One of the computers I have is an HP Omen 870-290nz with Intel Core i7-7700K processor, which offered a rather good price/performance ratio at the moment of purchasing. Unfortunately, it turned out that the machine - that by the way cost about CHF 2000 - had absolutely insufficient CPU cooling provided by a cooler, which immediately produced an impression of being far too small and cheap for an i7-7700K CPU.
The issue could remain unnoticed in general computer use, even if the CPU package temperature easily gets to 80-85 °C under moderate load. However, it becomes obvious when the machine is used for tasks like 3D rendering or when running several heavily loading the CPU tasks simultaneously. First of all, the machine begins to sound like a vacuum cleaner, while cooling is still insufficient. CPU temperature gets to 95 °C, thermal throttling begins, even then the temperature can rise up to 100 °C, in which moment the machine pips, signalling overheating and probably attempting to shut down with no success. At least, by the next boot it shows "A thermal shutdown has occurred" BIOS message.
I decided to replace the original cooler, roughly measured the space available for it, and then decided to use the Noctua NH-U9S, which height is 12.5 cm. Indeed, after installing the cooler it was obvious that this is very close to the maximum size of a cooler which still fits into the chassis.
To install the new cooler it was necessary to remove the graphic card, including its holder, and the chassis fan, and to unscrew the motherboard. After that it was possible to turn the motherboard into a position allowing access to the backplate of the CPU socket without further disassembling and/or disconnecting any cables. It turned out that the holes in the backplate were too small for allowing the rods of the additional backplate used to mount the cooler to get through them, and the original backplate had to be dismounted to enlarge its holes. For dismounting it one has to unscrew two screws having somewhat exotic heads. To accomplish this task, I used a Torx (6-point star-shaped) screwdriver with a diameter of 4mm. I am not sure if it was supposed to be used with those screws, but it fit enough to be able to unscrew and subsequently screw them on. After dismounting the original backplate, I enlarged the holes in it using a drill with a 4mm bore, mounted the original backplate back, put Noctua's additional backplate over it and screwed the motherboard back on. The rest of the operation is hardly worth describing, one should probably mention that Noctua's cooler came with thermal paste and there was no need to order the latter separately.
Now I am quite happy with the machine: with moderate load its CPU temperature is now usually below 60°C, while the cooler is practically silent. Under maximum load during 3D rendering, etc, the CPU temperature is usually 85-90°C, reaching 95°C in peaks, while the fan is barely audible. Temperature of 90-95 °C is probably still too high, but Intel Extreme Tuning Utility now reports no thermal throttling. The temperature does not get higher and the machine does not try to shutdown because CPU temperature reaches 100°C.
Resume: HP Omen 870-290nz computer comes with a perfectly insufficient cheap CPU cooler, which makes it practically impossible to use the machine under really heavy load. Moreover, since the machine appears to be unable to perform supposed shutdown upon detecting overheating of the CPU, the latter can even die while executing such tasks. The issue can be fixed by replacing the cooler with a decent one, Noctua NH-U9S appeared to be sufficient. It will hardly be possible to close the case with a larger cooler: with 12.5 cm of Noctua NH-U9S there is hardly more than 5-10 mm of "unused" space. To mount a Noctua NH-U9S cooler into HP Omen 870-290nz, one will have to remove the graphic card and the chassis fan. One will also need a mentioned above Torx screwdriver to dismount the original CPU socket backplate and a drill with a 4 mm bore to enlarge the holes in the latter.
Other questionable "features" I noticed with this machine
Updating BIOS of the machine
HP has a download page for this model, offering two BIOS updates, for ROM Family SSID 2B4B and ROM Family SSID 828A, however, none of them is compatible with my machine, which has ROM Family SSID 830C. Fortunately, Google search found a HTML file at ftp://ftp.hp.com/pub/softpaq/sp86501-87000/sp86731.html and the guess that mentioned there SP86731.exe executable should reside in the same FTP directory was correct and the update was done using that file.