Tech is at a 3-ton R-410A TXV system on a 68 F afternoon in October.

What went wrong:

1. Misdiagnosed low-side pressure as a refrigerant charge problem. - At 68°F ambient, 105 psi suction isn’t necessarily “low” for R-410A — it depends on the evaporator load and superheat. The tech should have checked superheat first, not assumed undercharge.

1. Added refrigerant without verifying superheat or airflow. - Charging by suction pressure alone is incorrect. Proper 410A charging requires measuring superheat (for fixed orifice) or subcooling (for TXV), along with airflow and load conditions.

1. Overcharged the system. - Subcooling of 22°F is well above the typical 10–15°F target for most TXV systems. This indicates liquid backup into the condenser, reducing efficiency and risking compressor damage.

1. Ignored system performance indicators. - No mention of checking superheat, delta T, or airflow — all critical before adding charge.

Correct procedure (per ACCA and manufacturer guidelines):

• Verify airflow (0.4–0.6 FPM across coil).
• Measure evaporator superheat or condenser subcooling.
• Confirm outdoor ambient and design pressures.
• Charge to manufacturer’s specified subcooling (usually 10–15°F for TXV).

Bottom line: The tech guessed instead of measuring. Overcharged systems fail faster. Always charge by the numbers, not by feel.

He overcharged the system by using suction pressure and line sweating as charging indicators on a TXV system. This is a critical error.

What he did wrong:

1. Ignored the metering device: TXV systems must be charged by subcooling (or weigh-in), not suction pressure or superheat. The TXV maintains constant superheat; adding refrigerant only raises head pressure and subcooling.
2. Misdiagnosed "low" suction: At 68°F ambient, 105 psi suction (~34°F SST) is normal. Design SST is typically 35–45°F below return air temp (e.g., 75°F RA → 30–40°F SST). He chased a ghost problem.
3. Created a new problem: 22°F subcooling is excessive (target is typically 10–12°F ±3°F per manufacturer). This indicates liquid backing up in the condenser, raising head pressure, increasing compressor work/amp draw, and reducing capacity/efficiency. It risks compressor damage (liquid floodback if TXV hunts) and wasted energy.

Correct procedure (per ACCA Standard 5 / Manufacturer specs):

1. Verify airflow (400 CFM/ton), clean coils, correct fan speed.
2. Let pressures stabilize (10–15 min).
3. Measure liquid line temp and high-side pressure (convert to saturation temp).
4. Subcooling = Sat Temp – Liquid Line Temp.
5. Add/remove refrigerant only to hit manufacturer’s target subcooling.

Result: System is now overcharged. Recover refrigerant until subcooling matches the nameplate spec.