My Rube Goldberg setup in action.
Tl;Dr - I need a larger cooler.
Icy things go in the cooler, along with a pump, and water.
Up top, a thermostat sensor thingy is in a stainless steel jacket, immersed in the beer.
When the magical black box decides the beer is too warm, it turns on the pump below, and chilled water courses through a stainless steel coil, also immersed in the beer.
It works great if the icy things never warm up, thus defying thermodynamics.
But nature bats last. In this case waking up this morning to find the beer at 73.8 degrees, not 67.0
This in a 70 degree room.
What happened: the icy thingies absorbed all of the heat they could. In a perfect world, that would be at 67-68 degrees, imperfect insulation notwithstanding.
In my world, the magic box sensed the beer temp drifting up, and as it's supposed to do, turned on the pump.
The pump delivered 68 degree water, then 69 degree water, then...
The imperfect insulation of the pump motor added heat to the formerly chilled water, which added heat to the beer. A positive feedback loop.
My beer chiller turned into a beer heater.
Fortunately the yeasties didn't seem to mind too much, as they are merrily devouring the sugars. That episode may mean off flavors, but I won't notice.
Still, the beer temp is supposed to drop to 65 in 5 more days, so a larger cooler is in order. That or a $500 glycol chiller.