an attempt to balance the two temperatures (which is another of the main reasons there is life on Earth, by the way). If the colder location is horizontal or even below the heat source, that's the direction the heat will go.

So why do we say that heat rises? Because hot air and hot water rise – the medium holding the heat has its own properties and, in the case of both air and water, a hotter medium is lighter because the molecules have expanded. Consequently, the hottest water (or air) is always at the top of a storage tank – that's why heating elements are always at the bottom, where they'll do the most good. This principle, called stratification, is used to great advantage in solar thermal systems.

The amount of heat transferred between two materials (per second) increases if the two temperatures are further apart – heat is transferred quicker from an almost-boiling liquid to an almost-frozen one than from two liquids within a few degrees of each other. Thus, we need to design systems that present the biggest temperature difference (or "delta-T", ΔT) at all heat exchange points.