A wood fired hot tub in the winter is designed to be used and the heating system drained. It has port plugs that will allow you to drain the heater and keep the water in the tub but that is only used for over night not for any long period of time. For extended periods it is recommended to have a backup electric heater. Heat up times in winter will depend on the water temperature as well as the size of the tub and size of the heater. To control the heat, you need to get familiar with the heating times based on the type of wood you are using. When it reaches temperature you then shut off the air supply to the heat to shut down the fire.