All Precedent and Large Foundation units with Symbio 700 controls and two compressors utilize a manifolded set of compressors with only a single circuit in the unit. Compressor 1 (large comp) is on the left and Compressor 2 (small comp) is on the right. Precedent units (3-5T) utilize a single compressor and some of these units may also utilize a 2 stage compressor.
Odyssey units can be ordered in many configurations:
- Single Compressor, single or 2-Stage
- Dual Compressor, single manifolded circuit
- Dual Compressor, dual circuit
Compressor staging varies based on tonnage, unit type (VVDA, cooling, heat pump) and control type (thermostat vs zone sensor). For more information on compressor staging, visit the links below:
- Precedent with Symbio 700 Controls Compressor Staging and Turn Down Percentage
- Odyssey Dual Circuit Condenser Units Compressor Staging
Unit safeties
All Symbio 700 controlled units utilize the same basic safety features for each unit. Below is a brief description for each set of safeties, associated diagnostics and troubleshooting.
High Pressure Cutout (HPC) and Discharge Temp Limit (DTL)
Every compressor will have a normally closed High Pressure Cutout (HPC) and a normally closed Discharge Temperature Limit Switch (DTL) on the discharge line of that compressor. These safeties are wired in series with the compressor contactor 24vac. A HPC or DTL trip (safety opens) would remove the voltage from the compressor contactor and can produce the following diagnostics:
- Compressor 1 Proving Trip
- Compressor 2 Proving Trip
- Compressor 1 Proving Lockout
- Compressor 2 Proving Lockout
If the HPC or DTL trip while the compressor is running, both compressors will shut down for 15 minutes. After the 15 minute timer has expired, the system will auto reset and begin compressor staging as necessary. This will produce a Compressor Proving Trip alarm and can happen up to 3 times in a row. If it happens a 4th time, the unit will lockout compressor operation and set a Compressor Proving Lockout requiring a manual reset.
Typical Precedent
- For details on troubleshooting Compressor Proving Issues, visit these links:
HPC ratings vary based on the refrigerant type:
- R410 Open 650 psi / Close 550 psi
- R454B Open 600 psi / Close 500 psi
DTL ratings also vary based on the refrigerant type:
- R410 Open 230 deg / Close 180 deg
- R454B Open 245 deg / Close 195 deg
HPC/DTL Troubleshooting
HPC trip troubleshooting should start with visual inspection of the condenser coil, validating condenser fan operation, TXV operation and checking pressures.
- A dirty condenser coil will obstruct air flow through the coil and reduce the surface area for heat transfer.
- Condenser fan operation varies based on unit type, number of condenser fans and outdoor temperature. Most units with fixed speed compressors utilize fixed speed/contactor started condenser fans.
- On Precedent units with a single fixed speed condenser fan, the fan will run anytime the compressors(s) are running. (a) (b)
- On Precedent and Large Foundation units with two fixed speed condenser fans, Condenser Fan 1 will run anytime there is a compressor running, Condenser Fan 2 operation is based on running cooling capacity and outdoor temperature. (a)
- On Precedent units that utilize variable speed condenser fans, these vary condenser fan speed based on running cooling capacity and outdoor temperature. These fan utilize a 0-10vdc signal from the Symbio 700 P12 connector.
- On Odyssey units with a single fixed speed condenser fan, the fan will run anytime a compressor is running. (a)
- On single-circuit Odyssey units with two fixed speed condenser fans, both condenser fan run anytime a compressor is running. (a)
- On dual-circuit Odyssey units with two fixed speed condenser fans, Condenser Fan 1 runs with Compressor 1, Condenser Fan 2 runs with Compressor 2. (a)
- (a) Precedent heat pump units do not run the condenser fan in defrost.
- (b) Some R454B heat pumps with modulating hot gas reheat utilize fixed speed fans that will cycle off based on ambient temperature when the unit is in reheat
- Restrictions in the system will cause high head pressures.
- Filter dryer - a stopped up filter dryer is a sign of debris, bearing material, burned oil or organic contamination.
- The presence of bearing material in the system can come from compressor bearing damage due to liquid refrigerant in the oil and can be caused by missing or malfunctioning crankcase heaters and continued operation with liquid refrigerant flooding back to the compressor.
- The presence of burned oil is typically from extended compressor operation at high temperatures, failed compressor or poor system cleanup after compressor failure/replacement.
- The presence of debris and/or organic contamination in the system is typically from poor cleaning or evacuation practices during system repair.
- Filter dryer - a stopped up filter dryer is a sign of debris, bearing material, burned oil or organic contamination.
- TXV operation - TXV sensing bulb location should be checked to ensure that it is the optimal location. TXV operation can be checked by holding the sensing bulb and watching the pressures, a working valve should open as it warms up. For units with two TXVs on the evaporator coil, testing would require that one TXV sensing bulb be placed in water or in a media that has a fixed or steady temperature to get that valve to "hold" its operation while holding/warming up the other bulb.
- Every Precedent, Large Foundation and Odyssey condenser unit have charging and pressure charts located on the inside of the compressor access panel. This chart also shows target super heat and/or sub cooling numbers along with the factory charge amount.
DTL trip can be caused by any scenario that also causes HPC trip. Operating with a restriction causing low refrigerant flow back to the compressor can cause compressor overheating leading to DTL trip.
Low Pressure Cutout (LPC)
All Precedent and Large Foundation units utilize a single LPC on the suction line mounted before the tee in the suction manifold near the compressors. Odyssey Condenser is similar for single circuit units, but dual circuit units will have two LPC switches, one on each suction line near the compressor. When the LPC trips, it can produce the following diagnostics:
- Diagnostic: Circuit 1 LPC Trip
- Diagnostic: Circuit 2 LPC Trip
- Diagnostic: Circuit 1 LPC Lockout
- Diagnostic: Circuit 2 LPC Lockout
Typical Precedent
If the LPC trips while the compressor is running, both compressors will shut down for 3 minutes. After the 3 minute timer has expired, the system will auto reset and begin compressor staging as necessary. This will produce a Diagnostic: Circuit x LPC Trip alarm and can happen up to 3 times in a row. If it happens a 4th time, the unit will lockout compressor operation and set a Circuit x LPC Lockout requiring a manual reset.
LPC Ratings vary by model, type (HP or Cooling only) and refrigerant. The list is too extensive for this document. For assistance with switch ratings, visit https://www.tranesupply.com/ and use you unit serial number to locate the switch and ratings (typically ratings are in the part description).
LPC Troubleshooting
LPC trip troubleshooting should start with visual inspection of the evaporator coil, air filters, air flow, TXV operation and checking pressures.
- A dirty evaporator coil reduces the surface area available for the heat transfer from the air to the refrigerant. This will also cause issues with potential liquid slugging back to the compressor and reduced refrigerant flow to the compressor.
- Dirty air filters and low air flow due the duct work restrictions or improperly set indoor supply fan will produce the same issues as a dirty coil.
- TXV operation - TXV sensing bulb location should be checked to ensure that it is the optimal location. TXV operation can be checked by holding the sensing bulb and watching the pressures, a working valve should open as it warms up. Units with two TXVs on the evap coil, testing would require that one TXV sensing bulb be placed in water or in a media that has a fixed or steady temperature to get that valve to "hold" its operation while holding/warming up the other bulb.
- Every Precedent, Large Foundation and Odyssey unit have charging and pressure charts located on the inside of the compressor access panel. This chart also shows target super heat and/or sub cooling numbers along with the factory charge amount.
Frostat (FOS)
All Precedent and Large Foundation units utilize a normally closed Frostat on the suction line at the evaporator that is wired directly to the Symbio 700. When the Frostat trips it will produce:
- Diagnostic: Frostat Trip
Typical Precedent
Odyssey does not utilize a Frostat on the evaporator for frost protection, but Odyssey does have a Frostat input on the AHU Indoor Options Module that can be "INSTALLED" in the Symbio 700 configuration. This Frostat input on the Odyssey AHU Indoor Options Module is repurposed as a high temp limit for the VFD on SZVAV AHUs with model number digit 15 = D. The purpose of the high temp limit is to shut the compressors down (specifically on heat pump mode) if the temp at the VFD > 135°F (resets @ 105°F). If this FOS (High Temp Limit in this case) trips (closes) it will produce the same:
- Diagnostic: Frostat Trip
On Precedent, Large Foundation and Odyssey, when the FOS trips, all compressor operation is stopped and the indoor fan will stay on at low speed to thaw the coil (to cool the VFD on Odyssey AHU). This is an auto-reset alarm and once the FOS resets (closes), the compressors will begin staging up as necessary. This alarm does not have a hard lockout/manual reset mode, it can trip indefinitely with auto-resets.
Condensate Overflow
Precedent and Large Foundation can have an optional Condensate Overflow Switch (COF) installed either at the factory or in the field. This switch is a basic 2 wire normally open (close on high water) float switch. It is mounted in the corner of the condensate pan near the drain connection and wired back to the Indoor Options Module P15 (white). The only exception to this is on Precedent 3-5T with A0 cabinet, this unit does not utilize an Indoor Options Module at all and instead has a standard Customer Connection Module with the COF wired to P12 (black). When the COF trips it can produce:
- Diagnostic: High Condensate Level Detected
- Diagnostic: Condensate Overflow Lockout
When the COF trips (closes) Symbio 700 will shutdown all unit operation including the indoor fan. Once the COF resets (opens), the unit will start back up normally. There is not reset time, it is based on how fast the drain clears. The switch can trip up to two times in a 72 hour period and auto-reset each time, this will produce a Diagnostic: HIgh Condensate Level. If the switch trips a third time in 72 hours, Symbio 700 will produce a Diagnostic: Condensate Overflow Lockout and will require a manual lockout. Note, the 72 hour timer is based on the last COF trip.
- If COF trips and resets two times in 24 hours, the 72 hour timer started at the first trip
- Once it has been 72 hours since the first trip, the timer is reset to when the 2nd trip occurred
- If the COF does not trip again within 72 hours of the 2nd trip, the timer is reset to 0 until the next COF trip
Odyssey units do not utilize a COF as part of the configuration, requirements for a COF would involve a aftermarket switch tied into the Equipment Shutdown Input on the Symbio 700 J18. If a smoke detector is also required on the unit, it will need to be wired in series with the aftermarket COF. For more information on Equipment Shutdown Input see this article → Symbio™ 700 - Emergency Stop Input
Unit not responding to thermostat control
Precedent and Odyssey units shipped from the factory with model number digit 21 = 1/2/3, will ship with the configuration set for a zone sensor for space control instead of a thermostat. Unit with model number digit 21 = 0 will be shipped configured for a conventional thermostat.
A unit configured for a zone sensor without a zone sensor connected will show a Space Temperature Active of 1,000,001°F and have the following diagnostics:
- Space Temperature Active
- Space Temperature Arbitrator
For information on how to switch from a zone sensor to a conventional thermostat, see the link below:
Symbio™ 700 - How to Change from a Zone Sensor to a Thermostat Control
Precedent units configured as VVDA (discharge air control) for the System Type can not be set to utilize a conventional thermostat without changing the System Type to CVZT or VVZT (space temp control), but be warned that a VVDA unit changed to CVZT/VVZT with VAV boxes installed is not recommended. CVZT/VVZT are space control methods and will not control fan speed to duct static, fan speed will then be based on cooling stages and/or load in the space and there will be not protection for the ductwork from over pressurization.
On a VVDA unit, 24vac on S700 J21-4 (G/VAV Changeover) will put the unit into MAX HEAT mode and the unit will try to start heating 6 min later.
Large Foundation units all ship configured for conventional thermostat control only.
WARNING:
Information in this article is intended for use by individuals possessing adequate backgrounds of electrical and mechanical experience and who comply with all federal, state, and local laws, rules, orders, or regulations related to the installation, service, or repair of a heating or central air conditioning product. Any attempt to install, service, or repair a heating or central air conditioning product may result in personal injury and/or property damage. The manufacturer or seller cannot be responsible for the interpretation of the information contained herein, nor can it assume any liability in connection with its use.