In a closed piping system (such as a chilled or hot water system in a tall building), is the water pressure at the top less than that at the bottom? Bernoulli's equation would lead you to believe that it is, but I cannot find anything explicitly stating that this applies to closed systems as it does for open systems like plumbing fixtures or sprinklers. I have found a lot saying that in closed systems like these the pump is sized for friction losses only without mentioning a need to make sure you have enough to have some minimal pressure at the top. This leads me to believe the opposite, that the pressure at the top should be equal to that at the bottom, so I am confused. For example, say you have a pump in the basement of a tall building that circulates water to the top floor. Say the theory is true, that the pressure at the top will be less than in the basement. If you have 100 psi leaving the pump you might have only 70 at the top, then it would increase in pressure on the way back down and be 100 minus friction losses on the inlet of the pump. The pump would only need to make up those friction losses and bring it back to 100 psi. On the other hand, if you only had 30 psi in the basement you would have 0 at the top and the system wouldn't work right, so in sizing the pump you would also have to verify that the pressure at the top is greater than zero (otherwise your flow would stop, right?). I've looked through my Fluid Dynamics and HVAC textbooks, ASHRAE papers and all over the web trying to figure this seemingly simple question out and I'm completely confused why I can't find an explanation. If you know the answer please let me know what I need to do to look up to find a full explanation of the physics of the situation. Thanks.