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Radiators aren't meant to heat buildings via radiation, they're meant to heat buildings via convection.
If the radiator were painted a dark color, the radiant heat would make you uncomfortably warm on the side facing the radiator, while leaving you cool on the side away from the radiator. By painting them a light color, they minimize the heat radiation.
By minimizing the heat radiation, the heat convection is relatively maximized. The radiator heats the air in its vicinity; that air circulates around the room. |
