Same question but the system has a TXV.

Charging a TXV System: Targets & Measurement Points

With a TXV (Thermostatic Expansion Valve), charging is based on superheat and subcooling, not fixed pressures.

Charging Target:

• Subcooling: Typically 10–15°F (varies by manufacturer; check nameplate or specs).
• Superheat: Should match the rated superheat listed on the TXV or manufacturer data (usually 8–12°F).

Where to Take Readings:

1. Suction Line (Low Side): - Temperature: Insulate and measure at the service valve or evaporator outlet. - Pressure: Connect gauge to suction service port. - Superheat = Suction Temp – Saturated Temp (from pressure).

1. Liquid Line (High Side): - Temperature: Measure at the liquid line near the TXV inlet (after the filter/dryer). - Pressure: Connect gauge to liquid line service port or high-side gauge. - Subcooling = Saturated Temp (from pressure) – Liquid Line Temp.

Steps:

1. Recover refrigerant if overcharged; add if undercharged.
2. Adjust charge until: - Subcooling = 10–15°F - Superheat = 8–12°F (or per TXV specs)
3. Verify stable superheat across load changes.

Notes:

• Ensure proper airflow across the condenser and evaporator.
• Use correct refrigerant type and oil compatibility.
• If readings are unstable, check for refrigerant restrictions, oversized TXV, or metering issues.

Safety: Always wear PPE and follow EPA 608 guidelines. If unsure, consult manufacturer specs or a licensed technician.

With a TXV, you charge to Subcooling (SC), not superheat. The TXV maintains a fixed superheat at the evaporator outlet; you cannot adjust it via refrigerant charge.

1. Charging Target: Target Subcooling

• Source: Read the manufacturer’s data plate on the outdoor unit. Typical range: 10°F–15°F.
• Do not use generic "rule of thumb" charts.

2. Where to Take Readings (Liquid Line Only) You need Saturation Temperature (from pressure) and Line Temperature at the condenser outlet.

• Pressure: Connect your high-side hose to the liquid line service port (King valve or dedicated port) on the outdoor unit.
• Temperature: Clamp your thermocouple on the liquid line pipe within 6–12 inches of the condenser outlet / service valve. Insulate the probe (cork tape/foam) to negate ambient air influence.

3. The Math $$ \text{Subcooling} = \text{Saturation Temp (from gauges)} - \text{Liquid Line Temp (probe)} $$

• Add refrigerant to increase SC.
• Remove refrigerant to decrease SC.

Critical Pre-Checks (Per ACCA QI / ANSI/ACCA 5 QI-2010):

1. Airflow verified first: 350–400 CFM/ton. Check static pressure (TESP) and blower table. Wrong airflow = wrong SC target.
2. Clean coils: Both indoor and outdoor.
3. Metering device confirmed: Ensure it is actually a TXV (not a piston) and the bulb is strapped tight/insulated on the suction line.
4. Conditions: Outdoor ≥ 65°F (prefer 70°F+). Indoor return ≥ 70°F DB / 60°F WB. System run 15+ mins stable.

Verification: Once SC hits target, verify Superheat (SH) at the compressor inlet (suction port) is 20°F–35°F (compressor cooling). If SH is low (<10°F), you have a TXV/airflow issue, not a charge issue.

Ref: EPA 608, ACCA Standard 5, Manufacturer Charging Charts.