High Superheat and High Subcooling on a Restaurant Split System
A 3-ton R-410A split system serves a restaurant dining room. Outdoor ambient is 95°F, return air at the unit is 76°F dry bulb and 62°F wet bulb, and the blower is moving a verified 1,200 CFM. The condenser coil is clean, the outdoor fan is running at full speed, and the compressor is drawing 15.8 amps against a nameplate RLA of 16.4. Measured pressures are 115 psig suction and 390 psig liquid. Suction line temperature at the service valve is 62°F; liquid line temperature is 97°F. The sight glass at the receiver outlet is clear, and there is no measurable temperature drop across the filter-drier. Supply air is 57°F. The unit has a thermostatic expansion valve with an external equalizer. The owner says the space never quite reaches setpoint in the afternoon and the compressor runs continuously. A previous technician topped off the charge twice this season.
1. Compute superheat and subcooling from the readings and identify the single fault that best explains all of them. Name at least one competing diagnosis you must rule out and state the measurement that rules it out.
Read the worked answer
Calculations (R-410A):
- 115 psig suction ≈ 37°F saturation. Superheat = 62 − 37 = 25°F (target for a TXV is roughly 8–12°F).
- 390 psig liquid ≈ 116°F saturation. Subcooling = 116 − 97 = 19°F (target often 8–12°F).
Interpretation: The evaporator is being starved (25°F superheat) while the condenser is being flooded with liquid (19°F subcooling). That combination is the signature of underfeeding at the metering device - a stuck or plugged TXV, a blocked inlet screen, or a failed power element/bulb charge.
Ruled out - low charge: an undercharged system shows low superheat and low subcooling, usually with bubbles in the sight glass. Here subcooling is 19°F and the glass is clear, and two top-offs did not change the complaint.
Ruled out - liquid line restriction: a plugged drier or kinked line produces a measurable temperature drop across the restriction (a good drier is under ~3°F) and flashing downstream. This drier shows no drop.
Ruled out - weak compressor: a failing compressor gives high superheat with low subcooling because it cannot build a liquid seal, and amps typically fall well below RLA. Amps here are near nameplate.
2. The previous technician added refrigerant twice. Predict what each measured value does after adding charge, and explain why the readings still support your diagnosis.
Read the worked answer
Adding charge to a system that is already at 19°F subcooling pushes more liquid into the condenser:
- Subcooling rises further (e.g., 19°F → 24°F or more), and discharge pressure climbs with it. At 95°F ambient, head pressure rising toward 400+ psig squeezes the compressor toward its high-pressure limit.
- Superheat barely moves, because the bottleneck is downstream of the liquid line - the TXV still passes only what it can pass. A point or two of change is common and is what fools technicians into 'topping off' again.
- Amps creep upward as head pressure rises, which on a hot afternoon can trip the high-pressure control or the internal overload.
So the two top-offs did not fail because the system 'needs more' - they failed because charge is not the variable that is wrong. Rising subcooling with unchanged superheat is the confirming evidence: liquid is stacking in the condenser because the evaporator will not accept it.
Also note the efficiency penalty already present: 25°F superheat means the evaporator is only partly wetted, so capacity is well below 3 tons even though the compressor runs continuously.
3. List the repair sequence and the commissioning checks you would use to prove the fix, including what charge method is appropriate afterward.
Read the worked answer
Repair sequence:
- Recover the refrigerant into a clean cylinder; do not vent. Weigh what you remove and compare with nameplate - this also tells you how much the two top-offs added.
- Pump down or isolate, then open the liquid line at the TXV. Inspect and clean or replace the inlet screen; check the bulb for a cracked or lost charge and confirm it is clamped tightly on clean, bare suction line at the correct clock position with good contact.
- Replace the filter-drier whenever the circuit has been opened.
- Leak-check, evacuate to 500 microns and hold (decay test), then weigh in the nameplate charge - the only reliable method when the correct charge is unknown after tampering.
- Confirm the distributor tubes feed evenly and the coil frosts/sweats uniformly end to end.
Commissioning at design conditions (~95°F ambient, 62°F WB return): superheat 8–12°F at the TXV sensing bulb, subcooling 8–12°F or per the manufacturer's charging chart, amps at or below RLA, discharge pressure consistent with ambient plus 15–25°F, and a 18–21°F air temperature split. Record all of it so the next technician has a baseline.