Likely cause of heater being stuck on?

Question

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.

Answer 1

Note the little lever at the bottom of the picture from amazon.

It has auto, open and manual open positions.

You can push the lever and lock it to manual open but generally the lever should be in the auto position if no heat is being called for or the open position if heat is being called for.

When the zv's fail and the thermo is not calling for heat, the lever will be loose. If properly functioning when heat is not being called for the lever will have resistance but you can push it to the open position (just as if the zone valve motor was pushing the lever to that position).

When you go to replace these things older valves required you to shut off the water. Newer ones allow you to remove the screws of the zv motor assembly and replace the motor assembly with out shutting off the water. If you have to replace it, it is generally just the zone valve motor but it could also be the valve that the zone valve is turning as well or both.

Answer 2

It's either the zone valve, or the wiring between the thermostat location and the zone valve.

Unfortunately, it appears that the installer has a different interpretation of "in a neat and workmanlike manner" than I do. That is a rather ugly mess of wires.

That part (the zone valve actuator) has been discontinued. The replacement is claimed to be 40003916-026 by one supplier I checked. Position is supposed to be normally closed. But you'd want to verify whether the trouble is with the actuator or the actual valve, so you'll know whether you need just a new actuator, or a new valve and actuator. The latter is not a lot more expensive than the former.

Before you go for a replacement, though, you need to check that the problem persists when the wires are removed from the valve, to eliminate the possibility that the thermostat wires have been shorted, rather than that the valve is defective. Note well the warning on the cover. This may have both 24V and 120V wiring, from a quick look.