What did the "wet side" of the vessel look like when you hooked it up? Was it all stoned up with mineral? Did you wash it out first/
Yes basic questions but we have to start someplace.
100 degree water -- measured by ??? just the gauge on the boiler -- how about your hand? 100 degrees is bath water -- 120 is hot enough to touch but you don't want to hang on long - hotter is well just hotter
at what temp is the supply vs the return pipes to the boiler -- is there a difference?
is the pump -- pumping??? Choke the flow -- a lot and see if the temp starts rise quickly. If it does your pumping at too hi a gpm -- if not check the pump again or open a wash out plug and check inside for mud.