A positive head shower pump is used where water would normally flow from the shower head without assistance from a gravity fed system. The pump senses the flow of water and automatically starts when the shower is used, to augment the flow. In some situations the water tank is below the level of the shower outlet. When the shower is turned on no water would therefore flow because there is no gravitational incentive. This is where the negative pressure pump is required; using diaphragms an air pressure difference is sensed when the tap is opened and the pump is switched on. Because of the extra components required, negative pressure pumps are more expensive. Hot Water & Central Heating do not normally recommend the fitment of a shower pump unless there are no alternatives, because they tend to be noisy and vibrate excessively.