The night before filling our spa for the first time, since we were too excited to sleep, we went into some calculations, including if it would worth it to use the water tank. Our water tank is a 60gallons, and we keep it at 60C (140F). After emptying it into the spa, it would take it almost an hour to be back to its temp, and by now the spa would almost be filled up, so only 60gallons out of 600 would come from hot water tank. We found out that it would raise the overall temperature of our water before heating of about 2C, not much, and not worth it, since it raises 4-5C/hour with the heater. And it didn't factor in the fact that doing that, you can bring in rust that is often present in old hot water tanks, and bacterias that grow in the hot water tank. (I just received with my electrical bill today an article about hot water temps, and each year 100 people in Quebec get pneumonia from Legionaria.)
In our case, thermodynamics laws were very close to reality, since our spa heated in a little less than the 6 hours we calculated. (Gotta use all those university classes for something usefull after all
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