Hydronic baseboard heaters use heated water or another heated liquid to control the temperature in a room or area. Positioned along the base of a wall, these heaters draw in cool air at the base, which is heated and pushed out of the top of the heater to warm the room. Low external operating temperatures allow these heaters to be installed flush against a wall or baseboard without damaging the structure.

The technology behind hydronic heaters has been used since the 1940s and has evolved from cast iron radiators. Modern hydronic heaters are lighter and easier to install than their predecessors. In a built-in system, water is piped from a central boiler to the hydronic baseboard heaters, with a return pipe channeling cooled water back to the boiler for reuse. Heaters can be separated into zones, with thermostats for each area, allowing for better control over the temperature. This type of system is permanent, and a range of baseboard heater covers are available to coordinate with a room's decor without obstructing heat. This type of heating system is usually added when a house is being built; the plumbing involved makes it difficult to add to an already existing structure.

Electric hydronic baseboard heaters have a heating element and liquid sealed within the heater, so they do not have to be linked into the household plumbing. These heaters are wired into the household power supply, and most can be wired to a remote thermostat. Electric hydronic baseboard heaters are easier to install to an existing structure than one linked to household plumbing, but the wiring required does not allow for easy relocation of a heater.

Portable electric hydronic baseboard heaters are also available. These heaters can plug into any standard household outlet. Portable heaters are usually smaller than those wired into the household power, and heat a smaller space. Temperature is controlled by a thermostat directly on the unit.

Hydronic baseboard heaters have a number of advantages. They are usually quiet and efficient. Being closed systems, these types of heaters don't require the owner to periodically add liquid. Additionally, hydronic heating does not dry out the air, or spread dust or allergens.

Along with these advantages, however, there are some potential disadvantages. Due to their location on the baseboard, these heaters take up wall space, and should not be blocked by furniture, curtains, or other obstructions. Also, hydronic baseboard heaters heat a space relatively slowly, and unlike forced air heating systems which can be linked to an air conditioning system, hydronic baseboard heaters are necessarily separate from a cooling counterpart.