Airflow over passive radiators

[akis]

I have been using a free standing, electrical heater to heat my room (those on wheels), and as it got a lot colder recently, I turned on another passive electric unit mounted on the wall. As I leaned over to adjust the settings, I was amazed how much hot air was rising from the wall mounted unit, whereas the free standing one never produces any air flow to speak of. The wall unit runs much hotter, you would burn yourself if you could touch it.

I have also discovered that the addition of a fan behind the free standing radiator warms the room much quicker than without the fan (it is a fan I use in the summer).

I also have noticed that a heat pump company (Daikin) now produce a radiator (convector) with an added fan which "works better at lower temperatures".
 
Based on these observations, would I be correct to assume that between two equal size and shape radiators, without fans, one say heating to 40C and the other to 70C, the one at 70C will produce the larger airflow (more air circulated over time) ?

[Berni]

Yeah you need a tall column of hot air in order to create a convection updraft.

I am guessing the wall mounted unit is taller and has less surface area, so it can create a taller and hotter column of hot air.

When it comes to electric heaters, they will always put the rated amount of heat power into the room. Increasing airflow just makes the heater run cooler at the same power because the heat is moving into the room quicker. The actual issue with heating a room is that hot air rises up to the ceiling, while people walk around on the floor where the cold air is, so the hot air doesn't do much good if its it above your head. This is why mixing the air helps with making the room feel warmer. Sticking a fan into a electric heater helps that process along, and it has useful feature that you can choose where the hot air is blown, so that most of the heat goes towards you, rather than just into the room.

When it comes to feeling warm while saving power, the best thing i found is a heated floor mat. They only take about 50W, but because your feet are directly on it, it feels really warm. So it is like a highly selective heated floor where you can heat just the spot where you are.

[akis]

I am guessing the wall mounted unit is taller and has less surface area, so it can create a taller and hotter column of hot air.

No they are the same size more or less. The only difference is that the wall unit is burning hot whereas the free standing unit never gets so hot (I presume to stop you scolding yourself)

I have edited my post to make it more clear.

[Siwastaja]

Yes, convection needs a large temperature difference. I don't know what the exponent exactly is, but closer to 2 than 1. Same applies to design of heatsinking in electronics; if you have high ambient temperature or need low component temperature, so cannot afford large dT, you have to use a fan because a heatsink dimensioned for natural convection would be just massive. Similarly, natural convection even from small radiators is fine when you can afford high temperature (e.g., resistive heating, burning wood/gas/oil in boiler), but heatpumps where COP plummets at large dT you need much (more than linearly) larger radiators, or preferably, fan coil units.