If the water stays in the hot tub for a longer time in winter, the most important thing is to ensure, that the water would not freeze in the barrel. In case it happens, it may damage the wooden hot tub.
Here are a few steps you can take to prevent the water from freezing:
* Heat up the furnace from time to time;
* Use a heating cable and temperature sensor to prevent the freezing;
* Empty the wooden bath barrel;
* Put large pieces of styrofoam into the water, the ice pushes the pieces of styrofoam together and the barrel will not be damaged;
* Use a filter pump in the hot tub to circulate the water.