We get called to high-pressure trips more than almost any other refrigeration fault. A pub in Clapham, a restaurant in Soho, a cold store in Barking — the story is always similar. The condensing unit has been running fine for months or years, then one day it trips on the high-pressure safety switch and won’t restart. The temperature in the cold room or display case starts climbing. Someone resets the switch, the unit runs for ten minutes, and trips again.
High-pressure trips exist to protect the compressor. When the discharge pressure exceeds a safe limit — typically 28-32 bar for R410A systems, 25-28 bar for R449A and R134a — the pressure switch opens and shuts the compressor down. It’s not a fault in itself. It’s the system telling you that something is preventing it from rejecting heat properly.
The Usual Suspects
Blocked Condenser Coil — The One We See Every Week
The condenser coil rejects heat from the refrigerant to the outside air. When it’s blocked with dirt, grease, pollen, leaves, cardboard fibres, or kitchen exhaust residue, airflow drops and the refrigerant can’t dump its heat. Discharge pressure climbs until the safety switch trips.
In a commercial kitchen environment, condenser coils can go from clean to critically blocked in 8-12 weeks. On rooftop units, autumn leaves and summer pollen are the main culprits. On units at ground level, litter, packaging material, and urban debris accumulate against the coil.
The fix is cleaning. Brush off loose debris, then chemical coil cleaner and a low-pressure rinse. It’s a standard part of every PPM visit and it’s the single most effective thing you can do to prevent high-pressure trips.
Failed Condenser Fan
If the condenser fan motor has seized or the capacitor has failed, there’s no airflow across the coil even if it’s clean. The unit will trip on high pressure within minutes of starting. You can usually tell by looking at the outdoor unit — if the fan isn’t spinning when the compressor is running, that’s your problem. Fan motors and capacitors are van stock items for us — most of these repairs are done on the first visit.
Refrigerant Overcharge
If someone has topped up the refrigerant without checking for a leak first, the system may now be overcharged. Excess refrigerant raises the discharge pressure. This is a surprisingly common problem after visits from less experienced contractors who treat every symptom with “add more gas.”
High Ambient Temperature
Condensing units in enclosed plant rooms, behind walls with poor ventilation, or positioned where hot exhaust air from other equipment blows across the condenser will struggle on hot days. The unit was probably borderline on the hottest days when it was new and clean — add two years of dirt to the coil and it tips over the edge. Solutions: improve ventilation, relocate the unit, or fit an additional condenser fan.
What You Can Check Before Calling
- Is the condenser coil visibly dirty or blocked? (Look, don’t touch — the coil fins are sharp)
- Is the condenser fan spinning when the unit tries to run?
- Is there adequate airflow around the condensing unit, or is it boxed in?
- Has anything been placed against or near the coil (boxes, bins, equipment)?
If the coil is clean, the fan is running, and there’s good airflow, the problem is likely internal — refrigerant overcharge, a restriction in the circuit, or a partially blocked expansion valve. That needs an F-Gas engineer with gauges and diagnostic equipment. Call 020 3974 1419.