TXV System Charged by Suction Pressure
A newly installed 4-ton R-410A split air conditioner with a TXV is not cooling adequately. Indoor conditions are 75°F dry bulb and 63°F wet bulb; outdoor air is 95°F. Suction pressure is 100 psig, and suction-line temperature at the service valve is 48°F. Liquid-line pressure is 280 psig, and liquid-line temperature is 89°F. The compressor nameplate RLA is 18.0 A; measured current is 13.2 A. Supply air is 62°F and return air is 75°F. The installer reused the existing line set after a nitrogen purge but did not weigh in the charge; he added refrigerant until suction pressure reached 100 psig. The liquid-line filter drier shows no temperature drop, and no sight glass is installed. The outdoor unit is a single-stage unit.
1. Using a PT chart, calculate superheat and subcooling. Interpret the readings and state whether the system is likely overcharged, undercharged, or restricted.
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Using an R-410A PT chart:
- Suction saturation at 100 psig is about 29°F. Superheat = 48°F − 29°F = 19°F.
- Liquid saturation at 280 psig is about 92°F. Subcooling = 92°F − 89°F = 3°F.
For a TXV system at these conditions, expect roughly 10–15°F superheat and 8–12°F subcooling, depending on the manufacturer. The combination of high superheat and low subcooling means the evaporator is being starved and the liquid line is not fully liquid. The low compressor current (13.2 A versus 18.0 A RLA) and small 13°F temperature split support a low-capacity condition.
The absence of a temperature drop across the liquid-line filter drier argues against a blocked drier, but it does not rule out a restriction elsewhere. Because the installer charged by suction pressure rather than weighing in the charge, undercharge is the leading suspect. However, a liquid-line restriction, a kinked line, or a TXV problem must be ruled out before simply adding refrigerant.
2. The installer wants to add refrigerant until suction pressure reaches 120 psig. Explain why this is incorrect for a TXV system and what charging method you would use.
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On a TXV system, the valve modulates to maintain evaporator superheat. Suction pressure therefore varies with indoor wet-bulb temperature, airflow, outdoor temperature, and load - it is not a charge indicator. Adding refrigerant until a target suction pressure is reached can overcharge the system at one condition and still leave it undercharged at another. Overcharge raises subcooling, raises compressor current, and can flood the condenser and compressor.
The correct approach is:
- Recover the refrigerant, evacuate, and weigh in the charge specified on the nameplate, adjusting for line-set length and any factory charge allowance in the installation manual.
- Operate at steady conditions and verify subcooling at the liquid-line service port against the manufacturer's target (often 8–12°F at 95°F outdoor).
- Verify superheat, supply-to-return temperature difference, and compressor amps.
If a sight glass is installed, it can confirm a solid liquid column, but subcooling is the primary charging check on a TXV system.
3. After the correct charge is weighed in, superheat remains 19°F and subcooling remains 3°F. What other causes would you investigate, and how would you test them?
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Persistent high superheat and low subcooling after a verified weigh-in point to a liquid-line or TXV problem rather than charge:
- Restriction in the liquid line: check for a temperature drop across the filter drier (a rise of more than a few degrees suggests a restriction), kinked tubing, or a blocked TXV inlet screen.
- TXV bulb problem: verify the bulb is securely clamped to the suction line, insulated, and not corroded or lost its charge. A bulb that has lost charge usually drives the valve closed and produces high superheat.
- TXV stuck closed or misadjusted: check valve inlet and outlet temperatures and pressures. A temperature drop across the valve body can indicate a restriction.
- Undersized or restricted line set: confirm line sizes and look for a crimped or collapsed line, especially since the old line set was reused.
- Compressor valve inefficiency: less common on a new unit, but low amps with poor capacity and abnormal pressure relationships should prompt a pump-down or performance test.
Correct the restriction or valve problem, replace the filter drier, evacuate, and recharge by weight.