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.
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Try fixfault.com to fix convector heaters.