I am having problems with one zone of a two-zone heating system. The thermostat does not function; previously this happened intermittently, and turning on the valve manually while the thermostat was set high enough started the furnace. Now, turning on the valve does not help either; the only way to get heat into this zone is to open the valve manually and turn up the thermostat in the other zone so that the furnace starts.

Should the valve opening turn on the furnace independently of the thermostat - i.e. is this clearly a valve or valve wiring problem? Or could it be a thermostat problem?