This video shows a home-made outdoor wood burning boiler system used to heat a garage.The materials you need for this project are Pex crimper,Barrel stove kit,Pex tubing,Pex clamps,A windows fan,A regular car radiator,Coolant,A bucket,PVC polytube,Submersible pump.This uses an indoor car radiator and fan and some underground tubing to cycle the water through.Drill two holes in one corner of the wall in your garage. Remove the pol heater and fix the pipes up with a few pex crimpers.
You can even use hose clamps, since they are less expensive. Make sure that the pex pipes are well covered with dirt and soil underneath so that nobody trips on them. The part that attaches itself to the garage wall can also be covered with an insulator to prevent leeching.