Heat Exchanger water to air temperatures

I have been having a debate with a coworker regarding heat exchanger fluid temperatures and while I thought I firmly grasped the principles, I was not arguing my point very well.

I am looking at an air to water coil for an air handler. I was selecting the coil based on a leaving air temp of ~60F (it's an industrial space with a lot of ventilation). I am using condensing boilers so I sized for 180 leaving water temperature with 140 return on the coldest day which gives a 40 degree temperature delta. On the air side, I am bringing in 5 degree air (EAT) at a rate of 16000 CFM on one of the air handlers. My coworker wants to lower the temperature to 140 supply and 100 degree return, his reason being that it would still be a 40 degree temperature delta on the water side so the thermal output would be the same. I believe the hotter supply temperature will result in more rapid heat transfer but I didn't really have any good argument against his logic. He was simply relating the two rules of thumb equations for air and water: [Btu/hr]=1.08*CFM*deltaT and [Btu/hr]=500*gpm*deltaT to prove his point. My argument was that this does not account for the heat transfer taking place from one side of the coil to the other, but I was not able to adequately illustrate my side to the point that he would agree.

First of all, am I even right about operating the system at 180 degrees being a more efficient solution, and, if so, how do I fit the heat exchanger heat transfer into the system of equations?

• Are you right about what? Please start a new paragraph and ask your question. The two situations lead to different coil selections. There is enough size here that you both could probably retire off the savings from a professionally engineered and optimized system. Jan 7, 2018 at 20:06
• I clarified what I was asking about a little bit. Hopefully it is more clear now. Jan 8, 2018 at 4:58
• I am not familiar with those rules of thumb, what is the definition of delta T in those equations? Jan 8, 2018 at 20:55
• The delta T on the water side is the difference in the temperature of the water entering the heat exchanger and the temperature of the water leaving the heat exchanger. On the air side equation (1.08...) the delta T is the difference in entering air temperature and leaving air temperature. Jan 9, 2018 at 23:59
• Yes a higher water temp will be more heat transfer at the air handler but it will and also be more expensive to heat water to a higher temp. It would be wasteful to have leave a 40 degree exit delta. Exchangers are relatively cheap. Jan 10, 2018 at 15:19