I've been curious about the feasibility of using aluminum flex to protect thhn/thwn wires feeding heat pumps. I know that the code allows such wire to be installed in wet environments..I'm just wondering about the possibility of corrosion of the flex connection at the heat pump. I've never seen a problem with old installations done that way, but others have pushed hard for using "seal tite" flex..presumably for the crush resistance and physical damage protection thereby effected. Inspectors in my area don't seem to have a problem with such short pieces (6ft) of aluminum flex outdoors..but something tells me that that isn't a sound idea. Anybody have some experience along these lines?

I'm not a fan of Aluminum flex & where it is used doesn't change my opinion.

However, 348.12(1) allows flex to be used in wet locations with the proper type of conductors when the flex is installed so thatliquid is not likely to enter raceways or enclosures to which the conduit is attached.

In all likelyhood, the flex installation you describe contains an equipment grounding conductor, so the corrosion shouldn't be that much of a problem.

Around here, things can get quite competitive, but no one is so cheap that they have to save 20 cents a foot on the cost of sealtight & maybe $1 to $2 for the connectors.