Hmmm. Several things about this discussion bother me.
The quoted hot water temperatures are high. Yep, one can achieve these easily enough in a pressurized system. I've worked with such in the past. But where is THIS system installed, and why are they using a pressurized system? Pressurized systems, of adequate pressure to allow heating water temps as described, are expensive and generally only used when situation dictates it's use.
Next, coil manufacturer's claim that coil only tested and suitable for the ONE given supply and return temp, and may fail under any other condition is at best bogus and silly. How in the world do you do a startup and shut down if that's the case? Did they ship it to the site in a sealed and enviormentally controlled container to maintain temps within specs? That's ridiculous. Normal such a rating as they are arguing would be the conditions under which they can guarantee a specific performance of heat transfer. And is otherwise meaningless. It certainly can not reasonably mean that the coil will physically fail under any other circumstances whatsoever.
What are the coil manufacturer's instructions concerning warm up, cool down rates and times? In high performance, high energy systems, where thermal shock is a concern and issue, it is normal for equipment maker to provide specific operating info, stating something like "Do not allow equipment to warm up (or cool down)faster than 20 degrees per minute." Something to that effect. One sees these types of instructions commonly in high energy systems which damage due to overfast heating or cooling of system is expected to cause damage or shorten expected and design operational lifetime of equipment. Did coil manufacturer provide such instructions, if not, then they don't have much of a leg to stand on in their claims that the too low return temp damaged their coil. If they truly designed coil to such close (and flimsy) tolerances, they should've been clear about this. Actually, they should get into another line of business.
Now, that return temp you got is a problem. Shouldn't have caused the failure. One would expect that upon startup or shut down one might well see such a difference between supply and return. But that return temp shouldn't be "normal". That is, shouldn't occur normally during operation of properly designed and controlled equipment. I tend to agree with another poster that coil might be oversized. But, really, you haven't given much info for us to really know.
In my experience, which isn't all that great IMHO, there are questions here. Normally I'd expect that a coil that needs to have a specific supply and return water temp within some faily close tolerances, would achieve this by varying GPM, with control one control point looking at HW supply temp, keeping it steady. And another, the one for the coil, watching return temp and modulating to maintain that, as versus watching air temp. Another control of some sort, bypass damper or whatever, taking care of maintaining air temp parameters.
This is an interesting discussion. But raises a lot of questions in my mind.
In any event, pin hole leak as you describe sounds more like a manufacturing defect to me that anything else. ie Welder or brazer got a bit carried away with heat attaching strap, dug too deep into material of pipe. Thermal expansion and contraction, not unexpected ... quite normal, as one would get this in start up and shut down conditions even if everything else was working right, broke free strap and pulled away too thin spot on pipe.
Just my opinion.
In any event, that big of a deltaT shouldn't be during normal operations. Coil likely too big. Or maybe control valve on inlet, and inlet on top of coil? I've seen that screwup before.
