If your house has a basement or any rooms below grade, the floor is probably graded to collect any water seepage into a sump ~ a pit in the floor. An electric sump pump drains the water out, activated by a float that triggers the pump when the water level reaches a certain level.
Some houses collect sewage in a small underground tank by the side of the house. There’s a “grinder pump” that turns on when the sump is full, pumping the effluent into pressurized sewer lines that transport the waste to a central sewage plant.
In commercial buildings, a sump pit collects sewage, effluent, or water drained off from floor drains. Displacement-type floats on a fixed linkage turn the pump on and off. As the water level increases, the weight of the porcelain float is reduced by the weight of the fluid displaced. This decreases the weight on a spring. When both floats are underwater, the spring expands to activate the switch, turning on the pump. As the level decreases below the upper floats, the switch remains on, and the pump continues to operate until the level drops below the bottom displacer. The full weight of both floats then expands the spring sufficiently to deactivate the switch. Some systems have a third float that triggers a visual alarm to indicate fluid levels above the normal maximum level.
The size of the sump and the spacing of floats are calculated to minimize the pump cycles. A small sump, or one that fills up rapidly, will cause the pump to operate frequently, for short periods. This causes energy conservation projects excessive wear on the pump and uses more energy— because of the high inrush current needed to start the motor—than a larger pump running less frequently. Some systems come pre-designed for the application, with the sump basin, pump, and floats- matched to the usage; others are custom-designed. The latter requires more careful attention to each component.
Pre-designed systems are always the best bet for designers not well versed in the dynamics of the system. Either way, the system should be adjusted to minimize the pump run time.
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Excerpt from Energy Conservation Projects.