Trane Heat Pump: 4TWR
Re: Compressor Kicks On and Off in Cooling
The compressor and condenser fan motor cycle on and off rapidly.
- Often accompanied by burnt compressor contactor points from excessive cycling.
► The Low and High Pressure switches are wired in series with the "Y" output from the defrost board to the compressor contactor. A tripped switch will de-energize the contactor when it opens and re-energize once the switch closes. (No lockout)
Qty |
Possible Causes (From Most to Least Likely): |
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High Pressure - Overcharged |
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Low Pressure - Low Refrigerant Charge |
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High Pressure - Low Outdoor Airflow |
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Low Pressure - Low Indoor Airflow |
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Low Pressure - Bad Cooling Expansion Valve |
CHECKOUT:
1. Check Refrigerant Pressures
- Turn off power to the outdoor unit by pulling the service disconnect or turning off the breaker.
- Hook-up refrigerant gauges to the pressure ports on the unit.
- Re-apply power to the outdoor unit.
- Start the compressor with a call for cooling from the thermostat.
- Make sure the compressor and condenser fan start correctly.
- Some units have a Fan-Cycling control that may delay the starting of the condenser fan.
- No Fan Operation = Bad Condenser Motor/Capacitor or Bad Fan-Cycling Control
- Observe refrigerant pressures as the system operates.
- Normal R410A Pressures = 100 to 150 psig Suction, 200 to 400 psig Head.
2. Check High Pressure Issues
- If the Head Pressure exceeds Trip Point within 1 minute:
- Suspect Bad Overcharge or No Outdoor Airflow
- If the Head Pressure gradually climbs to Trip Point:
- Place an insulated thermistor on the liquid line.
- Calculate Sub-Cooling = (Liquid-Line Gauge Temperature - Actual Liquid-Line Temperature)
- Sub-Cooling > 15 deg + High Head Pressure + High Suction Pressure = Overcharged
- Sub-Cooling > 15 deg + High Head Pressure + Low Suction Pressure = Bad I/D Expansion Valve
- Sub-Cooling < 2 deg + High Head Pressure + High Suction Pressure = Low Outdoor Airflow
3. Check Low Pressure Issues
- If Suction Pressure drops below the Trip Point within 1 minute:
- Attempt to add refrigerant to the system.
- Suction Pressure + Head Pressure Climbs = Low Refrigerant
- A Liquid Line Sight Glass should have bubbles until proper charge is achieved.
- Head Pressure Climbs but Suction Pressure stays low or drops = Bad I/D Expansion Valve
- A Liquid Line Sight Glass should be clear.
- If Suction Pressure gradually drops to the Trip Point:
- Place an insulated thermistor on the Suction Line.
- Monitor the Suction Line Pressure and Temperature as the system operates.
- Suction Pressure Drops + Suction Line Temperature Drops = Low Indoor Airflow
- A Liquid Line Sight Glass should be clear.
- The Suction Line may begin to freeze.