Hydronic models operate in conjunction with a closed loop forced-hot-water heating system and typically operate only during the period that the central heating system is operational. The hydronic towel radiator is heated by circulating warm water from a boiler, which typically operates from home heating oil, natural gas, or lpg propane depending on the boiler. Towel radiators are not made to operate off of a domestic hot water system. The one exception to this rule is the Runtal Fain which is made entirely of stainless steel and may be connected to the domestic hot water system.
Hydronic towel radiators are most often used as the main source of heat for a room (particularly bathrooms, kitchens, entryways and laundry rooms), and proper sizing is a priority. A good "rule of thumb" value for a room is 40+/- BTUH per square foot of floor area, assuming a single outside wall with one window, and a standard ceiling height. For a more accurate heat loss value, consult with a qualified heating specialist.