If necessary, you can close the ducts in the hot tub with duct plugs. You can empty the heater (there is a plug for that in the front part of the heater) after the ducts have been closed. That way you will not have to empty the hot tub but will be able to protect the heater from freezing. Use 50 mm plugs, with adjusting screws. Please refer to the instruction manual of the heater for extra information about the freezing risk.
If you close the ducts of the filter system it is possible to do maintenance work (i.e change filter balls) without emptying the hot tub from water. You may also close the ducts if you are not going to use the filter sytem for a while or you want to remove it. Use 38 mm plugs, without adjusting screws.