Negative Head Twin Impeller Pumps – Over 2.0 Bar may sound a little daunting so we’ll split it into sections for you. The bar is the boost or pressure of the pump itself, more than 2.0 bar will be quite a high pressure and the higher the number the bigger the boost. Twin impeller simply allows you to boost both the hot and cold water supplies to your outlet equally and are ideal if you suffer from low pressure on both hot and cold feeds. The negative head part refers to where the cold water tank is compared with the outlet in the bathroom, a positive pump would be needed if the shower head was below the storage tank outlet and vice versa for a negative pump. Negative pumps should work fine on both positive and negative applications but we would advise always checking with a qualified installer.