Balcony Condenser Recirculation on a 96°F Afternoon
A 3-ton R-410A residential split system serves a second-floor condominium. On a 96°F afternoon the owner reports the unit runs continuously and the space never drops below 78°F. You measure: return air 78°F dry bulb / 63°F wet bulb; supply air 62°F; suction pressure 118 psig with a 58°F suction line temperature; liquid pressure 415 psig with a 108°F liquid line temperature. Ambient at the condenser inlet grille reads 104°F, while a thermometer six feet away at the balcony railing reads 94°F. The condenser coil is clean, the fan runs at full speed, and the compressor draws 21.4 A against a nameplate RLA of 18.0 A. The balcony is enclosed on three sides by a six-foot wall, and the unit discharges horizontally toward that wall from about 30 inches away. The filter is new, the blower is set for 1,200 CFM, and the evaporator coil is clean.
1. Calculate superheat and subcooling from the readings and state whether the charge should be adjusted as a first step.
Read the worked answer
R-410A saturation at 118 psig is roughly 40°F, so superheat = 58 − 40 = 18°F. Saturation at 415 psig is roughly 119°F, so subcooling = 119 − 108 = 11°F.
Interpretation:
- 11°F subcooling sits squarely inside the normal 8–12°F band, so the condenser has a solid liquid seal.
- 18°F superheat is at the upper edge, consistent with a reduced evaporator load rather than a starving metering device, since airflow is correct at 1,200 CFM and the coil is clean.
Neither number justifies adding or removing refrigerant. Adding charge would raise head pressure and amp draw on a compressor already at 119% of RLA, pushing it toward overload. The number that actually explains the complaint is the 10°F difference between the 104°F air entering the condenser and the 94°F air six feet away: discharge air is short-circuiting back through the coil, so the condenser is rejecting heat into air hotter than design ambient. Elevated condensing pressure, elevated amps, and lost capacity all trace back to that air-side problem, not to charge.
2. What mechanism produces the 21.4 A draw, and what corrective actions do you specify before touching the refrigerant circuit?
Read the worked answer
The mechanism is discharge-air recirculation inside a three-sided enclosure. The fan discharges horizontally into a wall 30 inches away; the air strikes the wall, spreads, and part of it returns to the coil inlet. Because the coil sees 104°F instead of 94°F, condensing temperature and pressure rise, the compressor's compression ratio increases, and mass flow plus amp draw go up. Capacity falls at the same time, which is why the space never reaches setpoint.
Corrective actions, in order:
- Eliminate the short circuit: rotate the unit 90 degrees, add a discharge plenum or duct kit that carries air above the wall, or cut a relief opening/louver in the wall so discharge air escapes.
- Verify manufacturer clearances for coil inlet and discharge; a coil can be within the listed wall clearance and still fail if the discharge path is blocked.
- Confirm fan RPM, blade pitch, and rotation are correct, and that no debris or stored items sit behind the unit.
- Only after the air-side fix, recheck pressures and amps; adjust charge only if subcooling falls outside the manufacturer's target at the corrected ambient.
3. What measurement set after the repair proves the diagnosis was correct?
Read the worked answer
Repeat the same measurements under the same weather and compare:
- Condenser inlet temperature should drop close to free-air ambient (within about 2–3°F). A reading of 96–98°F at the inlet when the railing reads 94°F confirms the short circuit is gone.
- Head pressure should fall by roughly 25–30 psi. With R-410A near 120°F saturation, each 1°F of condensing temperature is worth roughly 2.5–3 psi, so a 10°F reduction predicts a drop from about 415 psig to roughly 385–390 psig.
- Compressor amps should fall below the 18.0 A RLA, typically into the 16–17 A range at 96°F ambient.
- Supply-to-return temperature split should widen as net capacity recovers, and the space should begin pulling down.
If inlet air is corrected and head pressure and amps fall accordingly, the charge, coil, and compressor are exonerated and no refrigerant was ever needed. If head pressure stays high with correct inlet air, then investigate non-condensables, an overcharge, or a mismatched/undersized condenser.