Issue:
Low Superheat diagnostic
Resolution:
RTAA, RTUA, RTWA
Low superheat is annunciated when the suction superheat (difference between compressor suction and saturated evaporator refrigerant temperatures) is less than or equal to 2 F for more than 2400 degree F seconds. For RTAA 70-125 the superheat setpoint should be set to 4 F and for RTAA 130-400 the setpoint should be 8 F. For RTUA 70-125 the superheat setpoint should be set to 4 F. For RTWA 70-125 the superheat setpoint should be set to 6 F.
If either of the saturated evaporator sensor or compressor suction sensor are reading incorrectly, this could cause the low superheat. Complete checkout (including ohming each leg to ground) of the saturated evaporator refrigerant sensor and compressor suction sensor should be done.
Check out the electronic expansion valve (EXV) for proper operation. You can do this by checking the suction superheat while the unit is on. If metering correctly, the EXV will maintain the superheat setpoint within 2 F of the setpoint. If the superheat is very erratic and outside the setpoint +/- 2 F, suspect a bad EXV. To further test, after running the chiller for a period of time (to allow refrigerant to condense in condenser), shut the circuit you are troubleshooting down and immediately check the sight glass on that circuit and verify that after the EXV finishes ratcheting down, there is no refrigerant flowing through the sight glass. The EXV is commanded to close after the circuit shuts down and there should not be any refrigerant flowing through it. If you see that there is refrigerant flowing, replace the EXV.
Notes:
Change out both the Saturated Evaporator Temperature sensor and the Compressor Suction Temperature sensor since they are a matched pair.
Resource Available:
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Tech TV Video(s):
- N/A
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Reference Literature: (Electronic copies can be pulled from e-library at Trane Residential (tranetechnologies.com))
- IOM:
- Service Guide:
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Other:
- RLC-SVD03A-EN
Changes:
Other Question/Problem/Cause Statements:
- Will a bad saturated evap temperature sensor cause a Low Superheat diagnostic?
- Low superheat
- The Saturated Evap Temp sensor was reading 60 degrees high and the suction sensor was reading okay. Therefore the calculation made for the superheat would have been a negative number and the Low Superheat diagnostic would be generated.
Facts:
- Air-Cooled Chiller (Charmes) (RTAA)
- Air-Cooled Chiller (Pueblo) (RTAA 130-400 Ton)
- Air-Cooled Chiller (Pueblo) (RTAA 70-125 Ton)
- Compressor Chiller (Pueblo) (RTUA)
- Water-Cooled Chiller (Pueblo) (RTWA)
Document Identifier: DOC-81277
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.