Hot-air systems supply a current of air from heating-tubes in a furnace, in much the same way as the hot water is supplied in the above systems. The air is set free into the room to be warmed, and if not too warm the effect may be agreeable. But the most pleasant, warmth is experienced by radiation, and not by actual contact with a warm atmosphere. Also air currents behave in a way that has not yet been fully explained, frequently confining themselves to a small portion of a room without spreading out so as to affect the whole.
In fact, the practical difficulties attached to hot air systems of heating have not yet been overcome.