Heating with air is less efficient in transporting heat with water. So you either need to move more air or heat it up to a higher temperature. The first one is

Heating with air is less efficient in transporting heat with water. So you either need to move more air or heat it up to a higher temperature. The first one is obvious why it’s a bad idea, the second is a bad idea as at a certain point the dust in the air is burning up leaving a bad smell. So practically you can heat a house with air heating alone only when they are extremely well insulated or in a mild climate. The passivhaus-Standard in fact is defined by heating through air being enough.