Connection to Pentair minmax 100 pool heater

[aandsc]

I got a pool heater (for free) to replace my old one but it didn't come with any connection for the water. I've connected the propane and faked out the safeguards, so I know it flames up (it's an old millivolt thermopile valve). I've purchased a "Pentair 471083 1-1/2-Inch Threaded Connection Quick Flange II Replacement Kit MiniMax 75/100 Pool and Spa Heater" from Amazon which gets me part way.

It looks like I'll need a 1 and 1/2 inch threaded PVC pipe. I think I can get that from a local plumbing supply (I'll take suggestions on what to buy!), but what do I use to seal it? I'm pretty sure regular teflon/plumbers tape will be useless; my concern is the differing expansion between the PVC and the rubber (or plastic?) of the flange. I think silicon rubber may be usless too (because of the temp changes). I'm thinking I need a sealant that stays in a somewhat flexible state....

My basic question is: What is normally used to go from flange kit to regular 1.5" PVC? I can handle things from there.