Modifying a wood furnace to increase efficiency

[Daox]

My cousin just dropped me off a nice little wood furnace for my garage. Yep, another freebie! But, it does need work. From the name plate on it it looks like its from the early 80s. It is VERY simple which is great. However, it does not have a dedicated inlet for combustion chamber air. Its also pretty small so I'm guessing a lot of heat will escape out of the flue.

I am looking to refurbish this furnace as well as modify it to increase the efficiency. I am looking for ideas! It'll obviously need a dedicated intake for the combustion air. I'm not sure what can be done with the flue to increase efficiency so I'm looking for ideas there. I am also thinking of running some plumbing to it to work as an auxiliary water heater since I'm sure it'll provide more heat than is needed to heat my garage. Anyway, I don't know a lot about furnace design or anything so if anyone has any links to info or anything it would be great.

Here are the pictures:

Front of the furnace. The rust is not only from use, but sitting outside for a while.







Here you can see the problem with the unit. It has several holes in the back. I'll just weld them shut.



This is the back side. Again you can see the holes.



A nice perspective shot. The one hole under the exhaust flue is fairly good in size. Nothing that can't be fixed though.



Here is the unit without the cover over it. There is a blower that attaches under the unit and blows air over the combustion chamber and the hot air exits the top via a large duct. Pretty simple design.



Here is where the blower attaches on the bottom.



This is where the air comes up from the bottom and around the sides.