What's the difference between AG and IG heater???

[The Mermaid]

Guess I could re-state my question. I'm looking for a 150,000 btu gas pool heater that can be used for AG pool. All I can find with that output are classified for IG pools. What's the difference?

[Rangeball]

My guess is simply larger capacity and heavier dutier.

I've often wondered the same thing about AG vs IG automatic vacuums. Your AG pool has the capacity of my IG.

[The Mermaid]

I want to buy a heavy-duty heater and like I said, I don't mind spending a little more money if I get something that is really good quality. But what I don't want is to pay big bucks for something that I get home and then find out can't be installed on my AG pool. But everything that I see for AG is about 100,000 btu. And according to my calculations I would want at 150,000 btu minimum.... all of which are called "in-ground pool heaters." I will have to talk to someone that sells them at the pool store I guess, but that is always a last resort because I get such bad information from them so often. That's why I posted the question here first to see if someone might know the answer.