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Control of first-order thermal plants with inherent time lags by means of hystereitc ONÂ¿OFF controllers is investigated. Open-loop pulse-duration-modulation control of the same class of plants is briefly analyzed. A scheme is investigated where the plant is controlled on an open-loop basis by a controller which is included in a feedback loop containing a model of the plant. The parameters of the model can deliberately be made different than those of the process. By a suitable choice of these model parameters better performance criteria can be obtained, especially when they exist due to the distributed nature of thermal processes and to the location of temperature sensing units.