I recently had to drain my second floor radiators because I needed to remove the Master Bedroom radiator. Now a plummer had showed me in the past how to drain my pipes, but never did show me how to refill them.

How do I refill my radiators with water?

Shovelbum

Most radiant heating systems will have an automatic water make up near the boiler, this will replace any water lost but you may have air trapped in the high parts of the system.
This air can be removed by a preinstalled automatic air relief or simply by cracking the union (pipe connection) at the radiator and allowing the air to escape.The air will go to the highest part of the system. Hope this is helpful. Mike