I have a 16x32 inground concrete pool. I need to replace my 185,000 BTU Comfortzone NG heater. It is installed inside an unheated pool building on concrete floor with venting through the roof. I was considering the Raypak NG 206 BTU model. Should I spend the extra $70 for the cupro-nickel heat exchanger? Do I need to use the Raypak indoor stack kit or is my existing venting sufficient? It looks similar to the Raypak Kit. What are your thoughts and recommendations on all of this. Thanks.