Watch this video from This Old House to learn how to fix a sweating toilet.
1. Turn off water and drain the pipes.
2. Cut a tee-fitting into a nearby hot-water water pipe.
3. Dry-assemble the various copper pipes and fittings to the anti-sweat valve.
4. Solder together the assembly, making sure the valve is positioned vertically.
5. Install the soldered valve assembly so that it connects the hot-water pipe with the cold-water line leading to the toilet.
6. Solder the valve assembly to the existing pipes.
7 .Turn the water on and check for leaks.
8. Adjust the valve to allow just enough hot water into the toilet to raise the temperature of the water in the tank.