Analysis of the performance of a tankless water heating combo system: Space heating only mode

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 system that is used for residential space heating. To quantify the effect of the outlet water temperature from the water heater on the time-averaged thermal efficiency of a tankless water heater-hydronic heater combo system, an experimental method was developed. With the experimental assembly, it was found that the thermal efficiency of the tankless water heater during space heating varied monotonically between 39% and 95% for outlet water temperatures settings respectively between 38 °C and 60 °C. Energy losses from the flue gases represented 3%-47% of the total energy that was supplied to the system for the same respective outlet water temperature settings. An exergy analysis of the air handling unit was performed and results are presented. The results suggest tankless water heater-hydronic air heater combo systems can be an efficient configuration for space heating at tankless heater outlet water temperature settings of 60 °C. Water temperature settings below 60 °C produced transient variations, including cycling, in the system that became important and system performance deteriorated.