The distribution of heat in an electric radiator is the result of particle diffusion within the heating medium and heating medium circulation. As a result of both of these processes hot liquid particles mix with cool particles and distribute heat across the radiator.
The collector in which the heating element is installed is the hottest part of the radiator and the pipes in the lower part of the radiator usually remain cool. This is expected behaviour.
The cool pipes at the bottom are a result of the 7 cm so called dead (not heated up) zone in the heating element. The dead zone allows safe operation of a heating element installed through a T-piece in a radiator connected to a central heating installation. When the heating element is connected through a T-piece the dead zone covers the length of the T-piece, however, when the heating element is installed directly in the radiator, the zone moves upward resulting in the lower temperature of one or two lower pipes. In addition, in narrow and high radiators the upper part can also remain cooler due to the air cushion (protection against excessive pressure rise), in particular at lower temperature settings and while the radiator is warming up.