Commercial Rooftop Units Critical Interlock

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

Published: 2026-05-20

🔍 Symptoms Checklist

⚠️ Compressor trips on high head pressure (650+ PSIG R-410A)
⚠️ Condenser fan motor not rotating or rotating slowly
⚠️ Outdoor coil feels excessively hot to touch
⚠️ Unit may restart after 30-minute auto-reset cooldown

🛠️ OEM Replacement Parts

Part Name	OEM Part Number	Est. Price
Condenser Fan Motor (1/2 HP, 1075 RPM)	`LB-98765A`	$340
High Pressure Switch (Auto-Reset, 650 PSIG)	`LB-HPS650`	$85
Fan Motor Run Capacitor (7.5 μF, 440V)	`LB-CAP075-440`	$22

📋 Interactive Diagnostic Procedure

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

1 Test Condenser Fan Motor & Capacitor

Lennox Energence units use PSC condenser fan motors. First, spin the fan blade by hand (power off) — it should rotate freely with no bearing noise. Then test the run capacitor: discharge safely, measure capacitance. A reading below 90% of rated value (e.g., <6.75 μF for a 7.5 μF cap) indicates capacitor degradation — the motor will run slow or fail to start. Replace the capacitor first ($22 fix vs. $340 motor) — this resolves ~60% of alarm 27 cases.

2 Inspect Condenser Coil for Airflow Restriction

A dirty or blocked condenser coil elevates head pressure independently of fan operation. Inspect the coil surface for: cottonwood seed buildup (common in May-June, can reduce airflow 40%+), impacted dirt between coil fins (use a fin comb and coil cleaner), or bent fins from pressure washing. Clean the coil using a commercial HVAC coil cleaner (alkaline for outdoor coils) and verify head pressure returns to normal range before restarting.

3 Verify High Pressure Switch Calibration

The auto-reset high pressure switch is designed to open at 650 ±15 PSIG for R-410A systems. If the switch is tripping at lower pressure, it may be drifting out of calibration. Connect a calibrated pressure gauge to the liquid line service port and monitor pressure during compressor operation. If the switch opens at <600 PSIG, replace it — a miscalibrated HP switch causes nuisance trips that technicians often misdiagnose as refrigerant overcharge.

4 Check Refrigerant Charge & Non-Condensables

Overcharge elevates head pressure at all operating conditions. Recover and weigh the charge — compare against the unit nameplate charge adjusted for line set length. If charge is correct but head pressure remains elevated at normal outdoor temperatures (e.g., 450+ PSIG at 85°F ambient), perform a non-condensable test: let the system stabilize at 70°F ambient with the condenser fan off, measure the saturation pressure at ambient temperature. If the measured pressure exceeds the PT chart saturation pressure by >5 PSIG, non-condensables are present — recover, evacuate to 500 microns, and recharge.

🔗 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

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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 27: High Pressure Cutout — Condenser Fan Failure

🔍 Symptoms Checklist

🛠️ OEM Replacement Parts

📋 Interactive Diagnostic Procedure

🔗 Related Error Codes — Commercial Rooftop Units

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⚡ Fault-Induced Energy Waste Estimator

References & Industry Standards