I got a PM from the OP, but I'm gonna throw this out there anyway and another option.
I live in the City (I'm thinking I'm the only one here that does) and my shop is a detached garage that faces the ally. I still use as a garage most of the time since I don't have a driveway at my house. The house was built in 1929 and is a great house. The garage was in bad shape though. So in 2001 we tore it down and built a new one 576 SqFt with a nice wide 18' X 8' door and offset it so I could build the shop on the east wall. I partially insulated the garage (east wall, insulated garage door plus part of the south wall and part of the north wall.) Now the summer months I don't mind, but I hate the cold. So... How to heat this space?
I checked the building code and made a huge discovery. The St. Louis Fire Department made it a building code that detached buildings on the property (not the main dwelling) cannot be heated with any type of standing flame unit. Sealed or not... Vented or not... Not allowed. I think the NG Company as well will not allow you to extend a gas line past 10' of the main dwelling. So... Long story short... Using Gas to heat the shop was pretty much out. Also learned that any electric heater that runs on 110VAC eats 1500watts and produces 5100BTUs. Not nearly enough for a 576SqFt shop with open rafters.
So I found and installed this...
The brand is QMark, the model number is MUH102 and it's a 220VAC electric heater that puts out 31,500BTU for 10Kw/Hr of power. It runs like a furnace where the fan runs til all the generated heat is blowned to the space being heated.
The plus of this unit is there's no flame to contend with so it meets building code. It also makes finishing a piece worry free since I can leave the unit set at 60* overnight for the finish to cure and not worry about what happens with the flame. There is also no flu to deal with and no moisture generated from this unit. Gas units can generated moisture when they fire up.
The minus of this is that you'll need 8Ga or 6Ga wire to wire it up (I used 6Ga on a 220VAC 40A circuit), and depending on you electric rates, can be a bit pricey to run. I'm lucky where I am cause electric & gas cost about the same. I also keep the unit at 50* when I'm not in there and that seldom changes. I may crank it up to get above the dew point temprature outside (to protect the tools) and I may raise it when the cars are brought in from the rain or snow to get the mosture out. Works pretty good.
Cost wise... I can give you a comparison. During the summer to cool the house to 77*, the electric bill runs from $120.00 to $175.00 depending on the outside temprature. The heater doesn't run nearly as often as the A/C during the summer, but depending on the temprature and such, the winter electric bill as been from $85.00 to $130.00. The QMark is the most efficient electric heater out there where as 100% of the generated heat is used to heat the space.
This is what I do, and it is another option. It's not a cheap option, but it is a solution that does not need a flame and that helps me sleep at night with a piece finishing or being glued up.