While the gas connection for pool heaters is 3/4", more than a few feet of it will not provide enough gas. Typically, 1.25" or 1.5 inch pipe, depending on the distance from the meter is needed. A failure to do this will result in starving the heater, creating soot in the combustion chamber and accumulating on the heat exchanger, preventing any exhaust gasses from flowing out.

As for RayPak and Rheem, I gave up with them. They have some pretty significant issues with Mean Time Between Failure (MTBF), especially with heat exchangers, internal bypasses, less than idea chems, and rust.

I prefer Pentair Master Temp heaters. Even though they cost a little more, the MTBF is much longer. They don't have all the sizes I'd like to see in the HD models though. The HD models are needed with salt cells for the cupro-nickle heat exchanger they use. If you can provide at least 400 cubic feet of gas though, the 400K BTU model is as bullet proof a unit as I have seen.

In terms of how much gas is used, the overall amount is very nearly the same to get a job done with most sizes of pool heater. With the way water is heated, the higher BTU models just get the water warmed faster and keep it there.

Scott

Scott, thanks for the insight I know you're the expert. Hope your health is improving.
Judy