Table 64 — Condenser Fan Staging
FAN RELAY |
|
| 48/50A UNIT SIZE |
| |
051,060 | 060 with MCHX | ||||
| |||||
OFC1,4* (MBB - RELAY 6) | OFM1 | OFM1, OFM2 | OFM1, OFM2 | OFM4 | |
OFC2 (MBB - RELAY 5) | OFM2 | OFM3, OFM4 | OFM3, OFM4, OFM5, OFM6 | OFM2 | |
OFC3 | NA | NA | NA | OFM3 | |
OFC1* | NA | NA | NA | OFM1 |
* For size 60 ton units with MCHX condensers, MBB – Relay 6 drives OFC4 and compressor contactor B1 or B2 auxiliary contacts drive OFC1.
For 060 size units with MCHX condensers, there are four outdoor fans, two for each independent refrigerant circuit. The control cycles two stages of outdoor fans for each circuit, one fan per stage, to maintain acceptable head pressure.
When a circuit A compressor has been commanded on, then OFC3 is energized via the normally opened auxiliary contacts on the compressor contactors. The auxiliary contacts are wired such that turning on either circuit A compressor will energize OFC3. Contactor OFC3 will remain on until all circuit A com- pressors have been commanded off. If SCTA is above the HPSP or if OAT is greater than 75 F, then condenser fan A (MBB Relay 6) will be energized (CD.F.A = ON) turning on OFC4. Condenser fan A will remain on until all compressors have been commanded off, or SCTA drops 40 F below the HPSP for greater than 2 minutes and OAT is less than 73 F.
When a circuit B compressor has been commanded on, then OFC1 is energized via the normally opened auxiliary contacts on the compressor contactors. The auxiliary contacts are wired such that turning on either circuit B compressor will energize OFC1. Contactor OFC1 will remain on until all circuit B com- pressors have been commanded off. If SCTB is above the HPSP or if OAT is greater than 75 F, then condenser fan B (MBB Relay 5) will be energized (CD.F.B = ON) turning on OFC2. Condenser fan B will remain on until all compressors have been commanded off, or SCTB drops 40 F below the HPSP for greater than 2 minutes and OAT is less than 73 F.
Failure Mode Operation — If either of the SCT or DPT sen- sors fails, then the control defaults to head pressure control based on the OAT sensor. The control turns on the second fan stage when the OAT is above 65 F and stages down when OAT drops below 50 F.
If the OAT sensor fails, then the control defaults to head pressure control based on the SCT sensors. The control turns on the second fan stage when the highest active circuit SCT is above the HPSP and stages down when the highest active cir- cuit SCT drops 40 F below the HPSP for longer than 2 minutes.
If the SCT, DPT, and OAT sensors have all failed, then the control turns on the first and second fan stages when any com- pressor is commanded on.
Compressor current sensor boards (CSB) are used on all units and are able to diagnose a compressor stuck on (welded contactor) condition. If the control commands a compressor off and the CSB detects current flowing to the compressor, then the first fan stage is turned on immediately. The second fan stage will turn on when OAT rises above 75 F or the highest ac- tive circuit SCT rises above the HPSP and remain on until the condition is repaired regardless of the OAT and SCT values.
ECONOMIZER INTEGRATION WITH MECHANICAL COOLING — When the economizer is able to provide free cooling (Run Status→ECON→ACTV = YES), mechanical cooling may be delayed or even held off indefinitely.
NOTE: Once mechanical cooling has started, this delay logic is no longer relevant.
Economizer Mechanical Cooling Delay — This type of me- chanical cooling delay is relevant to the all machine control types.
If the economizer is able to provide free cooling at the start of a cooling session, the mechanical cooling algorithm checks the economizer’s current position (Run Status→ ECON→ECN.P) and compares it to the economizer’s maxi- mum position (Configuration→ECON→EC.MX) – 5%. Once the economizer has opened beyond this point a
Economizer Control Point (Run Status→VIEW→EC.C.P)
—There are 4 different ways to determine the economizer control point when the economizer is able to provide free cooling:
If no mechanical cooling is active and HVAC mode = LOW COOL
EC.C.P = Setpoints→SA.LO + Inputs→RSET→SA.S.R
If no mechanical cooling is active and HVAC mode = HIGH COOL
EC.C.P = Setpoints→SA.HI + Inputs→RSET→SA.S.R When the first stage of mechanical cooling has started
EC.C.P = 53 F plus any economizer suction pressure reset applied
When the second stage of mechanical cooling has started EC.C.P = 48 F plus any economizer suction pressure reset
applied
Heating Control — The A Series ComfortLink™ con- trol system offers control for 3 different types of heating systems to satisfy general space heating requirements:
Variable air volume (VAV) type applications (C.TYP = 1, 2, 3, or 5) require that the space terminal positions be commanded to open to Minimum Heating positions when gas or electric heat systems are active, to provide for the unit heating system’s Minimum Heating Airflow rate.
For VAV applications, the heat interlock relay (HIR) func- tion provides the switching of a control signal intended for use by the VAV terminals. This signal must be used to command the terminals to open to their Heating Open positions. The HIR is energized whenever the Heating mode is active, an IAQ pre- occupied force is active, or if fire smoke modes, pressurization, or smoke purge modes are active.
SETTING UP THE SYSTEM — The heating configurations are located at the local display under Configuration→ HEAT. See Table 65.
Heating Control Type (HT.CF) — The heating control types available are selected with this variable.
0 = No Heat
1 = Electric Heat
2 = 2 Stage Gas Heat
3 = Staged Gas Heat
Heating Supply Air Set Point (HT.SP) — In a low heat mode for staged gas heat, this is the supply air set point for heating.
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