Head is the height at which water pumps can raise liquid. *Please bear in mind that most of our customers are dealing with an enclosed solar hot water system. One where the hot water panels are on their roof. In an enclosed system, that is one completely void of air, head becomes almost a moot point. Hence, as the saying goes, what goes up must come down. Water in a solar hot water heating system that is coming down off the roof creates a syphoning effect on the water going up to the roof, thus effectively offsetting the head pressure. In such cases the flow rate is normally the primary concern.