Best way to quickly cool down the CHS tank's contents?

Let's say one wants to change the coolant on a 2nd generation which was driven the night before. The majority of the coolant is at (or maybe below in the A.M.) ambient, but the coolant in the CHS tank is at 180F. Let's say avoiding burns is a primary goal, so the temperature of the fluid coming out the valve under the tank should be at all times below 120F. One could wait several days for the tank to cool off, or one could turn the car on briefly one or more times to force it to pump the tank out so that it could cool in a uninsulated location.

What would be the quickest way to do this to meet the "always less than 120F" goal?

If the car is turned on (READY, ICE starts) once it will pump the hot coolant out and replace (mostly) / mix (some) with the cooler fluid. I have no idea how fast the tank fills or empties. The "filling the tank noise" after the car shuts off lasts, what, 20 seconds? That suggests that it probably empties in that time or less, since the mechanical pump will be active once the ICE starts. The whole system holds 9 qts and the CHS tank holds 1.1 qt. Let's say that the cabin heat is set to MAX and the fan is on, so that the heater core fluid mixes too. This could turn on the electric heating elements (counter productive) but they aren't going to do much in 20 seconds. So ignoring their heat contribution, and that of the motor, and assuming that everything outside the tank was at 70F, the final temperature is (7.9*70 + 1.1*180)/9 = 83.4F. If the heat is left off, and the heater core holds 3 quarts, instead we get (4.9*70 + 1.1*180)/6 = 90F. So either way it is safe. Worst case the car pushes 1.1 quarts into the tank, and it does not mix, and then closes the valve, also the heat is off. So now there is 1.1 qt of cool coolant that can't mix with the hot coolant. Even so: (3.8*70 + 1.1*180)/4.9 = 94F. Still good.

Is 20 seconds enough though?

That will move the fluid around, but it may not be long enough to mix it very well. There could be a big slug of 180F liquid sitting in parts of the system. If at this point the drain is opened that fluid won't have time to come to equilibrium and might not mix all that much in the center on the way out. (If the cooling system was one big hose, for instance, it would mix very little. I have done that particular experiment many times with garden hoses which have sat in the sun all day.) So it could still be very hot when it goes out the drain, failing the <120F condition.

 

Well, I do have thick neoprene gloves that would be relatively safe to open the valve. But what I don't have is a lift and a big funnel on a tube that goes into a large safe container that can be rolled right under the outlet. Instead the car is up on jack stands in the drive way and after it drains there would be a big plastic tub full of very hot fluid. Hmm, on the other hand, in the car sitting overnight scenario, and all 6 qts of fluid fit into one container, the final mixed temperature wouldn't be any hotter than the calculations above. It would only be dangerous if the outlet valve fell apart or squirted past the side of the tub, or something of that sort.

Still, probably safest to just run the car for 20s (or whatever it takes to circulate the hot fluid out of the tank) and then open the valve wearing nitrile gloves, which provide a lot more dexterity than the clunky big neoprene ones.