefore, it is nowpossible to create any curve desired with the addition of oneoutdoor relative humidity sensor and the options forchangeover now available. See Fig. 9 for an example of acustom curve constructed on a psychrometric chart.UNOCCUPIED ECONOMIZER FREE COOLING — ThisFree Cooling function is used to start the supply fan and usethe economizer to bring in outside air when the outsidetemperature is cool enough to pre-cool the space. This is doneto delay the need for mechanical cooling when the systementers the occupied period. This function requires the use of aspace temperature sensor.When configured, the economizer will modulate during anunoccupied period and attempt to maintain space temperatureto the occupied cooling set point. Once the need for cooling hasbeen satisfied during this cycle, the fan will be stopped.Configuring the economizer for Unoccupied EconomizerFree Cooling is done in the UEFC group. There are threeconfiguration options, FC.CF, FC.TM and FC.LO.Unoccupied Economizer Free Cooling Configuration(FC.CF) — This option is used to configure the type of unoc-cupied economizer free cooling control that is desired.0 = disable unoccupied economizer free cooling1 = perform unocc. economizer free cooling as available dur-ing the entire unoccupied period.2 = perform unocc. economizer free cooling as available,FC.TM minutes before the next occupied period.Unoccupied Economizer Free Cooling Time Configuration(FC.TM) — This option is a configurable time period, prior tothe next occupied period, that the control will allow unoccu-pied economizer free cooling to operate. This option is onlyapplicable when FC.CF = 2.Unoccupied Economizer Free Cooling Outside LockoutTemperature (FC.LO) — This configuration option allowsthe user to select an outside-air temperature below which unoc-cupied free cooling is not allowed. This is further explained inthe logic section.Unoccupied Economizer Free Cooling Logic — The follow-ing qualifications that must be true for unoccupied free coolingto operate:• Unit configured for an economizer• Space temperature sensor enabled and sensor readingwithin limits• Unit in the unoccupied mode• FC.CF set to 1 or FC.CF set to 2 and control is withinFC.TM minutes of the next occupied period• Not in the Temperature Compensated Start Mode• Not in a cooling mode• Not in a heating mode• Not in a tempering mode• Outside-air temperature sensor reading within limits• Economizer would be allowed to cool if the fan wererequested and in a cool mode• OAT > FC.LO (1.0° F hysteresis applied)• Unit not in a fire smoke mode• No fan failure when configured to for unit to shut downon a fan failureIf all of the above conditions are satisfied:Unoccupied Economizer Free Cooling will start when both ofthe following conditions are true:{SPT > (OCSP + 2)} AND {SPT > (OAT + 8)}The Unoccupied Economizer Free Cooling Mode will stopwhen either of the following conditions are true:{SPT < OCSP} OR {SPT < (OAT + 3)} where SPT = SpaceTemperature and OCSP = Occupied Cooling Set Point.When the Unoccupied Economizer Free Cooling mode isactive, the supply fan is turned on and the economizer dampermodulated to control to the supply air set point (Setpoints→SASP) plus any supply air reset that may be applied (Inputs→RSET→SA.S.R).ECONOMIZER OPERATION CONFIGURATION — Theconfiguration items in the E.CFG menu group affect howthe economizer modulates when attempting to follow aneconomizer cooling set point. Typically, they will not needadjustment. In fact, it is strongly advised not to adjust theseconfiguration items from their default settings without firstconsulting a service engineering representative.In addition, the economizer cooling algorithm is designed toautomatically slow down the economizer actuator’s rate oftravel as outside air temperature decreases.Fig. 8 — Psychrometric Chart forEnthalpy ControlCONTROL CURVE CONTROL POINT(approx Deg) AT 50% RHA 73B 68C 63D 58A50-544