Condensation inside Vitocell 300 tank

meerkat

We have a Vitocell 300 tank running indirectly off a Vitorond 100 boiler, both installed in early 2018, so four years old. A couple of weeks ago I noticed a narrow line of dried rusty water down the front of the tank, coming from behind the thermostat, and called the installing contractor. Upon removing the lid he found water sitting on top of what I assume (from the material) is the top of the styrofoam(?) insulating jacket. The jacket was completely discolored orange and the inside of the tank lid has areas of rust as well. At the end of the service call he instructed us to leave the lid off the tank for about a week in order to let the top dry out, and let them know if any water reappeared. So we did. No water collected during the test period (although the water temp was a couple of degrees cooler than normal.)

A few days after putting the lid back on, we checked it and found water in the same area again. (The system is in an unfinished full basement that never goes below freezing; average winter minimum down there is at least 50F.) It's pretty obvious that as soon as the lid is put back on, condensation begins inside the tank lid, drips down onto the top of the styrofoam jacket, and starts collecting there. When that water got high enough previously, it began leaking out behind the thermostat indicator.

Waiting for a call back from the installing contractor after giving them the update via voicemail.

Anyone else ever seen this happen, and if so, was it pegged to a failure of the insulating jacket (either in workmanship or install, and in either case that is out of the initial 2-year warranty) or of the tank itself (which obviously is still covered)?

Thanks!