On Saturday, September 2, 2017 at 10:24:13 PM UTC-4, Deodiaus wrote:
I just got back from the hardware store. The guy recommends using this copper flexible coated with plastic line. Is electrolysis corrosion an issue for copper and iron piping [which seems to be a common youtube presentation]? I see this done on hot water heaters.

Electrolysis generally happens in water, which supplies the electrolyte.
Gas transitions from steel to copper, brass, etc and I've never heard
of any dielectric unions, etc. Is that flexible line even of sufficient
size for the BTUs of the pool heater? They can be 250K, 400K, which
is like 4x a furnace. Is it rated for outdoor use? Whether codes allows
it or not, IDK. If it does, I would think it would have to be protected
from physical damage and if so, I don't see an advantage to just using
steel pipe. I have not seen it used here for a pool heater, but then
I haven't seen a lot of them either. You could probably use the
flex pipe made specifically for gas runs, ie the kind you can
run through a whole house, but AFAIK, that requires special tools
to work with. I looked into it awhile back for a project, but figured
it wasn't worth it and went with steel. But the only thing that matters
is what the local code is.

If it were me, I'd just either use what's there, with minimal change,
if it's in decent shape and looks OK or replace it with new steel.
How much is your time worth, figuring all this out? NAt gas isn't
very expensive here, but people with pool heaters wind up with bills
of $2K a season. Spending a few hundred to just get it done right
might be the best option.