Water heater thermal expansion tank?

First of all, great site! I found this forum some years ago when I started having to trouble shoot my Grundfos VFD due to the shortsighted engineered obsolescence of this type of system (I fell victim to the promises of greater efficiency with VFD when my conventional well pump failed after some 20 years). After 2 CU301 controllers failed (the first being replaced under warranty), I converted by system over to one utilizing a CSV, and never looked back.

Today, my 4.4gal pressure tank has become water logged (the tank that was gleaned from the Grundfos VFD system when I converted my system over). My local plumbing supply house has an Amtrol ProFlo thermal expansion tank (water heater safety tank) that sure seems like it can be used for my application. I mean, logically, it does what I need it to do. It operates in the same pressure ranges as my old tank, it has a bladder with air on one side and water on the other, what else is there? Amtrol customer (no)service was no help. I've poured over this site, and the internet, to determine if there's any reason why I CAN'T use this tank, to no avail. Just looking for verification from those in the know....

I don't know? Looking up Amtrol ProFlo thermal expansion tank on the internet says it's for hot water systems. I don't know why it wouldn't work for a pressure tank. I suggest you call Amtrol and ask them.

I called Amtrol, and unfortunately, could not get to a competent person to get a straight answer. The tech support simply read out the application of the "water heater safety tank" (thermal expansion tank) from the catalog. My issue is, it does the same thing, works the same way, operates in the same pressure ranges, and it even looks EXACTLY the same (except for the color and part number), so, if it looks like a duck, walks like a duck, talks like a duck, and acts like a duck, it must be a duck, right?

Not everything that looks like a duck is a duck. Sometime you have to look inside. A thermal expansion tank sometimes has a very small opening. It may still be a 3/4" pipe thread connection but, the hole inside the connection may only be as big as a pencil. If it has a regular size opening, it will work fine. If it has a tiny little opening, the water can't get in and out fast enough to work as a pressure tank. Of course you know that tank was probably damaged when the CU301 failed.