I know, it’s 9 years later, but chatGPT could help me a little on this concern.
In UE5 there is an option to disable “Realtime rendering”. The option is in the 3D viewport on the upper left corner (the “burger” menu). Shortcut is Crtl+R
This kind of fixes the fans spinning up whenever the viewport is the active window. But the fans still ramp up when you playtest your project. Hope that helps anyone.