Heat cost allocators are devices attached to individual radiators

Radiator

Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in automobiles, buildings, and electronics...

in buildings that measure the total heat output of the individual radiator. Heat cost allocators can be either electronic, where an electronic sensor is used to calculate the heat consumption, and evaporative, where a special, calibrated liquid in a capillary tube records the total heat output of the radiator.