Hi All, I'm doing a study on effects of outside temperature on the energy efficiency of air-conditioning systems in computer server farms/data centers. It will be a simulated study so I cannot use the standard definition of COP; COP = heat removed / work At the same time I also want to examine the effect of the outside temperature. There for I've been using the following derivable definition for COP; COP = 1 / (Th/Tl - 1) where Th = the temperature of the hot, outside environment and tl is the temperature of the inside. cool environment. The problem I have is that in certain places, the outside temperature will be lower than the inside temperature, giving me a negative value for COP?? Any comment on this, is a negative COP realistic given that I'm doing work to move heat from a warm environment to a cooler one? or is this incorrect? any help is appreciated.