I have a Q-Mark Model HBB-754 hydronic heater in a bathroom which was controlled by a Honeywell 7 Day Programmable TH-106 line thermostat. I thought the thermostat had failed in the on mode as heat could not be controlled. I replaced the thermostat with a used Honeywell 5-2 Day Programmable Thermostat RLV4305A and the problem was still there. Since the replacement was untested I tried another used one and the problem was still there. At that point I sat down with a cup of coffee and pondered the situation. After a while I removed a wire nut from the thermostat and restored power intending to measure voltage. The heater was still heating with the thermostat out of the circuit. At that point I decided that somehow the grounding wire had become a conductor (I don't have GFCI). I disconnected the heater and performed continuity checks on it finding no continuity line to line but one line showed continuity to ground. My conclusion is that the heating element failed somewhere near its midpoint (as heater got quite hot) and contacted the tube containing the element and heat transfer liquid so as to convert a 240v heater to a capable 120v heater via the un-switched line and the grounding wire. Things are not always what they seem. A replacement heater is on order.