I don't know how far North your are. If you are concerned, you could set your heat at the lowest setting AND run the pump. Frankly, it would have to be below zero and stay there for a couple days for me to be concerned about the water in a hot tub with the pump running, even with the heat off.
On the other hand, you definitely don't want the thing to freeze. If you are way up north and it will be below zero for several days, maybe you should dump the water.
Answers & Comments
Moving water rarely freezes. Is your spa outside?
I don't know how far North your are. If you are concerned, you could set your heat at the lowest setting AND run the pump. Frankly, it would have to be below zero and stay there for a couple days for me to be concerned about the water in a hot tub with the pump running, even with the heat off.
On the other hand, you definitely don't want the thing to freeze. If you are way up north and it will be below zero for several days, maybe you should dump the water.
Thanks for your question at FixYa.com