Answer: View the Antminer S9's specs: "Operating Temperature: 0°C to 40°C"
https://shop.bitmain.com/antminer_s9_asic_bitcoin_miner.htm?flag=specifications
I admire your adventurousness.
But, all Antminer owners really should carefully read the specs for their equipment. There are likely very specific reasons why Bitmain specifies these specs. So, operating in any temperature below freezing is likely, to very likely, have adverse or even damaging effects to your equipment.
https://shop.bitmain.com/antminer_s9_asic_bitcoin_miner.htm?flag=specifications
I admire your adventurousness.
But, all Antminer owners really should carefully read the specs for their equipment. There are likely very specific reasons why Bitmain specifies these specs. So, operating in any temperature below freezing is likely, to very likely, have adverse or even damaging effects to your equipment.