Our house is heated with hot water. 3 zones. 2 downstairs 1 upstairs. Each with own thermostat. Haven't used the upstairs zone in years since we are happy to sleep in the cold. We now have a guest and I decided to give her some upstairs heat. Zone valve won't turn. I assume for sitting so long. I can turn the others by hand. There's a little thumb wheel sticking out of each side of the zone valve for turning it. They are White Rodgers valves. Any suggestions as to how to free it up without having to shut down the whole boiler to remove the valve?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13