Four different thermo-economic techniques for optimum design of hot water piping systems are presented. They are as follows: the first one is a sequential optimization of pipe diameter based on minimization of total cost without considering heat losses and then of insulation thickness based on minimization of cost of insulation and heat losses. The second is simultaneous optimization of pipe diameter and insulation thickness based on the first law of thermodynamics and cost. The third is simultaneous determination of pipe diameter and insulation thickness based on maximization of exergy efficiency without considering cost. Finally, the fourth is simultaneous determination of pipe diameter and insulation thickness based on maximization of exergy efficiency and cost minimization. A case study is carried out for a hot water pipe segment, and the differences and merits of each method are discussed. (c) 2005 Elsevier Ltd. All rights reserved.