Definition - What does Negative Head mean?
Negative head is a condition of negative pressure or vacuum which occurs when the pump outlet is at the same level or above the level of the cold water storage tank. In a negative head situation, the pressure from gravity is insufficient to provide flow at the outlet and the flow speed at the outlet is usually less than 1 liter/minute.
Corrosionpedia explains Negative Head
In a negative head situation, there is poor natural flow of water. This may occur in shower heads situated above the height of the base of the cold water tank, or when the height between the base of the cold water tank and the shower head is limited.
Negative head may be experienced either when the distance between the outlet and the water storage is equal or less than 600mm, when there is poor natural flow, or when the combined hot and cold water flow is less than 2 liters per minute. In these cases a negative head pump may be required, this is usually a pressure-sensitive pump activated by a drop in the pressure between the pump and the device.
Some gravity is required to activate conventional pumps. However, the available gravity in a negative head system is insufficient to start the system and an external switching mechanism such as a negative head kit is used to initiate the pump startup.
Negative head may lead to air binding in pipes, pumps and water filters due to the excess oxygen in the water. The negative head situation posses a bigger problem when filtering cold water due to the high solubility of gases in water at low temperatures, well-aerated ground water or surface water.
The accumulation of air from the air binding process reduces the effective cross-sectional area, and hence the flow rate of the water through the pipe, and may lead to higher pumping costs or clogging of the pipes. In addition, the high concentrations of trapped oxygen mix with water and lead to corroding of pipes' interior surfaces.