I have tried understanding this and I just cannot grasp it so I'm hoping someone on here can explain it in layman's terms for me. I'm currently having a new pool and spa built in North Florida and I have included in my project a 400,000btu gas heater for the 7ft circle spa and I was told it could heat the pool a couple of degrees too with the oversized spillway from the spa to the pool. I don't have gas currently so I will have to get that separately which i've been quoted around $3,000 for a 250g underground tank filled with propane. My question is, does this make sense to do or should I just buy an electric heater that will heat both the spa and pool, if that's even a possibility as in the winter it can get into the 30s-40s sometimes at night? From what the PB said, even in that cold of weather the spa could get hot with the gas heater quickly. The pool is 15x30 in the largest spots as it's freeform style, 3.5ft to 5ft to 3.5ft sports style and the permit it says it's a 10,600 gallon pool but I am not certain if that is correct or not. Please help as from what I'm gathering propane is not cheap and if an electric heater could do both I think that makes most sense? Thank you for any advice in advance!