In the field, almost every condenser suffers from some kind of fouling problems. The chiller performance degrades naturally and gradually as fouling increases. Early identification of fouling in the condenser is essential to maintain an optimal chiller operation. In this paper, a method (virtual fouling monitor sensor) using low cost and commonly available onboard chiller measurements for monitoring the fouling status of the condenser is presented. The performance of the proposed virtual fouling monitor sensor was evaluated using lab data over a wide range of operating conditions in both normal and faulty conditions. Moreover, the proposed virtual fouling monitor sensor was also implemented and evaluated on a field chiller. The laboratory and field test results show that the proposed method gives a good and robust performance in terms of detecting the condenser fouling faults in chillers. In terms of possible applications, the proposed method has the potential to be implemented as an individual fouling monitor sensor/tool, incorporated within the commercial chiller FDD tool, or embedded in the control systems onboard the chiller to automatically monitor the condenser fouling status. (C) 2012 Elsevier B.V. All rights reserved.