I kind of chuckle whenever I hear the flammable vapor / open flame worry ....with any flammable vapor ignition risk is present whenever a switch is activated , every time an electric motor starts ,every time things are plugged in or unplugged , grinding steel , welding , the list is nearly endless ..I used to do body & paint in my garage , heated with a woodburner (Locke 1911 upright stove) long way from an airtight. The air/ fuel mix would have to be perfect to cause a problem , with the less than airtight construction of my garage , I never worried too much , but I did have outside combustion air to the stove & later to the natural gas furnace....my furnace is a 1990 $10 govt housing takeout , its 96% efficent & heats a 26x32 shop with 9' ceiling , 3-'9x8 overhead doors , 2- 36x 80 entry doors & a 20' x 7'' lean to greenhouse ( furnace is in greenhouse) I have no practical way of knowing what I spend on heat out there , but its not much