[Hauki]

I have a 25 year old Volvo with the block heater in and it goes on as soon as the temperature drops around 0°C. I even used it tonight in 4°C (just to test it was still working before winter hits really..)

There are no real laws about when to turn your block heater on, I do it when temp gets around 0, some do it in -10, some colder. It is down to you. Most people do it to look after their engines and so the heaters work quickly. I don't think it occurs to them how much fuel it saves. I do it for all the above.

Block heater is much better than oil heater, because a block heater heats both oil and water, whereas a oil heater can heat only oil. Not many people realise this. Of course it doesn't directly heat the oil, but indirectly by thermal conduction.

It does this by heating the water which then travels through all the capiliaries in the engine, all the while the water is transferring this heat to the metal block itself, all over the engine. When the engine is started the oil then flows through the block, which is hot, and the heat is then transferred to the oil by conduction. The oil is very quickly warmed up due to flowing through hot block.

As for using air heaters in front of, or underneath car, it might make 1 degree difference. My air con/inverter on the side of the house iced up last winter, I had to get out there with a 2kw hair dryer to try and heat the thing to melt the ice, it was futile. I had to hold the hairdryer almost touching the ice to make it melt, it took a loooong time. You are just wasting time heating air infront of the thing rather than heating the thing itself. You would probably benefit more from lining your bonnet/hood with foil to act as a radiant barrier to keep engine heat in the bay.