HVAC Error Library

Category: RTU Troubleshooting

  • High Head Pressure on Rooftop Unit – Causes and Fixes

    What Causes High Head Pressure?

    High head pressure indicates elevated refrigerant pressure on the condenser side of the system.

    This condition can trigger safety lockouts.

    Common Causes

    • Dirty condenser coil
    • Failed condenser fan motor
    • Refrigerant overcharge
    • Non-condensable gases in system
    • Blocked condenser airflow

    Troubleshooting

    1. Inspect and clean condenser coil
    2. Verify condenser fan operation
    3. Measure refrigerant pressures
    4. Check ambient temperature conditions

    Persistent high head pressure can damage the compressor.

    FAQ

    Can high outdoor temperature cause this?
    Yes, but the system should still operate within design limits.

  • RTU Trips Breaker on Startup – Causes and Solutions

    Why Does an RTU Trip the Breaker When Starting?

    If a rooftop unit trips the breaker immediately upon startup, the most common cause is excessive inrush current from the compressor or fan motor.

    Common Causes

    • Shorted compressor windings
    • Failed start capacitor
    • Locked rotor condition
    • Undersized breaker
    • Ground fault
    • Loose electrical connections

    Troubleshooting Steps

    1. Measure compressor winding resistance
    2. Inspect start and run capacitors
    3. Check breaker rating versus unit specifications
    4. Inspect wiring for signs of burning or insulation damage

    Repeated breaker trips require immediate diagnosis.

    FAQ

    Is this dangerous?
    Yes. Repeated breaker trips indicate a serious electrical issue.

  • Compressor Short Cycling on Rooftop Unit – Causes and Solutions

    What Is Compressor Short Cycling?

    Short cycling occurs when the compressor starts and stops rapidly without completing a normal cooling cycle.

    This can reduce efficiency and damage equipment over time.

    Common Causes

    • Low refrigerant charge
    • Oversized unit
    • Dirty condenser coil
    • Faulty thermostat
    • Pressure switch fault
    • Improper airflow
    • Electrical supply issues

    Troubleshooting Steps

    1. Inspect Thermostat Settings

    Verify correct temperature differential.

    2. Check Refrigerant Levels

    Low charge often causes rapid cycling.

    3. Clean Condenser Coil

    Overheating can trigger high-pressure trips.

    4. Inspect Pressure Switch Operation

    Repeated trips indicate underlying system imbalance.

    Risks of Ignoring Short Cycling

    • Increased compressor wear
    • Higher energy usage
    • Premature system failure

    FAQ

    Can short cycling damage the compressor?
    Yes. Repeated starts increase mechanical stress.

  • RTU Not Cooling But Fan Running – Causes and Fixes

    Why Is My Rooftop Unit Fan Running but Not Cooling?

    If your rooftop unit (RTU) fan is running but the space is not cooling, the most common cause is that the compressor is not operating, even though the blower motor is functioning normally.

    This condition usually indicates a refrigerant circuit or safety lockout issue rather than a fan failure.

    Common Causes

    The most frequent causes include:

    • Tripped high-pressure or low-pressure switch
    • Compressor contactor failure
    • Blown fuse in compressor circuit
    • Failed run capacitor
    • Thermostat not calling for cooling
    • Low refrigerant charge
    • Control board lockout

    How to Troubleshoot

    1. Verify Thermostat Call

    Confirm the thermostat is calling for cooling and sending a signal to the RTU.

    2. Check Compressor Operation

    Listen for compressor startup. If silent:

    • Check contactor engagement
    • Measure voltage at compressor terminals

    3. Inspect Pressure Switches

    If a safety switch is open, the compressor will not run.

    Reset only after identifying root cause.

    4. Check Refrigerant Pressure

    Low suction pressure can trigger compressor lockout.

    When to Call for Service

    If:

    • Breakers trip repeatedly
    • Refrigerant pressures are abnormal
    • Compressor hums but does not start

    Professional service may be required.

    FAQ

    Why does the fan run but not the compressor?
    The blower is often on a separate control circuit and may continue running during faults.