A steam valve is a globe valve with a fiber disc washer and it closes by turning the handle until it stops. It turns off the steam completely or you would have condensation problems in the radiator. A hot water valve turns off by rotating a cylinder to close the opening. The cylinder has a small hole in it to allow a minimal amount of water to flow through the radiator. A steam valve can be used for hot water, but a hot water valve cannot be used for steam.