The Critical Point About Heat Pump Service Ports

Standard Service Ports (Vapor/Liquid Line Ports):

• In heating mode, these will show equal or very similar pressures
• This is because both lines are on the high-pressure side when reversed
• The reversing valve has redirected flow, so both the vapor and liquid line service ports are essentially reading discharge pressure

True Suction Port (Third Port):

• This is where you'll see the actual low-side pressure
• This port reads the pressure before the compressor
• Shows the true pressure differential the compressor is creating
• This is the key port for proper diagnosis in heating mode

Why This Matters for Diagnosis

If a technician only checks the standard vapor and liquid service ports in heating mode and sees equal pressures, they might incorrectly think:

• The compressor has failed
• There's a refrigerant problem
• The system isn't working

When in reality, equal pressures on those two ports in heating mode is completely normal because of the reversing valve operation.

The true suction port is essential for:

• Verifying actual compressor performance
• Checking proper superheat
• Diagnosing low-side issues
• Confirming the system is operating correctly in heating mode