Macbook pro 2018 with Mojave overheating when connected to external monitor

The higher power GPU in some MacBooks Pro puts out a lot more wattage than the low power GPU.

The reason it gets so hot when hooked up to a monitor, even when you’re just running Word or Safari, is because the display outputs are hardwired to the dedicated GPU - if you’re using an external monitor, the dedicated GPU (graphic card) is in use regardless of how demanding the workload is. This puts more stress on the cooling system, resulting in higher-than-average temperatures ;) Thats the reason. I tried all my 6 Macbook Pro's and all does the same behavior. Switching to the dedicated graphic card and you can't just use the integrated iris.

The 2018 15" MacBook Pros all have dual-GPUs. The AMD GPU is required to be active when an external screen is connected. There is no way to use the internal (Intel) GPU for external screens.

Fan noise is one of the main nuisances of the MacBook Pros in my opinion. When I get too frustrated, I sometimes use Turbo Boost Switcher, which helps a bit. Volta is a similar App, but does not seem to work with 2018 MacBooks. Both Apps deactivate the CPU's turbo mode, which means less speed, but also less power consumption and less noise.