What is the best way to keep warm in your garage workshop?
Woodworking with cold fingers is not a pleasant or very precise undertaking. So, if you live in a place that gets cold in the winter, you're going to need to heat your garage workshop. There are a few ways to heat your garage, depending on your situation if it is a detached or attached garage.
If you are building new, by far the best way to heat your garage is to install radiant floor heating in a new concrete floor. Radiant floor heating is very good as most of the warmth is at your feet and not up at the ceiling. The concrete is a great heat sink and will release the heat more evenly than forced air heating.
If hot water radiant floor heating is not an option let's cross off all the garage heating options that you should not be considering.
Extending ducts from your existing heating system could draw exhaust and carbon monoxide into the rest of the house, not to mention dust and fumes from finishes. there should not be any return ducts in from the garage.
Next, portable electric space heaters do not really have the capacity to heat a large area and would be a hazard around sawdust. Plus you would constantly need to move it around which becomes a hassle.
Lastly on the list of really not recommended heaters is portable kerosene, oil, or propane heaters. They release moisture, soot, and carbon monoxide into the air as they burn. Besides their negative effect on your health, the moisture they create will not be good for your wood or your finishes.
Another way of heating your garage for woodworking is a wood stove. Sure, it would be inexpensive using your wood scraps, but it takes a while to heat up and they are not the safest things to have around wood dust and fumes.
The next step is to calculate how much heating power you will need to heat that garage space comfortably. This is measured in Btu or Kilowatts. (See the above link for that information.)
Now that you know how much heat you need, lets look at the heater options you have, basically what's left is electrical or propane. After you have found out how much heat output you will you can then go to the store and look at the different heaters you can purchase.
One note, if you do have to run a powerful electrical heater in the winter time, be aware of the power load it places on your electrical system. You may not be able to start the tablesaw and run the heater at the same time because the fuse might trip. (I say this out of experience unfortunately.)