The on/off valve for our very old baseboard convector (that is currently not heating, though input pipe is hot) will NOT budge. It is tightly closed. We've tried a vice grip even. The other on/off screws/valves throughout the house are equally tight and won't budge, but they are in the "on" position, thankfully. Doesn't appear to be corroded.

Any ideas on how to open? Thanks.

Marshall, November 2009