Carrier WeatherExpert 48N2 Controls, Start-up, Operation, Service And Troubleshooting Instructions

Also see for WeatherExpert 48N2: Installation instructions Installation instructions Controls, start-up, operation, service, and troubleshooting Installation instructions

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Carrier WeatherExpert 48N2 Controls, Start-up, Operation, Service And Troubleshooting Instructions Manual pdf 69 page image

69TEMPERING MODE — In a vent or cooling mode, theeconomizer at minimum position may send extremely coldoutside air down the ductwork of the building. Therefore itmay be necessary to bring heat on to counter-effect this lowsupply-air temperature. This is referred to as the temperingmode.Setting up the System — The relevant set points for temper-ing are located at the local display under Setpoints:Operation — First, the unit must be in a vent mode, a low cool,or a high cool HVAC mode to be considered for a temperingmode. Secondly, the tempering mode is only allowed when therooftop is configured for modulating gas, SCR electric heat, orhydronic heating (ConfigurationHEATHT.CF=3 or 4).Also, if OAT is above the chosen tempering set point, temper-ing will not be allowed. Additionally, tempering mode islocked out if any stages of mechanical cooling are present.If the control is configured for staged gas, modulating gas,SCR electric heat, or hydronic heating and the control is in avent, low cool, or high cool HVAC mode, and the rooftop con-trol is in a situation where the economizer must maintain aminimum position/minimum cfm, then the evaporator dis-charge temperature (EDT) will be monitored. If the EDT fallsbelow a particular trip point then tempering mode may becalled out:HVAC mode = “Tempering Vent”HVAC mode = “Tempering LoCool”HVAC mode = “Tempering HiCool”The decision making/selection process for the temperingtrip set point is as follows:If an HVAC cool mode is in effect, then the tempering coolpoint is SASP – T.CL.If not in effect and unit is in a pre-occupied purge mode(Operating ModesMODEIAQ.P=ON), then the trip pointis T.PRG.If not in effect and unit is in an occupied mode (OperatingModesMODEIAQ.P=ON), then the trip point isTEMPVOCC.For all other cases, the trip point is TEMPVUNC.NOTE: The unoccupied economizer free cooling does notqualify as a HVAC cool mode as it is an energy saving featureand has its own OAT lockout already. The unoccupied freecooling mode (HVAC mode = Unocc. Free Cool) will overrideany unoccupied vent mode from triggering a tempering mode.A minimum amount of time must pass before calling outany tempering mode. In effect, the EDT must fall below thetrip point value –1° F continuously for a minimum of 2 min-utes. Also, at the end of a mechanical cooling cycle, a 10-min-ute delay will be enforced before considering a tempering dur-ing vent mode in order to allow any residual cooling to dissi-pate from the evaporator coil.If the above conditions are met, the algorithm is free toselect the tempering mode (MODETEMP).If a tempering mode becomes active, the modulating heatsource (staged gas, modulating gas, SCR electric heat, or hotwater) will attempt to maintain leaving-air temperature (LAT)at the tempering set point used to trigger the tempering mode.The technique for modulation of set point for staged gas, mod-ulating gas, SCR electric heat, and hydronic heat is the same asin a heat mode. More information regarding the operation ofheating can be referenced in the Heating Control section.Recovery from a tempering mode (MODETEMP) willoccur when the EDT rises above the trip point. On any changein HVACMODE, the tempering routine will re-assess the tem-pering set point which may cause the control to continue or exittempering mode.Static Pressure Control — Variable air volume (VAV)air-conditioning systems must provide varying amounts of airto the conditioned space. As air terminals downstream of theunit modulate their flows, the unit must simply maintain con-trol over duct static pressure in order to accommodate theneeds of the terminals, and therefore to meet the varying com-bined airflow requirement. The unit design includes an option-al means of accommodating this requirement. This section de-scribes the technique by which this control takes place.A unit intended for use in a VAV system can be equippedwith a variable frequency drive (VFD) for the supply fan. Thespeed of the fan can be controlled directly by the ComfortLinkcontrols. A duct static pressure transducer is located in the aux-iliary control box. The signal from the pressure sensor is re-ceived by the RCB board and is then used in a PID control rou-tine that outputs a fan speed to the VFD.The PID routine periodically calculates the static pressureerror from set point. This error at any point in time is simplythe duct static pressure set point minus the measured duct stat-ic. It is the Proportional term of the PID. The routine also cal-culates the Integral of the error over time, and the Derivative(rate of change) of the error. A calculated value is then used tocreate an output signal used to adjust the VFD to maintain thestatic pressure set point.SETTING UP THE SYSTEM — Here are the options un-der the Local Display Mode ConfigurationSP. See Table 46.Static Pressure Configuration (SP.CF) — This variable isused to configure the use of ComfortLink for static pressurecontrol. It has the following options:• 0 (DISABLED) - No static pressure control by Com-fortLink controls. This would be used for a constant vol-ume (CV) application when static pressure control is notrequired or for a VAV application if there will be third-party control of the VFD. In this latter case, a suitablemeans of control must be field installed.• 1 (ENABLED) - This will enable the use of ComfortLinkcontrolsStaged Air Volume Control (SP.SV) — This variable enabledthe use of a CV unit with VFD for staged air volume control.Static Pressure Sensor (SP.S) — This variable enables the useof a supply duct static pressure sensor. This must be enabled touse ComfortLink controls for static pressure control. If using athird-party control for the VFD then this should be disabled.Static Pressure Low Range (SP.LO) — This is the minimumstatic pressure that the sensor will measure. For most sensorsthis will be 0 in. wg. ComfortLink controls will map this valueto a 4 mA sensor output.Static Pressure High Range (SP.HI) — This is the maximumstatic pressure that the sensor will measure. Commonly thiswill be 5 in. wg. The ComfortLink controls will map this valueto a 20 mA sensor output when the signal is 20 mA.Static Pressure Set Point (SP.SP) — This is the static pres-sure control point. It is the point against which ComfortLinkcontrols compares the actual measured supply duct pressure fordetermination of the error that is used for PID control. AdjustSP.SP to the minimum value necessary for proper operation ofair terminals in the conditioned space at full load and part load.Too high a value will cause unnecessary fan motor power con-sumption at part load conditions and/or noise problems. Toolow a value will result in insufficient airflow.ITEM EXPANSION RANGE UNITS CCNPOINT DEFAULTT.PRG TemperingPurge SASP–20-80 dF TEMPPURG 50T.CL Tempering inCool Offset5-75 ^F TEMPCOOL 5T.V.OC Tempering VentOcc SASP–20-80 dF TEMPVOCC 65T.V.UN Tempering VentUnocc. SASP–20-80 dF TEMPVUNC 50