I design and engineer houses and building compnents for a living, so I have been able to talk to all sorts of professional builders, heating contractors, and building inspectors to help in my decision on heating my own garage. First It sounds like your garage is connected to your house. If so, think twice about using any non-vented appliance that burns ANYTHING!(you really shouldn't burn anything in an unattatched garage without ventilation either, but by being attatched to your house, you are putting others at risk in addition to yourself!) I have wrestled with the same dilemma in my own house w/ attatched garage. First thing to do is finish and insulate the ceiling, then the walls. If you haven't already done this, it is amazing how much heat is held in from the vehicles being brought in from outside at operating temperature. I have decided on a good size overhead electric heater from a company named Farenheit (sp?) for around $200. The BTU output varies with how you wire it, but I know a guy who has one and it heats his insulated 26 X 24 garage 50-60 degrees above the outside temperature quite nicely here in green bay, wisconsin. It does need to be wired 220, and the guy I know who has this heater, wired it to be plugged into his welder outlet. I will hard wire mine. Electric heat is the safest, probably the cheapest to install (in a permanent application), but the most expensive to run! I am willing to pay more to run it if I know I am not putting myself or anyone else at risk of CO poisoning.
The father of a guy I know is an EMT, and he has seen more 'near miss' cases of carbon monoxide poisoning than you can believe, every thing from ice fishing shanties to hunting shacks, to idleing cars with rustholes in the exhaust, to the malfunctioning furnaces, to GUYS USING UNVENTED HEATERS IN ENCLOSED SPACES!!! He says that he knows he can't convince everyone to not burn fuel in an unvented appliance in a closed in space, so he asks that you also get a carbon monoxide detector to let yourself know when the danger level is at hand, and he also says the thing won't be malfunctioning, CO builds up faster than anyone thinks!! STAY ALIVE OUT THERE- LIVE TO DRIVE THAT PROJECT YOU LABOR OVER!!!!
You need to talk to your local building inspector, most municipalities have strict regulations when it comes to heating equipment, even if it's only used to heat your garage. If you do something in violation of a building code, and your house burns down, your home owner's insurance won't be covering you either. This is without a doubt a situation where you must DO IT RIGHT OR DON'T DO IT AT ALL!!
[This message has been edited by SSteve L (edited 11-18-99).]