The Thermal Cut off CANNOT cause a short to ground at all. IMPOSSIBLE!
It is isolated in the circuit and does NOT touch anything. So, that means I would BYPASS the 2 brown wires on the tstat and connect together with the Thermal installed. IF the fuse does NOT blow, your tstat is shorted to the aluminum tank. OR the 2 spades on the tstat are shorted in the tstat mount to the tank-----the spring for the tstat is not installed correctly. Have you pulled the black cover over the ECO and Tstat? If the fuse does blow with the tstat bypassed, then the Circuit Board would be the culprit. You may have stated but I missed it. Does the fuse blow immediately or ONLY when you start the ignition sequence? Doug