When I first installed my water heater I was running at around 50 degrees at the brush jets and went out one very very cold frozen February morning when I knew I shouldn’t have bothered, cracked a window!!
Ever since then I’ve turned it down so it’s lukewarm around 20-25 degrees now and I won’t work in subzero temps like that again, it’s not just cracking the windows it’s whst you leave behind freezing up afterwards I worry about