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Refrigerant diagnostic formulas
Used in RefCalc steps 3 (Coil Airflow) and 4 (Duct Measurements).
Coil airflow (step 3)
- Evap TD (air) = Return DB − Evap discharge
- Evap discharge = Return DB − Evap TD
- Cond TD (air) = Cond discharge − ODB
- Cond discharge = ODB + Cond TD
- Evap TD (water) = Evap In − Evap Out
- Cond TD (water) = Cond Out − Cond In
Duct measurements (step 4)
- BTU/hr = CFM × 1.08 × ΔT
- CFM (sensible) = BTU/hr ÷ (1.08 × ΔT)
- VP = TP − SP (in. w.c.)
- FPM = 4005 × √VP
- CFM (sizing) = FPM × duct area (sq.ft.) — enter area in sq.in. (÷ 144 for sq.ft.)
- CFM (traverse) = FPM × duct area (from VP path)
Air and oil: estimated oil temperature requires Discharge (°F) on the Setup step.