A non-linear model has been developed for a gas-fired furnace in which oil is heated. The model is applicable from minimum to maximum heat load of the furnace. The dynamics of the model have been compared to experimental results, which were obtained for a pilot-scale furnace. They are in good agreement. A cascade control with feed-forward action has been compared to single feed-back control. Proportional feed-forward action already gives much better results than the latter control.