Performance Comparison of Control Methods for Tankless Water Heaters

Abstract

Tankless water heaters (TWH) have several advantages and disadvantages when compared with storage water heaters. One disadvantage is that the outlet temperature of the water is difficult to control. To address this problem, the authors apply seven different control approaches and conduct experiments to test the effectiveness of each approach. The experiments are carried out on a commercially available electric TWH, using a test method and apparatus developed by the authors. The seven approaches include various configurations of feedback or feed-forward control, or both. One fairly simple approach devised in this project combines feedback and feed-forward control. This combined approach gives excellent performance, even when compared to a very sophisticated and complex approach (model predictive control). Furthermore, the approach does not impose a manufacturing cost premium for the subject TWH. The improvements in control that are possible with the approaches described in this paper are considerable when compared with approaches being used in electric TWH that are currently available to consumers.