High-Pressure Trip on a Roof-Well Split System
A 5 kW R410A wall-mounted split serves a communications room with a year-round sensible load. The outdoor unit sits in a screened roof well 1.2 m from a parapet, and two other condensers discharge into the same well. On a 28 °C afternoon the unit trips on its high-pressure switch, set at 40 bar gauge. A technician records just before the trip: suction 7 bar gauge, equivalent to about 5 °C evaporating; suction line 19 °C, giving 14 K superheat; discharge 38 bar gauge, equivalent to about 60 °C condensing; liquid line 57 °C, giving 3 K subcooling; and air at the condenser coil inlet 41 °C while the general roof ambient is 28 °C. The condenser fins are dusty but not blocked. The indoor filter is clean and the indoor fan is on high. The unit restarts after 20 minutes and runs for about 15 minutes before tripping again. The client asks whether the system is overcharged and wants gas added.
1. Interpret the readings, particularly the 41 °C coil-inlet air against 28 °C ambient and the 3 K subcooling. What fault do they indicate?
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The condensing temperature is about 60 °C and the discharge pressure is close to the 40 bar gauge trip point. That alone does not prove overcharge. The key reading is coil-inlet air at 41 °C while general ambient is 28 °C: the condenser is being fed hot air, almost certainly recirculated discharge from the other units in the roof well. The 3 K subcooling is low, not high. Overcharge would normally show excessive subcooling and a backed-up liquid line; this system shows a condenser struggling to reject heat. The 14 K superheat indicates the evaporator is not flooded, which is consistent with reduced mass flow at high condensing pressure. Adding refrigerant would raise the charge and condensing pressure further, shorten the run time and increase the risk of repeated trips.
2. Explain why adding refrigerant is likely to make the fault worse, and what immediate checks you would make before touching the charge.
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Charge is not the variable that explains a 41 °C coil-inlet temperature. If gas is added, the extra refrigerant has nowhere to condense quickly because the condenser is already receiving hot air. Condensing pressure and discharge temperature will rise, the high-pressure switch will trip sooner, and the compressor will run hotter with poorer lubrication. Before changing charge, verify condenser fan rotation and speed, fan capacitor and motor current, and whether all fans are running. Clean the dusty coil and confirm the screen is not restricting discharge. Measure coil-inlet temperature at several points and compare it with ambient; a large difference confirms recirculation. Check that the unit is not installed with its discharge blowing into the parapet or into another condenser. Only after airflow and recirculation are corrected should charge be assessed by subcooling at stable conditions.
3. Propose immediate and permanent corrective actions, with measurements that would verify the repair.
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Immediate action: clean the condenser, confirm all fans run at correct speed and direction, remove any temporary screen obstruction, and provide temporary ventilation or a spot cooler if the comms room load cannot be maintained. Permanent action: relocate the outdoor unit, duct its discharge away from the well, or rearrange the condensers so no unit draws another’s discharge. If relocation is not possible, add a discharge plenum or deflector designed for the available space. Verification: after correction, coil-inlet air should be close to general ambient, condensing temperature should fall well below the trip point at the same load, subcooling should settle within the manufacturer’s range, and the unit should run through the afternoon without tripping. Record suction and discharge pressures, superheat, subcooling and current at steady conditions.