New Rinnai boiler overheating very quickly but no hot water?

Hi all, I need some input from the professionals.

I recently had a solo Rinnai tankless boiler installed (IP060S). It's hard to tell from the picture but on the left I have two sets of pex manifolds each controlled by a Caleffi zone valve and both branch circuits are fed by a Taco VR1816. I purged all if not most of the air from the system, first from the secondary side, then from the boiler side with a sump pump and a couple of buckets.

I had my plumber come by to start up the unit because I was originally getting low water pressure error codes. He resolved that quickly but then came the main issue.

When there's a call for heat, the boiler fires but the water temperature goes up very quickly and immediately shuts off. On the first go I think the display was showing 255 F within 45 seconds from the call for heat, and there was steam coming out of the air vent inside of the boiler.

Just to give some more context, during the initial start up, the air vent was silent then it started hissing a bit, and gradually it became a loud gurgle both from the vent and the boiler. At one point it sounded like water hammering in the boiler while it was heating up, similar to what you would hear in a steam system, even though I bled the air out of the boiler.

The piping immediately under the supply side of the boiler was barely warm to the touch. And the fact that the water temperature was going up so quickly leaves me to believe that the boiler is heating up a small amount of water and that something is causing it to get stuck within the boiler. I can also feel/hear the pump running, but all of the supply side piping from the boiler to the pump is cold to the touch/barely warm.

Has anyone experienced this before or have any inclinations as to what could be causing this issue? Thank you in advance.