Simulating fuel-fired combination space and domestic water heating systems

HVAC systems that supply both space heating and domestic hot water are becoming more popular in residential buildings In North America. With these “combination systems” water from a fuel-fired storage tank is used to satisfy domestic hot water needs directly. Space heating is accomplished convectively, by circulating hot water from the tank through a water-to-air heat exchanger in a fan-coil system. An explicit plant modelling approach was used to represent a combination system within the ESP- r/HOT3000 simulation program. A simplified representation of the fuel-fired storage tank was employed in this initial phase of the research in order to explore the factors that significantly affect the modelling. As such, issues such as stratification in the tank, condensation of exhaust gases, cooling from ventilation air during burner off time were not included in the model. Measured data from a conventional power-vented water heater and from a high-efficiency condensing water heater were used to test and validate the model. Observations from these comparisons  demonstrate the shortcomings and strengths of this modelling approach that utilizes the simplified representation of the fuel-fired storage tank. The paper will detail the modelling approach employed, contrast the model and measured results, and summarize a literature survey on the m delling of boilers and fuel-fired storage tank models.