Performance of a Combination Heating System for Residential Applications

Abstract

Advancements in water heating and control technology have improved the performance of tankless water heaters in recent years. A tankless water heater coupled with a hydronic air-handling unit functions as a novel system known as a combination (“combo”) system that is used for residential space and water heating applications. However, the performance of such a system has not been well studied to date. In this thesis, an experimental method was developed to analyze the performance of the combo system for space heating only operation and simultaneous (space and water heating) operations. To quantify performance of the combo system, the effect of the outlet water temperature from the water heater and the effect of the flow rate of water for domestic usage on first law efficiency and hydronic heat exchanger second law efficiency were studied. With the experimental assembly, it was found that the thermal efficiency of the tankless water heater during space heating and simultaneous operation ranged between 39% and 95%. Two adverse behaviors, short-cycling and flow-cycling, were observed in the combination system operation and were found to have a negative impact on system efficiency. The hydronic heat exchanger exergy analysis results suggested that the combo system has comparable second law efficiencies when compared to other conventional hydronic space heaters. The results of the first and second law analyses suggest that combination systems have equivalent performance similar to mid- and high efficiency furnaces and conventional water heater units, provided that adequate control strategies are in place to optimize combination system operation.