A drain valve on my hydronic heating boiler is completely stuck. The valve does not have a bonnet, the stem seats in a valve body. A gland nut covered some kind of a packing which was like rubber and crumbled easy. The handle end of the stem is broken off. The valve leaks slowly, only a bit faster than evaporation. When I discovered the leak the valve was encrusted with calcium deposit end rust already eaten through the metal wall. I tried to use big wise grip and 12” pipe wrench on the stem without any success. I suspect that even when I unscrew the stem it will be unusable. One reason is broken off handle. To replace the whole valve I’m afraid to do something to thread in cast iron body of the boiler. The valve is installed using some kind of plumber putty that is now hard as a rock. Any advice how to approach this problem? Another question would be how to drain water from the boiler when the drain valve is unoperative?