h80i group questions

I had a h80i installed as part of a prebuilt system a couple of years ago - i updated Corsair Link and since doing so when running quiet mode the fans seem to fluctuate a lot even when not doing much.

On checking the group the two H80i fans were assigned to it said they were assigned to CPU Package - I've changed it to the H80i coolant - as I assume the fans should be regulating that temperature rather than anything CPU related?

Yes, that is correct. The fans help remove heat from the coolant and they can only directly affect that value. The CPU temperature is secondarily affected and the coolant temperature will serve as the base or lowest possible CPU temp with zero voltage. Take two identical machines doing the same thing and the one with a +5C coolant temp will have average CPU temps 5C warmer.

Coolant temp is also slow to change up and down, so that should take the annoying fluctuations out. The only catch is the entire system is affected by room and case temperature. You may start to see higher fan speeds in Summer when everything is upshifted +5-10C. That doesn't necessarily mean you can remove more heat at those faster speeds, so usually some Summer tinkering with the curve is required. You can do simple things like a direct shift up 5 or 10C to match your room temp change or figure out your normal peak coolant value, set the fans to a comfortable speed at that point, then a steep incline on the other side for an auditory warning if things are outside normal bounds.