These items work much like the convector radiators and heaters but instead of an open cavity in which air can flow through the heating element it is a sealed space filled with oil. This is heated by the element and then the warmth is convected through the metal casing to the surrounding air, the grilled exterior allows the maximum volume of air to come in contact with the metal and heat up, rising through the grills creating a draw. The one benefit of the Oil filled radiators is that the also supply a small amount of Radiant heat meaning if you stood at the side of it you would feel the warm more as you skin can sense infra red heat.
Most people presume you are able to get more heat from an oil filled radiator than a simple convected one, which is incorrect; you are able to get the same out puts from either dependant upon the size of the heating element within the model.
There are a couple of issues which are commonly mistaken, firstly the Oil within the heater has a high Specific Heat Capacity (SHC) this means that it is able to absorb a lot of heat before it starts to give heat back out into the room (much like water – the sea having a similar temperature in winter as summer because of the sea absorbing heat through summer and releasing it in winter) Because of this there is a small delay after it is turned on before any heat can be felt, this delay is relative to the volume of oil, size of heating element and pressure of the oils within. Although this delay on such a small volume is never more than a few minutes this can be a problem if the heater has been purchased for speed of heat.