The main reason why convection heating systems are not the best way to go is the simple fact that heat rises. Notice how it is always cooler in the basement than it is upstairs? That is a perfect example. The following graphic, courtesy of Total Insulations, shows what happens to heat.
Though convection systems (typical heating that blows air through vents) do provide heat, most of it floats to the top or gets lost in cracks of from windows and doors.
So why is it that hot air always rises above cold air? Simply stated, cold air is heavier.
When something is hot, it has a lot of stored energy. Molecules and atoms move quicker as the temperature rises; the faster they move, the more space they take up making their density less than a cooler object. Becaue hot air takes up more space than cold air (the cooler the air, the more density it has), lighter and less dense hot air will rise above the heavier and denser cold air.
The hot air vs. cold air concept is the problem with convection heating. General space heaters fall into this category. They do exactly what their name suggests: heat a space. When a space heater is on, it blows the warm air through a vent into an open space. The goal of the space heater is to warm the air in hopes that the objects (including people) will catch the heat also. Since heat rises, many people end up uncomfortable in the convection-heated environment, rendering the space heater practically useless. Convection heat is essentially a lot of wasted hot air.
