Commercial Rooftop Units Critical Interlock

Lennox Energence RTU Alarm 31: Low Suction Pressure — Refrigerant Charge Loss

Published: 2026-05-02

🚨 Critical Diagnostic Alert

Alarm 31 on the Lennox Energence commercial rooftop unit is triggered when the suction pressure falls below the low-pressure switch cutout (typically 40 PSIG for R-410A, corresponding to approximately 20°F saturated suction temperature). The Prodigy control board locks out the affected compressor circuit and logs the event.

Unlike the evaporator frost protection alarm (Alarm 27), which is airflow-related, Alarm 31 typically indicates a refrigerant charge problem — either a leak has developed, or the system was undercharged during a previous service event. The diagnostic priority is: leak test first (find the source of the charge loss), repair the leak, then recharge. Simply adding refrigerant without finding the leak guarantees a repeat service call.

🔍 Symptoms Checklist

⚠️ Suction pressure drops below 40 PSIG (R-410A) during operation
⚠️ Compressor short-cycles: runs 2-3 minutes, trips, restarts after cooldown
⚠️ Evaporator coil partially frosted (uneven frosting pattern)
⚠️ Cooling capacity noticeably reduced — space temperature drifts upward

🛠️ OEM Replacement Parts

Part Name	OEM Part Number	Est. Price
Low Pressure Switch (Auto-Reset, 40 PSIG)	`LB-LPS40`	$75
Schrader Core Removal Tool (for leak repair)	`LB-SCHR-TL`	$45
R-410A Refrigerant (25 lb cylinder)	`LB-R410A-25`	$185

📋 Interactive Diagnostic Procedure

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

1 Perform Electronic Leak Search

Low suction pressure accompanied by low subcooling (below 5°F) confirms refrigerant undercharge from a leak. Use an electronic leak detector (heated diode or infrared type, sensitivity <0.1 oz/year) to scan all braze joints, Schrader valve cores, and the evaporator and condenser coil tube sheets. The most common leak points on RTUs: 1) Evaporator coil U-bends (corrosion from condensate), 2) Condenser coil tube sheet joints (vibration), 3) Schrader valve cores (missing caps). Mark all leaks found, repair with 15% silver brazing rod (not soft solder), pressure test to 150 PSIG with nitrogen, evacuate to 500 microns, and recharge to nameplate specifications.

2 Verify Low Pressure Switch Setting

A LPS that trips prematurely (at higher-than-rated pressure) causes nuisance trips. Connect a calibrated pressure gauge to the suction line service port and monitor pressure during compressor operation. If the switch opens at >50 PSIG (when the rated trip point is 40 PSIG), the switch has drifted out of calibration and must be replaced. A switch that's tripping correctly but frequently indicates the root cause is genuinely low suction pressure — not a switch problem.

3 Check Airflow Before Adding Refrigerant

Low suction pressure can also be caused by low evaporator airflow, not just low charge. Measure the supply air CFM and compare to the unit's design airflow (350-450 CFM/ton). If airflow is below 300 CFM/ton, fix the airflow issue first (filters, belt, blower motor) before evaluating refrigerant charge. Adding refrigerant to compensate for low airflow creates an overcharge condition that will cause high-pressure trips once the airflow issue is resolved.

🔗 Related Error Codes — Commercial Rooftop Units

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

Commercial Rooftop Units Warning (Reduced Capacity)

⚡ Fault-Induced Energy Waste Estimator

When HVAC equipment runs with active error codes, it operates inefficiently and wastes electricity. Estimate the monthly cost of delaying repairs. All calculations run locally in your browser.

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References & Industry Standards

ASHRAE 15 — Safety Standard for Refrigeration Systems
ASHRAE 34 — Refrigerant Designation & Safety Classification
AHRI 550/590 — Water-Chilling & Heat Pump Packages
SMACNA — HVAC Duct Construction Standards
Manufacturer Service Manuals — Carrier, Trane, York, Daikin, Lennox

Lennox Energence RTU Alarm 31: Low Suction Pressure — Refrigerant Charge Loss

🔍 Symptoms Checklist

🛠️ OEM Replacement Parts

📋 Interactive Diagnostic Procedure

🔗 Related Error Codes — Commercial Rooftop Units

Trane RTU Error ERR04: Evaporator Frost Protection Active

Carrier WeatherExpert RTU Alarm A001: Supply Air Temperature Sensor Failure

Lennox Energence RTU Alarm 27: High Pressure Cutout — Condenser Fan Failure

York Predator RTU Alarm 3H: Compressor Thermal Overload Trip

Carrier WeatherExpert RTU Alarm A002: Return Air Temperature Sensor Failure

Trane RTU Error ERR07: High Compressor Discharge Temperature —...

York Predator RTU Alarm 4A: Condenser Fan VFD Fault — Variable Speed

Carrier WeatherExpert RTU Alarm A005: Economizer Actuator Feedback Error

⚡ Fault-Induced Energy Waste Estimator

References & Industry Standards