Good Guess Nick BUT I would think a flow control valve should be checked before thinking it is electrical problem.
Heat rises my good man and the flow control is there to prevent stratification of the hotter water traveling through out the system.
Now considering many systems are neglected and dirty it would not take much sediment to cause the flow control plunger to fail in the open position.
Hey nick you got to ask a heatin guy as they know everything about nithing huh?
Look at all those underslab heating failures P&H guys are known for huh dude