You also need to decide how big a bill you want to pay. Gas rates are down a lot over the past decade, but it's still true that most people who try to heat their pool with gas, do so until they get their 2nd late fall gas bill . . . and then the heater gets turned off.

If you want to WARM your pool -- that is, extend the season a bit, and keep the pool warm during rainy periods in summer -- that is a far more practical goal, for most people.

Either way, a 3,000,000 BTUH heater is massive overkill. I 'warm' (10 degree boost) a 200,000 gallon commercial pool very satisfactorily with a 400,000 BTUH gas heater.