Walk-in Cooler Not Holding Temperature in Summer
A restaurant walk-in cooler, 8 ft x 10 ft x 8 ft high, is served by a 1.5 HP medium-temperature condensing unit with a matching air-cooled evaporator. In July the box runs 44°F instead of the 38°F setpoint, and the compressor runs continuously. The evaporator coil ices at the bottom third; the sight glass shows bubbles; suction pressure is 48 psig (R-404A, ~22°F sat) and head pressure is 290 psig. The condenser is on a roof, coils visibly dusty, adjacent to a hot exhaust fan discharge. The door gasket is torn along the bottom. Ambient at the condenser is measured at 118°F. The box has a 1/2 HP evaporator fan and a defrost clock set for two 30-minute defrosts per day.
1. Prioritize the faults and explain how each contributes to the symptom.
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Rank by contribution and safety:
- Dirty condenser + 118°F ambient near exhaust: High head pressure (290 psig is high for R-404A, ~118°F condensing) reduces capacity and raises compression ratio. This alone can prevent pulldown. Clean coils and relocate/redirect the exhaust.
- Torn door gasket: Continuous sensible and latent load; the box cannot maintain 38°F. Replace gasket.
- Bubbles in sight glass + low suction (48 psig, ~22°F sat): Indicates undercharge or restriction. With a partially iced coil, low suction is also a symptom of low airflow. Verify subcooling/superheat before adding refrigerant - do not 'top off' blindly.
- Bottom-third icing + only two defrosts/day: Ice on the lower coil suggests poor airflow or a defrost termination issue; ice reduces airflow and capacity, driving low suction. Check defrost termination sensor and heater, and evaporator fan operation.
Logical order: clean condenser and fix ambient first (cheap, high impact), then gasket, then evaluate charge with clean coil and stable box, then address defrost/airflow.
2. What measurements would you take, and in what order, to separate a charge problem from an airflow/defrost problem?
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Sequence (after cleaning condenser and replacing gasket):
- Let the box stabilize with the evaporator fully defrosted - icing masks airflow.
- Superheat at evaporator outlet: With a properly charged system and clean coil, superheat should be in the design range (typically 6–12°F for a TXV). High superheat + low suction = undercharge or restriction; low superheat + flooding = overcharge.
- Subcooling at condenser outlet: Low subcooling (<5°F) with bubbles supports undercharge.
- Airflow across evaporator: Check fan blade, motor, and coil fin blockage; measure delta-T across coil (design typically 8–12°F for MT).
- Defrost check: Verify termination temp, heater continuity, and that the clock actually initiates/terminates. Ice pattern on the bottom third often means the coil is not fully cleared or airflow is short-cycling.
- Head pressure vs ambient: Compare to manufacturer's chart; 290 psig at 118°F ambient is near the upper limit - confirm condenser is clean and not recirculating hot air.
Only add refrigerant after these checks confirm a charge deficit; otherwise you mask the real fault.
3. The compressor is running continuously. What does that indicate about the control and what should you verify?
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Continuous run means the thermostat is calling and never satisfied, or the control is bypassed/stuck.
Verify:
- Thermostat/controller: Is it actually calling? Is the setpoint 38°F and the box 44°F (yes, so a call is legitimate)? Check for a stuck contactor or a jumped low-pressure control.
- Low-pressure control: If set too low, the compressor may run past a safe suction; if bypassed, it runs continuously even when the box is satisfied. Confirm settings and that it opens on pump-down if used.
- Capacity vs load: Continuous run with high box temp and normal charge indicates the system cannot meet load (dirty condenser, gasket, undersized unit, ambient).
- Defrost clock: Ensure it is not stuck in defrost or failing to terminate, which would keep the compressor running against ice.
A continuously running compressor is not automatically a bad compressor - it is a load/capacity mismatch signal. Diagnose the load side (ambient, gasket, airflow) before condemning the compressor.