Hi Pyro,
Our N. bicalcarata (a number of different clones), merrilianas, sumatrana, northianas, amps, raffs, echinostomas have all been subjected to low fifties. We had a heater flame out in February and the greenhouse was down to 48. No cold burn. Must admit, however, that these kinds of low temps (50-52 degrees F) are not consistent every night, but may run for two consecutive nights with the following nights being warmer, in the upper fifties. Like Dustin said, not recommended, but tolerated. Also, because they are in a greenhouse, there is no air movement, which helps. Most importantly, the days will see highs ranging from 86 to 95 F., with 80 percent humidity. As long as they get hot, muggy days, they are fairly tolerant of cool nights.