I've flashed both my routers to OpenWRT 18.06.1 yesterday and have noticed a big difference in operating temperatures. I had both devices running on stock firmware until now and never observed such a difference.
After flashing the images, I've immediately noticed the Zyxel NGB6817 beeing uncomfortably warm; the hotspot is around the SoC area (right half, heatsink on it). While the R7800 operates between 45 - 50°C and feels just a bit warm, the Zyxel is at around a constant 70-75°C in idle. Under light load, I've seen the Zyxel climb up to 80°C. I've used software readings and can confirm, there is a very clear difference between the R7800 vs NBG6817, as I have both devices sitting next to each other.
Looking at the picture at NBG6817 ToH page (https://openwrt.org/_detail/media/zyxel/nbg6817_serial.jpg?id=toh%3Azyxel%3Anbg6817), the heatsink doesn't look promising. Still, I wouldn't expect >70°C while the device is sitting there, clocked down to 384MHz (ondemand governor), doing nothing. I wonder, if the Zyxel is running at a higher SoC voltage, than the R7800.
Unfortunately, lm-sensors / lm-sensors-detect failed due to "no i2c interface found" message. Is there any other way to read SoC voltage?