It can be done. I am in the process of researching and will be heating mine the same way. I am on here for the same reason as you, which is to find out the best way. I have been in the heating business for 20 years so hook up I will be doing. I have been looking for best way to pipe it in and still have the use of the electric heater for the summer. If you get any info my email is alderfield@live.com. I would also share my thoughts with you as well. I do know that you need to figure how many btu's you need to send to it to be of any use. At 5.5 kw I just need to convert into btu's and that will be a good place to start. You need an exchanger of sorts as running your spa water through your boiler system would be just plain nasty.
