Condensing boilers use waste heat from flue gases to pre-heat cold water entering the boiler. Flue gases are condensed into liquid form, thus recovering their latent heat of vaporization, which results in as much as 10%–12% increase in efficiency. Modeling these heat transfer phenomena is crucial to control this equipment. Despite the many approaches to the condensing boiler modeling, the following shortcomings are still not addressed: thermal dynamics are oversimplified with a nonlinear efficiency curve (which is calculated at steady-state); the dry/wet heat exchange is modeled in a fixed proportion. In this work we cover these shortcomings by developing a novel hybrid dynamic model which avoids the static nonlinear efficiency curve and accounts for a time-varying proportion of dry/wet heat exchange. The procedure for deriving the model is described and the efficiency of the resulting condensing boiler is shown.

The field of energy efficiency in buildings offers challenging opportunities from a control point of view. Heating, Ventilation and Air-Conditioning (HVAC) units in buildings must be accurately controlled so as to ensure the occupants' comfort and reduced energy consumption. While the existing HVAC models consist of only one or a few HVAC components, this work involves the development of a complete HVAC model for one thermal zone. Also, a novel multivariable control strategy based on PI auto-tuning is proposed by combining the aforementioned model with optimization of the desired (time-varying) equilibria. One of the advantages of the proposed PI strategy is the use of time-varying input equilibria and zone setpoint temperature, which can lead to important energy savings. A comparison with a baseline control strategy with constant setpoint temperature is presented: the comparison results show good tracking performance and improved energy efficiency in terms of HVAC energy consumption.