What is the difference between positive head and universal (negative head) shower pumps?
Shower pumps for a gravity fed system are either positive head or universal (negative head). When choosing the pump you need it is important to know if you need a positive head or universal (negative head) pump as incorrect selection can mean the pump won’t work.
A positive head pump is designed to assist and improve the natural flow of water. A flow switch set to activate positive head pumps when a flow rate of 2 litres per minute (L/min) passes naturally through the pump. Therefore, a minimum flow rate of 2L/min must be achieved naturally (without a pump) from all outlets in order to install a positive head pump.
A universal (negative head) pump works in a different way to a positive head pump. These pumps are capable of creating flow where there ordinarily isn’t a natural flow, such as loft conversion with outlets higher than the cold water tank or where ‘up and over’ pipework has been installed. Other considerations where a negative head pump may be needed include restrictive outlets such as a shower with body jets or an electric shower, or anywhere where mains fed cold water meets gravity fed hot water.
These pumps will pressurise the pipework between the pump and the outlet, which, when the tap or shower is turned on, releases pressure which a sensor in the pump sees and activates the pump. Once the outlet is turned off, the pump runs on for a short time and re-pressurises the pipework ready for the next time. A negative head pump usually has a pressure vessel, which acts as a buffer to stop false activation.
If you are unable to achieve the natural minimum flow rate from all outlets that are to be pumped for a positive head pump, you will need to install a universal (negative head) pump. A negative head pump activates when it detects a drop in pressure