Rack Reference
How each part of the parallel compressor rack works, what "normal" looks like, and the numbers to keep in your head.
CO₂ Quick Reference
Approximate values for the field — always confirm against a CO₂ P/T chart and the equipment nameplate/manufacturer data.
Approximate — for quick field orientation only. Above the critical point CO₂ is transcritical (no condensation). High-side pressure in transcritical operation follows the controller / manufacturer's specified optimum high-side control strategy — use their setpoints, not this chart. Always confirm against a CO₂ P/T chart and manufacturer data.
| Temp °F | Press (psia) | Notes |
|---|---|---|
| -51°F | ~116 | Near triple point — caution |
| -40°F | ~145 | — |
| -22°F | ~207 | — |
| -4°F | ~286 | — |
| 14°F | ~384 | — |
| 32°F | ~506 | — |
| 41°F | ~580 | — |
| Point | Value | Notes |
|---|---|---|
| Triple point | 75.1 psia (≈ 60 psig) | -69.8°F — below = dry ice |
| Critical point | 1070 psia (≈ 1055 psig) | 87.8°F — above = transcritical |
| Flash tank / receiver | Controller setpoint | System-specific — never above receiver nameplate MAWP / relief setting |
| Transcritical high side | Varies with gas-cooler outlet °F | Controller-set, up to system design limit |
| Gas-cooler approach | Per design / commissioning spec | Over ambient (or wet-bulb with adiabatic pre-cool) |
| MT / LT SST | Per case requirements & controller setpoints | System design specification; convert with the PT data above |
| Item | Typical | Notes |
|---|---|---|
| High-side relief | Per nameplate / relief tag | Set by system design & manufacturer |
| Low/MT-side relief | Per nameplate / relief tag | Set by system design & manufacturer |
| Standstill MOP | Set below relief | Holds pressure with rack off |
| P-optimization (rule of thumb) | Higher outlet °F → higher target psi | Tune to manufacturer |
| Oil | CO₂-approved POE/PAG only | Moisture forms carbonic acid |
R-290 Field Cheat Sheet
The propane self-contained essentials to keep in your head — always confirm against the case nameplate and manufacturer data.
| Item | Value | Notes |
|---|---|---|
| Refrigerant | R-290 (propane) | A3 — FLAMMABLE |
| Charge size | < ~5 oz / 150 g | Charge-critical — use nameplate |
| Leak detection | LEL / combustible-gas or R-290-rated | NOT a standard halogen detector |
| Power | 120V plug-in (self-contained) | No rack connection |
| Before sealed work | Ventilate + kill ignition sources | No flame/spark/smoking; recover, never braze charged |
| Code | Means | Do |
|---|---|---|
| P1 | Room / thermostat probe fault | Open/shorted room probe — check probe & wiring |
| P2 | Evaporator probe fault | Defrost probe open/shorted — affects defrost termination |
| HA / LA | High / Low temperature alarm | Verify actual temp; check airflow, defrost, load |
| dA | Door open alarm | Door/curtain left open or door switch fault |
| EE / EF | EEPROM / data fault | Parameter memory error — reload parameters |
| cSd | Condenser overheat / probe | Clean condenser, check condenser probe & fan |
Codes vary by exact model/firmware — always confirm against the manufacturer's manual for the specific controller on the case.
Rack Controller Alarms
The common alarm codes and advisories for the rack / pack controllers you'll meet on supermarket systems — Danfoss and Emerson (CPC). These are whole-system controllers, distinct from the self-contained case controllers above.
| Code / Advisory | Means | Do | Guide |
|---|---|---|---|
| 1000 / 1001 | Too many resets (24 h / 1 h) | A fault keeps re-tripping and auto-resetting — stop resetting and find the root safety cause | — |
| 1100 | Serial timeout | Lost comms between controller and an I/O/expansion module — check LON/Modbus wiring, addresses, and power | — |
| 3001–3008 | SST / SDT out of range (low/high) | Saturated suction (SST) and/or discharge (SDT) temps off target — check pressures, staging, load, and the transducers | — |
| 3010 | Startup timeout | Pack didn't reach control within the allowed time — check compressor staging and safety chain | — |
| 3022 | Too many compressor starts | Short-cycling — widen the deadband/anti-cycle timers, check charge and load | — |
| 3024 / 3025 / 3026 | Min stop / run / restart time | Compressor timers preventing a stage change — normal protection; investigate if constant | — |
| 3027 | Start before unload | Staging sequence fault — check the unloader/VFD command order | — |
| 3302 | Discharge temperature high | Check discharge sensor, liquid/vapor injection or desuperheat, and the compression ratio | Diagnose |
| 3310 | Fan cycles exceeded | Gas-cooler/condenser fans cycling too often — check fan staging strategy and head-pressure control | — |
| 3400 / 7403 | Suction pressure low | Verify charge/restriction/load and calibrate the suction transducer before adding refrigerant | Diagnose |
| 3411 / 3431 / 7406 | Discharge pressure high / HP switch | Check gas-cooler/condenser fans & coil, the HP (Vhp) valve, and rule out non-condensables | Diagnose |
| 3500 | Oil level low | Check reservoir level, oil separator return, and the oil level regulators/floats | Diagnose |
| 3501 / 3502 / 3503 | Oil flow / pressure low, oil stop valve | Work the oil-management chain — filter, separator return, differential, and the oil stop valve | — |
| 3520 | Oil injection not effective | Injection not controlling — check the injection valve, its sensor, and the oil supply | — |
| 4003 | Motor current high | Compressor over-amping — check discharge pressure, supply voltage, and for mechanical binding | — |
| 4201 / 4202 | Motor phase error / phase sequence | Loss of a phase or wrong rotation — check the 3-phase supply and rotation before restarting | — |
| 4220 / 4221 | Motor frequency low / high | VFD frequency out of band — check the drive settings and its feedback | — |
| 4301 / 7304 | Motor temperature high / thermistor | Motor overheating or thermistor fault — check cooling, load, and the thermistor circuit | — |
| 6700 / 6701 / 6703 | Config: no file / CRC / read-only | Configuration file error — reload the parameter file/config for the controller | — |
| 6810–6816 | Hardware voltage / sensor-supply fault | Internal 3.3 V/5 V/24 V rail or sensor-supply fault — check the 24 V supply and sensor wiring | — |
| 6900 | Datalog error | Internal logging fault — usually non-critical; note firmware and reload if it persists | — |
Pack controller (AK-PC 572/772/782). Numeric alarm codes on the local display; read the code then check the matching safety input/sensor.
Codes and advisory text vary by exact model and firmware — always confirm against the manufacturer's manual for the installed controller.
VRF / VRV Error Codes
The common outdoor-unit error codes for the big VRF/VRV brands, with what they mean and where to start. Codes vary by model and generation — always confirm against the unit's service manual.
| Code | Means | Do | Guide |
|---|---|---|---|
| U0 | Refrigerant shortage / low pressure | Check stop valves fully open, leak-check, and confirm charge by weight for the pipe length | Diagnose |
| U1 | Reverse / open phase, wrong power wiring | Check the 3-phase supply and phase order at the outdoor unit before restarting | Diagnose |
| U2 | Power supply / main circuit voltage | Measure supply voltage on all legs under load against the nameplate tolerance | Diagnose |
| U4 / UF | Comm fault: indoor ↔ outdoor (F1/F2 DIII-NET) | Check F1-F2 wiring for polarity, loose lugs and shorts; keep it clear of power cabling | Diagnose |
| U5 | Comm fault: indoor ↔ remote controller | Check the remote-controller wiring and address; a one-off may clear on a power cycle | Diagnose |
| U7 | Comm fault: between outdoor units | Check the outdoor-to-outdoor transmission wiring and master/slave settings | Diagnose |
| E3 | High-pressure switch activated | Dirty coil / blocked airflow / overcharge / closed valve — verify before reset | Diagnose |
| E4 | Low-pressure switch activated | Usually low charge or a restriction — leak-check and confirm charge by weight | Diagnose |
| E5 | Compressor motor lock / overload | Do NOT keep resetting — check compressor windings and load; likely a tech/replace call | Diagnose |
| E6 | Compressor startup fault / overcurrent | Check compressor wiring, inverter output and that stop valves are open | Diagnose |
| E7 | Outdoor fan motor fault | Check for obstruction, the fan-motor connector, and the DC fan motor/driver | Diagnose |
| L4 | Inverter heatsink (fin) temperature high | Clean the inverter compartment/heatsink, clear airflow, and confirm its cooling fan runs | Diagnose |
| L5 | Inverter compressor overcurrent | Do NOT repeatedly reset — check compressor and inverter board; risk of permanent damage | Diagnose |
Never repeatedly reset compressor/inverter codes — that can cause permanent damage. Discharge the inverter DC bus and verify 0 V before any board or connector work.
VRF Layout — 2-Pipe vs 3-Pipe Heat Recovery
How the refrigerant network is piped decides what the system can do — and how you troubleshoot a "won't heat while another zone cools" complaint.
One liquid + one gas/suction pipe from the outdoor unit to all indoor units. The whole system runs one mode at a time — every zone heats, or every zone cools. A zone asking for the opposite mode sits in standby (that's a mode conflict, not a fault).
Three pipes feed BS / mode-change boxes, which route hot gas or liquid to each indoor unit independently — so some zones cool while others heat at the same time, and rejected heat is recovered between zones. If a site genuinely needs simultaneous heat + cool, it must be a 3-pipe system.
Illustrative schematic — confirm pipe count, BS-box locations and refrigerant type on the installed system.
Components & Field Examples
Tap a component for how it works, typical values, and an annotated example to match what you see in the field.
A parallel rack stages 2-6+ compressors on a shared suction header to match load efficiently. Capacity is controlled by staging compressors and/or unloaders/VFD lead compressor. Common oil management ties all crankcases to a reservoir.
- Suction group setpoint drives compressor staging.
- Lead compressor may be VFD or have unloaders for fine control.
- Shared oil system: separator, reservoir, filter, and level regulators.
- Each compressor protected by HP, LP, oil-failure, and motor overload.
Discharge 130-300+ psig depending on refrigerant/ambient; suction per group (e.g. low-temp ~ -20°F SST, med-temp ~ +20°F SST).
