How sad. If he was healthy for those 9.5 years I find it unlikely that he had internal issues that caused the death in addition to the cold temps. Yes, mid 50's is cold enough to kill a ball python. Especially since you don't know how long he was exposed to those temps. If the heating elements blew before he was able to digest that last meal then that would have just made things worse. Also, if you didn't have the two heating elements hooked up to a thermostat, then lowering the house temp would have lowered the temperature of the hot side of the enclosure by the same number of degrees. The heating elements alone, w/out a thermostat, just raise the temperature by a certain number of degrees from the base temp of the room. So any changes in the base temp of the room will affect the temperature on the hot side. Plus lowering the temp to 50 gave him a "cool side" of 50, and that alone isn't good for a BP to have to endure for 2 weeks. This is one of the reasons Ball Pythons are in no danger of becoming an invasive species. Even a brief encounter with a Cold Snap can kill them. Again, sorry for your loss, I really am, but please do learn from this. If you get a new Ball Python and take any future vacations, best to leave your pet in the care of a friend (at their house) rather than leave him unattended in a cold house. Not to mention, the thought of leaving a ceramic heater unattended for 2 weeks is scary. You are lucky you only lost your snake, and not your whole house...
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Slytherin Serpents
Have you been hugged by your snake today? 
Ball Pythons
1.1 Het Caramels (Edward & Bella)
0.1 Normal 66% ph Caramel (Rosalie)
1.1 Caramels (Crookshanks & Bellatrix)
1.0 Honey Bee (Aragog)
0.1 Hypo Pastel (Nagini)
0.1 Spider 66% ph Hypo(Nymphadora)
1.0 Mojave (Lucius)
1.1 Pastel Lessers (Jacob & Narcissa)
0.1 Pinstripe (Arwen)
1.0 Pewter (Salazar)
0.1 Bumble Bee (Alice)
1.0 Bell Pastel (Jasper)
0.1 African Dinker (Esme)
0.1 Het Pied
0.1 Hypo Cinny (Myrtle)
1.0 Cinny het Hypo poss het Albino
1.1 Pastel Dinkers (Snape & Xiomara)
0.2 Ghana Dinker Babies
0.0.3 CH Dinker Babies
Corns
1.1 Normals het Amel, Anery, Charcoal, Diffused, Hypo (Romeo & Juliet)
1.0 Blizzard het Diffused (Tumnus)
1.0 Blizzard poss het Anery, Hypo, Diffused (Cotton)
0.1 Anery Lavender het Amel (Lucy)
0.1 Plasma poss het Hypo (Victoria)
1.0 Plasma poss het Albino
1.0 Hypo Plasma
1.1 Amel Motleys (Harry & Ginny)
0.1 Lavender Motley poss het diffused
1.0 Lavender Pinstripe Motley het Amel
0.1 Fire poss het Anery, Charcoal, Hypo (LeeLoo)
0.1 Normal het Albino, Lavender, Anery, Charcoal, poss het Diffused
1.0 Albino het Lavender, Anery, Charcoal, poss het Diffused
1.1 Sunkissed poss het Plasma (Arthur & Molly)
0.1 Dilute Blue Motley (Myrtle)
1.0 Pastel Motley (Nick)
0.1 Diffused Caramel
1.0 Blizzard 66% poss het Diffused
1.0 Pewter het Albino
0.1 Fire het Charcoal
Thayeri Kings
0.1 Leonis phase (Sidney)
1.0 Milk Snake phase
Milks
1.1 Tangerine Albino Hondurans
0.1 Hypo Tangerine Albino Honduran