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You can open it as a temporary measure but opening it manually will usually engage the end switch in the valve head which will turn the boiler/burner on so the hat for that zone could run wild if you leave it open all the time. Replacing the head (most of the time you don't have to replace the entire valve) is the best option if the valve isn't responding to a call for heat and the thermostat and thermostat wiring for that zone is functioning properly.