Split AC Cooling Complaint in a South-Facing Office
A 5.5 kW nominal split system serves a 45 m2 south-facing office with 12 occupants and 8 desktop computers. The client complains it 'never gets cold' on summer afternoons. Measured data: outdoor 41 C, indoor return air 27 C, supply air 15 C, indoor relative humidity 62 percent. Suction pressure 4.2 bar gauge on R-410A, discharge 27 bar gauge, superheat at the evaporator outlet 6 K, subcooling 5 K. The indoor fan is on high, the filter is clean, and the outdoor unit sits in a recessed balcony with a solid wall 400 mm from the coil face and a second unit beside it. Design conditions were 24 C dry bulb, 50 percent relative humidity.
1. Separate the load-side problem from the equipment-side problem and quantify the evidence for each.
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Equipment side looks healthy. Superheat 6 K and subcooling 5 K are both in normal bands for a TXV system on R-410A, so the charge and metering device are behaving. A 12 K air temperature split (27 C return to 15 C supply) is at the upper end of normal, meaning the coil is transferring heat well. Suction at 4.2 bar gauge and discharge at 27 bar gauge are consistent with a working system under a high condensing load at 41 C outdoor.
Load side is the problem. The room is at 27 C and 62 percent RH against a 24 C / 50 percent design, so the system is not removing enough heat to reach setpoint. Contributing factors visible in the data and description:
- Condenser airflow restriction. A solid wall 400 mm from the coil face plus a neighboring unit in a recessed balcony causes recirculation and reduced heat rejection, raising condensing temperature and cutting capacity. Discharge pressure at 27 bar gauge on R-410A is roughly 47 C saturation, a high but plausible value at 41 C ambient that still points to poor airflow.
- Internal gains. 12 occupants at roughly 75 W sensible each plus 8 computers at roughly 150 W each contribute several kW of sensible load, and the occupants add latent load that explains the 62 percent RH.
- Solar gain through south glazing in the afternoon is the likely reason the complaint is time-specific.
Conclusion: the unit is functioning but is undersized for the actual load and is further degraded by condenser recirculation. The fix is not more refrigerant.
2. Recommend corrective actions in priority order and state what you would tell the client about expected outcome.
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Priority order:
- Relieve condenser recirculation. Relocate or duct the outdoor unit so the coil face has at least the manufacturer's minimum clearance (typically 600-1000 mm) and is not discharging into a solid wall or into the neighboring unit's intake. This is the cheapest, highest-leverage action and can recover a meaningful share of lost capacity.
- Reduce solar and internal gains. Solar film or shading on the south glazing, and a review of equipment scheduling. These are client-side measures that lower the load rather than the equipment's ability to meet it.
- Verify latent performance. At 62 percent RH the coil is not dehumidifying enough for comfort; check condensate drainage and confirm the fan speed is not so high that it bypasses dehumidification. Consider a lower fan speed or a dedicated dehumidification mode.
- Re-assess capacity. If after steps 1-3 the room still cannot hold 24 C on a 41 C day, the unit is undersized for the real load and a load calculation should be redone before recommending a larger unit.
What to tell the client: the equipment is working correctly and is not faulty, but the installation location and the actual heat load are preventing it from meeting the original design. Realistic expectation is that steps 1-3 will bring the room closer to setpoint but may not achieve 24 C on the hottest afternoons; a formal load calculation will determine whether a capacity upgrade is justified.
3. The client asks for a 'top-up' of refrigerant to make it colder. Explain why this is technically wrong and what could go wrong.
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The system's superheat and subcooling are already in the normal range, which is direct evidence that the charge is correct. Adding refrigerant would not increase cooling capacity; capacity is limited by condenser heat rejection and by the room load, not by the amount of refrigerant in the circuit.
What would actually happen with a top-up:
- Subcooling rises and liquid backs up in the condenser, reducing the effective condensing surface and raising discharge pressure further. With the unit already at 27 bar gauge and a restricted condenser, this pushes it toward the high-pressure cutout.
- Flooded evaporator. Excess charge drives superheat toward zero and can carry liquid back to the compressor, risking slugging, oil dilution and compressor damage.
- Higher current draw and possible nuisance tripping.
- Masking the real fault. The underlying condenser airflow problem remains and will worsen as outdoor temperatures rise.
The correct response is to explain that the measured superheat and subcooling show the charge is right, that the complaint is a load and installation issue, and to redirect the conversation to condenser clearance and load reduction. Document the explanation and the client's request in the service record.