Keeping the heater on low overnight versus shutting it down between swims

Tom, honest answer is I can't give you gas numbers because I don't run gas, we swapped to a heat pump years back, but the recovery versus steady-state question is the same physics either way and your neighbor's got a point. Holding a setpoint overnight is usually cheaper than a full cold start every time because the heater isn't fighting ambient loss from scratch, it's just topping off. The bigger the delta between your start temp and target, the more that fire-up method costs you per session, 25 seasons of it working doesn't mean it was the cheaper way, just that it didn't fail you.

What I'd actually check before messing with your binder routine is whether you've got a cover or at least rely on the screen cage to cut evaporative loss, because that's where the overnight hold really bleeds heat if you don't have one. Mine stays at temp all the time now since I swim every morning anyway, so for me it was never really a question of which method, just whether the heat pump could hold it cheap enough to leave running, and it has. If you want real numbers before switching habits, track your gas bill a few weeks each way, that binder habit of yours would actually make for a decent experiment.

Chuck's right about the delta being what costs you, but the piece missing is surface area and wind, not just whether you've got a cover. Heat loss off the water is basically proportional to exposed surface times the temp difference to ambient, so an uncovered 22k gallon pool bleeding from say 85 down to 70 overnight in a Katy breeze is losing a lot more BTU than the same pool sitting still at 78. That's the actual mechanism behind "recovery costs more," your Raypak has to replace all those degrees across the whole surface at once instead of just making up small losses continuously.

I don't run gas either so I can't give you the per-therm numbers, but for what it's worth the holding method only wins if the standby loss is actually smaller than the fire-up loss, and that math flips hard if you don't have a cover and it's windy that week. If you've got a cover, test the hold method with it on every night, if you don't, I'd bet your 25 season fire-up habit is closer to optimal than Chuck's experience with his heat pump running constant, those are different loss curves entirely.

Marcus, I'd push back a little on framing it as fire-up habit being "closer to optimal" without a cover, because you're still paying the standby loss either way during the hours the pool sits at ambient between your grandkids' visits, Tom's not holding 78 for 16 hours and then firing up, he's letting it drift all the way down and climbing back. The real comparison isn't hold vs fire-up in the abstract, it's whichever method loses fewer total BTUs over a full week, and a gunite pool with decent thermal mass sitting at ambient overnight before a cold Saturday morning start is losing heat the whole time too, just not to the heater, to the air. So the uncovered case may favor fire-up over holding at 78, but it doesn't mean fire-up is free, it means both are leaky and you're choosing which leak costs less.

This isn't my chemistry wheelhouse so forgive me wandering outside my lane, but on the practical side, if the actual complaint is grandkids waiting around, I'd just test a middle ground before committing to either extreme, hold it a few degrees lower overnight, say 72 instead of 78, with a timer that kicks the Raypak up a couple hours before the usual Saturday arrival instead of right at the door. That knocks down both the delta Marcus is talking about and the standby loss Chuck's worried about, and you can log it in that same binder for a few weekends and actually see which way the needle moves for your specific pool and your specific Marietta wind, rather than guessing from Katy or from somebody's heat pump that isn't even the same beast as a gas heater.

Not gas heated so I can't weigh in on the therms question, but the screen enclosure point Chuck raised about evaporative loss is real, anecdotally my pool holds temp noticeably better overnight than it did pre-screen at our old house, probably worth more than people give it credit for in this kind of math.