Question Is it safe to subject a liquid cooler to temps over 100C?

[Barney-]

I realise this temperature is way too high for a CPU or GPU. But in this case, I have a Peltier module between the GPU and the water block, so temperatures could be well in excess of the actual GPU temperature.

 

[Karadjgne]

Depends on the block. If an aio pump, no, there's not enough liquid inside, you'd be sending it to boil temps almost instantly, and thats not good for non-pressure fittings or the chemical makeup of the coolant. Acrylic or Nylon blocks will get toasty where the cold plate meets the block, you could end up with leakage there. A full copper block would be safe, unless using a fully aluminium setup, in which case an aluminium block would be fine. Only full custom loop blocks contain the volume and flow large enough to handle extreme temps.