Trane Rooftop Unit: YSC

Re: Cooling: High Discharge Pressure / High Head Pressure

As the system runs in the cooling mode, the discharge pressure exceeds the normal operating pressure.

R410A System

• High Head Pressure = Above 400 psig

CHECKOUT:

1. Check Refrigerant Pressures

• Hook-up refrigerant gauges to the pressure ports on the unit.
• Place an insulated temperature probe on the liquid line between the outdoor coil & indoor expansion valve.
• Allow the unit to run for at least 10 minutes.
• Monitor pressures and temperature as the system operates.
  • If pressure exceeds Normal Range or kicks-off on Hi Pressure safety in the first 3 minutes = Refrigerant Overcharge or No Condenser Fan Operation

2. Check Condenser Motor & Capacitor

• Disconnect power to the unit by pulling the service disconnect.
• Test the fan capacitor.
  • Replace the capacitor if less than 20% of rated capacity.
• Disconnect the condenser motor wiring and check the winding resistance and temperature of the condenser fan motor.
  • Common (BLK) to Run (VIO)
  • Common (BLK) to Start (BRN)
  • Run (VIO) to Start (BRN)
    • Any Open Windings + Cool Motor = Bad Condenser Motor
    • Any Open Windings + Hot Motor = Motor Internal Overload Open --> Allow motor to cool then recheck.
• Re-apply power and check the fan motor amperage.
• Observe the motor operation and check for unusual noises (bad bearings) or excessive temperature.

3. Check Sub-Cooling

• Record "Saturation Temperature" from the discharge pressure gauge.
• Subtract the Actual liquid line temperature reading.
  • Saturation Temp - Liquid Line Temp = Sub-Cooling
    • Sub-Cooling > 13 deg. + High Discharge Pressure + High/Normal Suction Pressure = Overcharged
    • Sub-Cooling > 13 deg. + High Discharge Pressure + Low Suction Pressure = Bad Expansion Valve
    • Sub-Cooling < 2 deg. + High Discharge Pressure + Hot Liquid Line = Dirty Outdoor Coil or Mis-Sized Fan Blade --> Clean the coil and re-check.