This paper describes an algorithm for simulating the steady state performance of direct expansion (DX) condensing units. The algorithm is intended for use in building energy analysis computer programs and is an improved form of a similar model implemented in the Building Loads Analysis and System Thermodynamics (BLAST) computer program (Ref I). The model resembles the algorithm of ASHRAE's publication Energy Calculation 2 -- Procedures for Simulating the Performance of Components and Systems for Energy Calculations (Ref 2) in that unit capacity and full-load power consumption are adjusted based on evaporator and ambient temperatures. However, the new algorithm differs from the algorithm of Ref 2 in several significant ways: (1) performance parameters are specified at a "design point"; (2) the algorithm includes the effect of part-load performance; (3) the effects of variables impacting DX unit performance are introduced one at a time; (4) dimensionless variables are used wherever possible; (5) machine characteristics, such as the U-factor area product of the condenser, are used; and (6) thermodynamic analysis of the refrigeration cycle is combined with empirical curve fitting to give more physical meaning to the form of the model.