System Pressure Diagnostic Calculator

The combined PT/SH/SC calculator computes superheat and subcooling and shows a 6-pattern interpretation. This diagnostic calculator extends the analysis with two more dimensions: condenser approach (discharge saturation vs ambient) and evaporator approach (return air vs suction saturation), then produces structured flags with severity, evidence, and ordered recommendations. The combined calculator answers 'is the charge correct?'; the diagnostic answers 'what's wrong and what should I do about it?'.

Condenser approach is the discharge saturation temperature minus the outdoor ambient. On a properly-running residential AC, the approach is typically 15-25°F (high-efficiency units near the low end) — the condenser needs that delta to reject heat to the air. An approach significantly above this range indicates the condenser can't reject heat as fast as the system is generating it: dirty coil, blocked airflow, non-condensables, or compressor inefficiency. Above ~40°F approach raises high-pressure-cutout risk and warrants stopping the system. These bands are hvacptcharts diagnostic heuristics drawn from OEM service practice, not a published standard; the equipment OEM service literature is the authoritative reference.

Evaporator approach is the return-air temperature minus the suction saturation temperature. For residential AC the approach is typically 20-40°F depending on indoor humidity — the evap needs that delta to absorb heat from the air. A HIGH approach (above ~40°F) means the coil can't pull the air down toward the refrigerant: low indoor airflow (dirty filter, failed blower) or evaporator starvation (undercharge, TXV restriction, blocked liquid line). A LOW approach (below ~20°F) means the air is over the coil too briefly relative to the boiling refrigerant: high airflow, overcharge, high indoor load, or a compressor not pumping. The approach is independent of charge in a way that SH is not, so it gives an orthogonal diagnostic dimension.

Target ranges for superheat and subcooling differ by metering device and application. TXV residential AC targets tight SH (8-15°F) with SC as the primary charge metric (8-12°F). Fixed-orifice residential AC uses a wider SH range read from the fixed-orifice charging chart — target superheat from the field approximation (3 × indoor WB − 80 − outdoor DB) / 2, indexed on indoor wet-bulb and outdoor dry-bulb. Walk-in cooler / freezer targets wider ranges (SH 8-20°F, SC 5-15°F). Without the system type the calculator uses a generic envelope; with it the flags are calibrated to what the specific equipment class expects.

The patterns reflect well-established HVAC field-service conventions — high SH + low SC = undercharge is the textbook fingerprint, repeated across decades of service literature. The calculator surfaces these patterns reliably from the input combination, but doesn't account for every real-world variable (system age, recent service, equipment-specific quirks, ambient changes during the reading). Treat the flags as 'here's what to investigate' rather than 'definitive diagnosis'.

Read them in priority order — alarm before concern before caution before OK. An ALARM flag (negative SH, negative SC, very-high condenser approach) demands action before continuing operation. A CONCERN flag identifies a likely cause and recommends specific investigation. A CAUTION flag is a less-clear pattern that warrants verification. Multiple flags often share root causes (overcharge flag + high condenser approach can both result from the same overcharge condition).

Yes — pick medium-temp or low-temp commercial system type. Target ranges adjust accordingly. Commercial systems have wider tolerance ranges on SH and SC than residential AC because case load, door openings, and defrost cycles introduce more variability into steady-state readings. For very-large industrial systems and transcritical CO₂, use the equipment OEM diagnostic procedures rather than these generic patterns.

Partially. The sub-critical low-side analysis works for CO₂ systems (R-744 saturation pressures below 87.8°F critical temperature are well-modeled). The transcritical high-side cannot be analyzed with the saturation-based logic — above the critical temperature there is no saturation pressure, so subcooling and condenser approach calculations don't apply. For CO₂ transcritical diagnostic work, refer to equipment OEM service literature; the patterns are different from sub-critical HVAC.

Severity is determined by both the magnitude of the deviation and the consequence. Zero superheat (slugging risk) is an ALARM because of immediate compressor-damage risk; high condenser approach during normal operation is a CONCERN because of long-term efficiency loss; slightly elevated subcooling alone is CAUTION because it might be early-stage and might be measurement error. The weighting follows established field-service severity-ranking conventions.