If you are Norwegian, Finnish, or Swedish, you might practice the fine holiday/seasonal art of hot-tubbing in freezing weather. While this may sound "off" to other people, it is perfectly normal in your culture and/or ancestral heritage. However, you could make some costly mistakes hot-tubbing in winter, all of which will require hot tub repair after the fact. Here are those mistakes and how to avoid them or make the damages less costly.
Leaving the Water in the Tub
Nope, nope, nope; leaving hot water in your hot tub in the cold winter air and not using the tub for more than a few days is a bad idea. Water will always freeze up, if given the chance. When water freezes, it expands, pushing beyond the boundaries of its container. In a hot tub that means that you could end up with ice shoving up out of the tub, or you could end up with a cracked tub that you can never use again without replacing it. Additionally, you would have some water stuck in the pipes and jets, causing them to break and/or explode.
If you like hot-tubbing in winter, just remember to empty the tub, drain the ports and jets, and make sure there is no water left in the pipes. This will save your hot tub, and allow you to hot-tub any time again in winter. Otherwise, your ancestral tradition of getting into a hot tub when temperatures dip below zero will have to end abruptly.
Leaving the Heating Elements Turned on and All the Way Up
Most hot tubs have adjustable heating. The heat is provided by heating elements under the hot tub. However, you can cause burning and melting of the tub and/or the panels surrounding the base of the tub if you crank the temperature and leave it on.
The tub will register the freezing temperatures outside, and attempt to work harder to keep the water in the tub at a consistent temperature. If you leave this go for several days, you could end up with a hot tub fire! It can happen, which is all the more reason to avoid it. Turn the temperature all the way down, or turn the heating elements off as you empty the tub. Avoid replacing the tub and any other parts that were scorched or melted by paying close attention to the temperature controls and heating elements.