There are two ways for backflow to occur: backsiphonage and backpressure. Backsiphonage happens when there is a negative pressure in a pipe, causing a vacuum, and water flows in the wrong direction. The vacuum can be caused by high water demand in another part of the system such as from firefighting or a water main break. Backpressure occurs when the pressure at one part of the system is greater than the pressure coming in from the water main. This can happen when a garden hose is connected to a faulty pressure washer, from an overheated water heater or boiler, or from the weight of water on a hill or tall building.