The air-cooled condenser designer calculates required condenser capacity and recommends a class-typical unit using the Total Heat of Rejection (THR) method from ASHRAE Refrigeration Handbook 2022, Chapter 35. THR = refrigerating capacity × (1 + 1/COP), adjusted for compressor heat of compression at the specified saturated discharge temperature (SDT). Hot-climate ambient derate is applied: medium-temperature systems lose roughly 2.0%/°C above the ASHRAE reference ambient of 95°F (35°C); low-temperature systems lose roughly 2.7%/°C. A hard floor of 60% rated capacity is applied regardless of ambient. The tool warns when saturated condensing temperature (SCT) reaches 131°F (55°C) — above this threshold compressor warranty may be voided and oil carryover risk increases. Validated against Bitzer Software and Heatcraft NROES unit selection for hot U.S. summer design conditions (Phoenix, Las Vegas, Gulf Coast).
References: ASHRAE Refrigeration Handbook 2022 Ch. 35; Bitzer Refrigerants Report 25 (2022); Heatcraft Refrigeration Product Selection Guide.
Size the condenser to reject the total heat of rejection (THR), which is the evaporator load plus the heat the compressor adds. As a rule THR is roughly 1.2 to 1.5 times the evaporator load. This calculator applies the ASHRAE THR method for your load, ambient temperature, and refrigerant, and derates for hot climates.
THR is all the heat the condenser must dump to the outdoor air: the heat absorbed in the walk-in plus the electrical work of the compressor. Condensers are rated at a specific temperature difference between the condensing temperature and ambient, so THR and design ambient together set the condenser capacity you need.
A condenser rejects heat by the difference between its condensing temperature and the outdoor air. On a hot day that difference shrinks, so the same condenser rejects less heat and the system loses capacity. Sizing for a high design ambient (with a derate) keeps the walk-in holding temperature through summer peaks.
Size for the refrigerant the system will actually run, because each has different pressure and heat-rejection behavior. For new 2026 equipment that usually means a low-GWP or A2L refrigerant such as R-454C; select it in the calculator so the THR factor and condensing conditions match. See the 2026 compliance guide for refrigerant rules.