Concept of improving or at least maintaining heat exchanger area efficiency has been widely adopted in previous studies which involving heat exchanger network retrofit case. However, application of the concept in retrofit case where fuel switching was taken as an option was less suitable. This study was carried to demonstrate the effect of existence of area efficiency constraint in retrofit case, which aims for CO 2 emissions reduction targeting for existing plant, via HEN retrofit and fuel switching. Two case studies were adopted in this paper to demonstrate the effect using mathematical programming approach. © 2011 IEEE.