Scroll Chillers Critical Interlock

York YLAA Scroll Chiller Alarm B5: Phase Loss / Phase Reversal Protection

Published: 2026-04-01

🚨 Critical Diagnostic Alert

Alarm B5 on the York YLAA scroll chiller is a system-wide lockout — the phase monitor relay has detected an incoming power problem and has prevented ALL compressors from starting. Unlike compressor-specific alarms (B2 overload, B3 high pressure), B5 disables the entire chiller. This is a protective function — running a 3-phase scroll compressor on single-phase power (one phase lost) or with reversed phase rotation will destroy the compressor within seconds.

The B5 alarm can be triggered by: a utility power issue (phase loss from a blown transformer fuse), a building electrical problem (loose connection in the distribution panel or at the chiller disconnect), or a failed phase monitor relay (false trip). The diagnostic sequence always starts with measuring incoming power at the chiller disconnect — never replace the phase monitor without first verifying the incoming power quality.

🔍 Symptoms Checklist

⚠️ Chiller will not start — B5 displayed on control panel immediately
⚠️ Phase monitor relay LED indicates fault (red instead of green)
⚠️ Compressor contactors do not energize
⚠️ All circuits are locked out — entire chiller is offline

🛠️ OEM Replacement Parts

Part Name	OEM Part Number	Est. Price
Phase Monitor Relay (3-Phase, 208-480V)	`YK-PMR-3PH`	$260
Compressor Contactor (3-Pole, 50A)	`YK-CONT50-3P`	$185
Main Power Fuse Block (200A, 600V)	`YK-FUSE-200A`	$350

📋 Interactive Diagnostic Procedure

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

1 Check Incoming Power at Chiller Disconnect

The YLAA's phase monitor relay continuously monitors all three phases for: phase loss, phase reversal, voltage imbalance (>3%), and under/over voltage (±10% of nameplate). Measure the voltage at the chiller's main disconnect (not at the building switchgear — the voltage drop in the feeder cable matters). All three phase-to-phase voltages should be within 2% of each other and within 10% of the chiller nameplate. If voltage at the disconnect is good but the phase monitor still trips, test the monitor itself.

2 Test Phase Monitor Relay Independently

The phase monitor relay can fail in the 'fault' state even with correct incoming power. Disconnect the relay's input terminals and apply a known-good 3-phase supply (or use a phase monitor tester). If the relay's output contacts don't change state with correct power applied, the relay has failed. Phase monitor relays have a finite service life — the internal electrolytic capacitors in the detection circuit degrade after 8-12 years, causing increasingly sensitive (false) trip behavior.

3 Verify Phase Rotation After Electrical Work

If the B5 alarm appeared after any electrical work on the building's distribution system — even work on a different piece of equipment — the phase rotation may have been reversed at the switchgear. Use a phase rotation meter to verify A-B-C sequence at the chiller disconnect. Never assume the phase rotation hasn't changed after electrical work — electricians don't always label phase conductors consistently.

🔗 Related Error Codes — Scroll Chillers

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

Scroll Chillers Critical Interlock

⚡ 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

York YLAA Scroll Chiller Alarm B5: Phase Loss / Phase Reversal Protection

🔍 Symptoms Checklist

🛠️ OEM Replacement Parts

📋 Interactive Diagnostic Procedure

🔗 Related Error Codes — Scroll Chillers

Carrier AquaSnap 30RAP Alarm 55: Low Suction Pressure — Freeze Protection

Trane CGAM Scroll Chiller Alarm LPC: Low Pressure Cutout — Circuit 1

York YLAA Scroll Chiller Alarm B2: Compressor Motor Overload

Daikin McQuay AGZ Chiller Alarm E1: Evaporator Flow Loss

Carrier AquaSnap 30RAP Alarm 51: High Pressure Cutout — Condenser Issue

Trane CGAM Alarm HPC: High Pressure Cutout — Circuit Protective Trip

Daikin McQuay AGZ Alarm E3: Compressor Motor Overload — Thermal Trip

Carrier AquaSnap 30RAP Alarm 52: Circuit Defrost — Freeze Prevention Active

⚡ Fault-Induced Energy Waste Estimator

References & Industry Standards