Fixed-Orifice Target Superheat (Charging)

The number a tech charges AGAINST, as opposed to the superheat the gauges measure. A fixed metering device does not hold a set superheat like a TXV, so the correct charge shifts with the load: Target SH = (3 x IDWB - 80 - ODT)/2, where IDWB is the indoor return-air WET-bulb and ODT the outdoor dry-bulb (F). On a hot day (63 F WB indoor, 95 F outdoor) the target is about 7 F; on a mild day (75 F outdoor) about 17 F. Charge until the measured superheat reaches this. The standard EPA 608 / manufacturer-chart charging method -- an empirical regression, valid roughly for outdoor >= 55 F; below that, or at a zero/negative target, do not charge by superheat (weigh in, or use subcooling on a TXV system). A field aid; the equipment charging chart and the manufacturer govern.

Target SH = (3 x IDWB - 80 - ODT) / 2, IDWB = indoor return-air wet-bulb (F), ODT = outdoor dry-bulb (F). Charge until measured superheat reaches this target.

The fixed-orifice target-superheat charging method, the standard EPA 608 / manufacturer charging-chart field regression, by name; empirical (a regression of the charging charts, not a first-principles derivation).

The (3 x IDWB - 80 - ODT)/2 target-superheat relation is public-domain and appears verbatim across free HVAC field-training references (AC Service Tech and equivalent); the wet-bulb and outdoor temperature are the technician's own measurements.

Estimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.

Field names used by the API: indoor_wetbulb_f, outdoor_drybulb_f, target_superheat_f

• Target superheat Target SH = (3 x IDWB - 80 - ODT)/2 (IDWB indoor wet-bulb, ODT outdoor dry-bulb, F)EPA 608 / manufacturer charging charts
• Fixed orifice only for piston/cap-tube systems; a TXV holds superheat and is charged by subcooling insteadcharging practice
• Valid range outdoor >= ~55 F and a positive target; otherwise weigh in the chargecharging practice