Below find help to avoid the spa from freezing and what to do to winterize your hot tub.
How do I protect the Spa from Freezing?
Normally the spa, when operating, will not freeze unless you turn it off.
If the power goes out any length of time or the unit fails, you need to act. Although water temperature will maintain for awhile (don't keep checking and allow heat to escape) you need to be proactive.
If you know you will not have your spa repaired quickly, you will need to either drain it (above) or protect it. In most climates, simply putting a heat lamp or even a shop light in the equipment area can provide enough heat from the bulb to keep things unfrozen. But the critical parts are the plumbing. For spas with a lot of foam, this too can be a blessing and curse. But making sure whatever method you are using, nothing hot is touching any equipment or wet area or foam. Do not place any electrical object in water. If you drain the spa you can also put the light/heater in the water area of the spa to warm from there into the pipes. But is short, do not depend on the draining to keep you safe. I have found spas that the owner "thought" the water was drained well but wasn't and bottom fittings and pipes were burst. This can be a major job. By assuring the spa/water is kept warm by either heat source in the equipment area or in a dry spa will be money well spent.
Another effective way to protect your spa from freezing if the spa shuts down is "pipe heater tape". This is a common pipe tape found at most hardware stores. They are designed to wrap around home water pipes in cold climates. Once you wrap them around the pipes you plug the tape in. It has a sensor and ONLY comes one when the pipe gets cold.
Wrap some of the pipes with one or two heater tapes, especially around the pump and heater. Then plug the heater in an outlet that isn't connected to the spa. That way if the spa caused the breaker to trip, the heater tape is ready to save the day. Of course you would want to make sure any wiring exposed to the elements were protected from the elements.
How to Drain Your Spa:
Your spa likely was designed to drain well when you use the built-in drain. But several areas may not drain well so you need to give them attention.
First, remove the left over water inside the spa's footwell area after draining. Some techs will use a wet vac and pull the remaining water out of each jet.
Second, depending on your spa, you have 1-3 pumps to be concern about. On each pump loosen the front fitting where the pump mates with the spa plumbing. On some pumps you will see a knob drain valve below the front fitting. This is there to allow remaining water to drain. Loosen or remove.
On smaller circulation pumps you have to pull the hose off the front of the pump. There will still be water in many pumps so if the weather is cold in your area, a wet vacuum can be used to suck the remaining water out of the pump.
Third, the heater. Loosen one or both fittings to the heater to allow the water to drain. If you find no water dripping out when you loosen, shake the connection to break a possible seal to allow water to drain.
Many homeowners will take their wet vacuum to each jet and suction fitting on the water side and suck any remaining water out of the piping. This or blowing air into the fitting is what a professional spa service company will do.
Finally, visually look at the lower piping to imagine if all the water is drained. If you can see where the water would drain to, you should be good to go.
What if the spa IS FROZEN?
There is frozen and there is frozen.
Remember, water freezes from top to bottom. Think a lake; frozen on top, fishes swimming below.
So a layer of ice on the top MIGHT not mean horror... but could.
Although ice floats, the exposed piping such as manifolds can freeze and burst. Typically, if they freeze it will not be until the spa warms up that you will see water. But if you find the ice on top early enough the spa's health might be save.
First option is likely your only option and that is to put a small heater inside the equipment area and turn on. Make sure its not touching anything and above the floor of the spa to avoid water leaking and it shorting out. Also make sure their is enough air flow to avoid overheating the heater itself.
Close up the access and hopefully the heater will bring the water up to a temperature to allow the spa to start unfreezing. Or at least save the piping. But as stated, if there is now a leak, it will flood the equipment area so you have to be assure that the heater doesn't touch water.
Also.. our experience if there is one crack in the piping there usually are more.
From time to time peek into the equipment area. In ideal situation you will see no water and the spa will get warm enough not to be frozen.
Then you can address what caused the failure or enough to drain for the winter. At least you can determine where you will go from here.
Second option is wait till spring. Often this is best because the part(s) needing fixing will require the spa to be drained anyway. But if you LOVE your hot tub.....