Hello,
Twin pumps should always operate with water (hot and cold) running through each end of the pump. As RPM mentioned above, there are seals in the pump - To be specific, they are mechanical shaft seals, one part is static, while the other is rotary and turns with the shaft. They are designed to self lubricate from the water, so if one end is dry, they will become damaged, leading to either a leak or seized pump. If you would like to give us a call on 01915162002, we can see which pump would be the best option for you.