Negative Head Twin Impeller Pumps – Up To 2.0 Bar may sound confusing so let me break it down for you. Starting with the bar, this is the pressure or boost of the shower pump, and up to 2.0 bar will be a low to medium boost. Twin impeller means you can connect a hot and a cold supply to the pump, boosting both supplies equally, great if you have generally poor pressure all round. Lastly negative head means that the shower head (or outlet) is above the cold water storage tank outlet. These will however work on a positive system.