I am not a pilot nor do I have any familiarity with the workings of aircraft (so please take this as pure speculation until a pilot can come in), but I believe

I am not a pilot nor do I have any familiarity with the workings of aircraft (so please take this as pure speculation until a pilot can come in), but I believe the ventilation comes through high-temperature bleed air. The bleed air is dehumidified, cooled through a heat exchanger, then fed into the cockpit. If the ECS went haywire, one failure mode be to not dehumidify the air, and also to run the heat exchanger at full blast (i.e., the "thermostat" basically broke). EDIT: according to https://www.navy.mil/local/pes/Comprehensive%20Review%20PE.pdf https://www.navy.mil/local/pes/Comprehensive%20Review%20PE.p... , ventilation is provided by mixing high temperature bleed air with cooled, dehumidified bleed air in whatever proportion is needed to keep the cockpit at the correct temperature. So assuming the cooled air is reaches -30C or so, an ECS failure that stopped mixing in any hot air would cause the event described.