Aircon Cuts Out, Blocked Outdoor Unit

Case Details

What We Checked

• Outdoor unit in a partly enclosed utility alcove — three sides enclosed by walls, open at the front only.
• New water heater platform installed above the outdoor unit six weeks earlier — its base sat 20cm above the top discharge grille, blocking the hot air exhaust path.
• Discharge air temperature measured at the grille — significantly above ambient, indicating hot air was being trapped and recirculated rather than dispersing.
• Ambient temperature in the alcove measured noticeably higher than the open corridor nearby, confirming heat accumulation in the enclosed space.
• Compressor surface temperature measured with a contact probe — elevated but within the range expected for thermal protection tripping, not internal mechanical damage. Current draw was normal when the unit was running.

The Diagnosis

The water heater platform installed six weeks earlier sat just 20cm above the outdoor unit's top discharge grille. When the unit ran, hot discharge air — typically 10 to 15 degrees above ambient — rose from the grille and hit the underside of the platform. Instead of dispersing upward, the hot air deflected back down into the alcove. The condenser intake drew it in again, creating a recirculation loop. The condenser was cooling its own exhaust instead of fresh ambient air, pushing discharge temperatures progressively higher. During morning hours, ambient temperature was low enough that the condenser could still reject heat despite the recirculation. By early afternoon, ambient temperatures peaked and the recirculation pushed compressor casing temperature past the thermal protection threshold. The switch tripped the compressor to prevent winding damage. Once the casing cooled — typically within 30 to 60 minutes — the switch reset and the unit restarted, only to trip again.

What Fixed It

We discussed the options with the client. The platform could be raised for adequate clearance, modified with perforations to allow airflow through, or fitted with a discharge duct to direct hot air out of the alcove. The client opted for the simplest option — raising the platform by 40cm. After the modification, we returned the following day and monitored the unit through the afternoon peak. The outdoor unit ran continuously from 1pm to 5pm without tripping. Compressor surface temperature stayed well below the thermal protection threshold. No parts, refrigerant work, or compressor repair was needed.

The afternoon cutouts stopped once the platform clearance was increased. No parts were needed and the compressor was not damaged. The client was told to keep at least 50cm of clear space above the discharge grille and to check before placing anything near the outdoor unit in future.

Why This Happens

Time-of-day cutouts and outdoor unit airflow.

• A unit that cuts out only in the afternoon almost always has an outdoor heat rejection problem. The pattern is the diagnostic clue: mornings fine, afternoons tripping, evenings recovering. An internal compressor fault — winding degradation, bearing wear, or valve damage — would not follow a time-of-day pattern. It would fail regardless of when the unit runs.
• The compressor's thermal switch trips when the casing temperature exceeds a factory-set threshold, measured by a contact sensor on the shell. Trapped discharge air in an enclosed space raises the effective ambient temperature around the condenser by ten degrees or more. This pushes the compressor past the threshold far sooner than it would in open air. The switch is a safety mechanism that protects against winding insulation breakdown.
• Any new structure placed above or around the outdoor unit can block the discharge airflow path. Water heater platforms, storage shelves, laundry racks, and plant boxes are common culprits in HDB utility areas. If the cutouts started around the time something was installed nearby, that timing correlation points directly to the cause.
• The compressor is usually undamaged after short-cycling thermal trips. The protection switch activates before temperatures reach the level that causes permanent insulation damage. However, repeated trips over weeks or months accelerate wear on the motor windings and starting circuit. Each locked-rotor start draws several times the normal running current. Resolving the airflow blockage sooner rather than later preserves the compressor's remaining life.