Towel Warmers can be hooked up to an existing hot water line, but not to a domestic hot water line. Hooking up the towel warmer to an existing domestic water line would compromise your drinking water and will likely violate local plumbing code.
With tons of stylish models made Amba and Runtal, you have lots of options to choose from. Just be sure to properly connect your towel warmers to your system and they'll be toasty in no time!
