Our house has baseboard heat that is split in to about 10 zones (one per room). A few days ago, one room (the master bedroom) has been stuck on, making the room much too hot. The wall thermostat shows the correct room temperature and the set point temperature (much lower than the current temp) and when it was turned off, and even when it was removed from the wall altogether, the heater kept going.
What is the likely cause of the failure? Is it somehow in the thermostat (which seems unlikely, since it kept going when I removed it)? Or is it in the switch in the basement (which seems to be a Honeywell V8043E1012). FYI, in the picture, I've turned the valve off.