Centrifugal Chillers Critical — Compressor Protection

Carrier 19XR Alarm 105: Low Evaporator Refrigerant Temperature Diagnosis

Published: 2026-07-24

🚨 Critical Diagnostic Alert

Alarm 105 on the Carrier 19XR centrifugal chiller indicates that the evaporator refrigerant temperature has dropped below the safety setpoint (typically 33°F / 0.5°C), triggering a compressor protection shutdown. This alarm prevents freeze damage to the evaporator tubes — a catastrophic failure mode that can crack tubes and flood the refrigerant circuit with water, requiring a complete chiller teardown and tube replacement ($30,000-80,000).

The root cause of Alarm 105 is almost always a mismatch between chiller capacity and building load: either the chiller is producing more cooling than the building needs (low load condition), or the chilled water cannot carry the heat away fast enough (low flow condition). The diagnostic priority is always: verify water flow first, then check refrigerant metering, then evaluate load matching.

Carrier — Cross-Reference

Available in Cities

Commercial Systems

⚠ Common Field Complaints — Carrier

Real-world issues reported by HVAC technicians and facility managers:

Coil leak on WeatherExpert RTU evaporator coils (2019-2023 vintages) — Carrier extended coil warranty to 10 years on affected serial ranges
i-Vu controller firmware bugs causing false alarm 104 (surge) on 19DV chillers — update to firmware v4.2.1 or later
Compressor short-cycling on 24ACC6 residential units when paired with non-OEM thermostats — requires Carrier Infinity or Cor thermostat for proper staging

🔍 Symptoms Checklist

⚠️ Leaving chilled water temperature drops below 36°F (2.2°C) setpoint
⚠️ Alarm 105 displayed on PIC II/III controller with manual reset required
⚠️ Compressor unloads to minimum capacity then locks out if condition persists
⚠️ Evaporator refrigerant saturation temperature below 33°F (0.5°C)

🛠️ OEM Replacement Parts

Part Name	OEM Part Number	Est. Price
Evaporator Refrigerant Temperature Sensor	`19XR-TEMP-011`	$285
Leaving Chilled Water Temperature Sensor	`19XR-TEMP-004`	$220
Electronic Expansion Valve (EXV) Actuator	`19XR-EXV-ACT`	$1,400

📋 Interactive Diagnostic Procedure

Click each step to expand detailed diagnostic instructions. Follow in sequence — each step builds on the previous one.

1 Verify Chilled Water Flow Rate

Alarm 105 is most commonly caused by insufficient chilled water flow through the evaporator. Check the differential pressure across the evaporator (should match the design delta-P, typically 10-15 ft head). If delta-P is low, investigate: pump operation (verify pump is running and impeller is intact), strainer blockage (open and inspect the Y-strainer), air in the system (bleed air vents at the highest points), or closed isolation valves in the chilled water loop. A flow switch proving circuit should also be verified — a stuck flow switch can cause the controller to falsely report low flow.

2 Check EXV (Electronic Expansion Valve) Operation

The EXV modulates refrigerant flow into the evaporator. If the EXV is stuck open or overfeeding, the evaporator pressure drops rapidly, causing the saturated refrigerant temperature to fall below the low-limit setpoint. Use the PIC controller service menu to command the EXV through its full range (0-100%). The valve should respond within 2-3 seconds. A sluggish or non-responsive EXV indicates actuator motor failure or debris in the valve port. If the EXV is operating correctly, verify the superheat setpoint — an artificially low superheat target will cause overfeeding.

3 Verify Building Load vs. Chiller Minimum Capacity

During shoulder seasons (spring/fall), building cooling load may fall below the chiller's minimum stable capacity (typically 10-20% of full load). The chiller cannot turn down low enough, causing the leaving water temperature to fall below setpoint. If this is the case, the solution is either a bypass valve that mixes supply and return water to maintain flow, or a buffer tank that adds thermal mass. Verify the chiller is not oversized for the current load condition by checking the actual cooling demand (BTU/hr) against the chiller's minimum capacity.

4 Inspect Refrigerant Charge Level

Low refrigerant charge can cause abnormally low evaporator temperatures because the reduced charge shifts the operating point. Check subcooling at the condenser outlet (should match the manufacturer's chart for the current operating conditions). If subcooling is low (typically <5°F), the system is undercharged. Use the Carrier pressure-temperature chart for R-134a to verify proper charge. Document any refrigerant added — repeated recharging indicates a leak that must be located and repaired.

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Frequently Asked Questions

Can I lower the Alarm 105 trip point to prevent nuisance trips during low-load operation?

No. The 33°F setpoint is calibrated to prevent evaporator tube freezing. Lowering it risks catastrophic freeze damage. Instead, address the root cause: install a chilled water bypass valve, add a buffer tank, or stage multiple chillers so that no single chiller operates below its minimum capacity.

How quickly does freeze damage occur if Alarm 105 is bypassed?

Evaporator tube freeze damage can occur within 60-120 seconds of the refrigerant temperature dropping below 32°F. The rate depends on the water flow rate and the temperature differential. This is why the controller does not allow a simple reset — the alarm requires investigation and root cause correction before restart.

🔗 Related Error Codes — Centrifugal Chillers

Other diagnostic guides in the same equipment category. Common failure patterns often share root causes — cross-referencing speeds up diagnosis.