My Intel NUC5i7 runs 70-80C under normal operation, 80-90C under heavy load. Intel says that is fine and it is allowed to go up to 105 no problem. I have also found the fan to be much louder than I expected.
It is important to understand the NUC cannot be considered a "desktop replacement." NUC is basically a laptop without a keyboard, display, or battery. Tasks that strain, for example. a MacBook will also strain a NUC. The dual core mobile processors in NUC are easily outperformed by comparable quad core desktop processors.
Disclaimer that I have not tried the NUC6 generation. Maybe it's a big improvement.
Hope that helps your decision.
The following video might be helpful for visualizing the NUC cooling situation. With a new case and fan, you might be able to achieve temperatures in the 70s with your NUC i7.
What type of tasks are being executed for a heavy load? It is rare that I run more than one program at a time. I use the term desktop replacement as opposed to HTPC. I'm actually replacing a laptop running i3-380M dual core 2.53 ghz with an ATI Radeon graphics card. It can run elements with no problems, is slower in lightroom and takes forever for lightroom to send to elements for edits. Also running 2nd 21inch monitor for easier viewing. That's why I was hoping that nuc i7 would work.
You say that your temp reaches 80 - 90C, does that cause any problems? When temp is high do you back down on tasks until fan slows down?
I did watch the video. I have to agree with some of the comments - if I have to rebuild the case, I might as well buy a larger desktop. It was very interesting though.
You can see more information about the Intel® NUC with the i7 processor here:
The unit can support up to 16GB of RAM DDR3L-1333/1600 1.35V SO-DIMM, it has the Intel® Core™ i7-5557U Processor, the temperature or T-junction is 105°C.
T-junction: It is the average temperature among the different digital sensors located inside the processor DIE. This is the thermal specification used for laptop processors.
In other systems you will see the T-Case temperature.
T-case: General processor temperature measured by a digital sensor located right underneath the heat spreader (out side of the DIE). This is the thermal specification used for desktop processors which is around 60°C to 75°C but usually they run at 40°C to 60°C under normal use.
This also depends on the type of software you use to check the temperature and also depends from the manufacturer what methods they install to check the temperature. If you take different system from different companies I’m pretty sure you will see different temperatures.
This information I provided above is for you to be aware of how the temperature work with this unit and for you to know that the temperature mentioned above by snowpine is completely normal.
Snowpine we have tested this unit several times and we don’t experience this fan noise, if you replace the unit I’m pretty sure that you can get a replacement for a unit that will not have this problem. This unit has three years warranty from the moment you make the purchase.
I hope this information can help.