Superheat Readings That Do Not Match the Charging Chart
A split system using R-32 is reported as 'not cooling well' in a first-floor office. Outdoor ambient is 35 °C, indoor return air is 24 °C dry-bulb and 17 °C wet-bulb. You measure suction pressure 8.0 bar gauge and liquid line pressure 26 bar gauge, suction line temperature at the service valve 12 °C, and liquid line temperature 42 °C. The manufacturer's charging chart, at these indoor and outdoor conditions, calls for 6 K superheat and 8 K subcooling. The unit has a fixed orifice (piston) metering device, a clean condenser, and a new filter. The line set is 15 m with a 6 m vertical rise to the indoor unit, and the suction line insulation is torn over the last 2 m near the outdoor unit.
1. Calculate superheat and subcooling from the readings and decide whether the system is undercharged, overcharged, or restricted. State any assumptions.
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Superheat is suction line temperature minus the saturation temperature at suction pressure. Using R-32 pressure-temperature data, 8.0 bar gauge is roughly 9.0 bar absolute; the saturation temperature at that pressure is approximately 4 °C. So superheat ≈ 12 − 4 = 8 K, against a target of 6 K - about 2 K high. Subcooling is saturation temperature at liquid pressure minus liquid line temperature. At 26 bar gauge (≈27 bar absolute), R-32 saturation is approximately 42 °C, so subcooling ≈ 42 − 42 = 0 K, against a target of 8 K. The pattern - high superheat and essentially zero subcooling - points to undercharge or a restriction, not overcharge. A restriction at the orifice would normally show high superheat with low suction pressure and could also starve the condenser of liquid, dropping subcooling; an undercharge produces the same signature. The torn insulation near the outdoor unit artificially raises measured suction temperature, inflating superheat, so the true superheat may be closer to target. The honest answer is: do not add refrigerant yet. Repair the insulation, re-measure, and confirm whether subcooling recovers; if subcooling stays near zero with a normal suction pressure, suspect a restriction or a slow leak rather than a charge fault.
2. Explain how the 6 m vertical rise and the fixed orifice affect your interpretation, and what you would check next.
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With a fixed orifice, the metering is pressure-driven and the charge is critical: the system has no receiver, so the condenser must back up enough liquid to feed the orifice. A 6 m rise on the suction line adds pressure drop and a small static head effect, which lowers the pressure at the evaporator relative to the service valve; measuring superheat at the outdoor service valve therefore reads lower superheat than the evaporator actually sees, not higher. That means the 8 K measured at the valve could correspond to more superheat at the coil, reinforcing the undercharge/restriction suspicion. The vertical liquid line also matters: a rise reduces liquid pressure at the orifice, which can cause flashing and poor feed. Next checks: confirm the actual refrigerant type and that no mixed gas was added; leak-test the circuit with an electronic detector and nitrogen pressure test rather than topping up; verify the piston size matches the indoor unit; measure superheat at the evaporator outlet if accessible; check for a partially blocked filter-drier by feeling for a temperature drop across it; and confirm airflow (static pressure, blower speed) because low airflow also distorts the charging chart. Only after those checks would I weigh in refrigerant by subcooling to the target.