MadZuri's last response was indeed what was wrong with the system. After reducing the boilers to 1 and adding additional dummy load (2 radars for now) the system starting working as intended.

Thank you guys so much for your help!

I was using a small pump to limit the amount of water going into the tank at the exact same rate it was being pumped out.
But just to satisfy my curiosity I adjusted my setup to reflect DaveMcW's advice (and MadZuri's earlier screenshot):

opamp-doublepumps.png

But unfortunately I'm still seeing ...

I'm trying to get this op-amp to work, but I'm experiencing "glitches" in the waterflow. Sometimes it seems theres a small drain on the watertank that causes backflow through the boilers.
This in turn causes some hot water to filter into the tank, heating it up by 1 or 2 degrees, flooding the system ...