The problem with the unit was that the coil had a little higher than design pressure drop at 20-25% low of design . We first thought the coil wasn't protected properly during construction and was impacted but that wasn't the case . The coil had more fins per inch than was specified . We told the engineer the coil was the problem but he didn't buy it , hence the crazy field visit by him and the soda can debacle . I am always totally against testing things in a way that is not consistent with the intended conditions . This includes holding the fan door open , testing with dirty filters , testing with slipping belts , etc . I am also not a fan ( no pun intended ) of just traversing a unit or testing the flow on a pump and not obtaining all data on that piece of equipment . I had a guy tell me all he wanted me to do was traverse the unit as he believes he is low and he just wanted to know how much low . If he already believes he is low why wouldn't he want to know what to do next ? Without all the data you cant . Cheers .