I have an old Woodmaster 434 outdoor wood furnace. I worked at Woodmaster for a year, so I'm familiar with most of the aspects of the systems.
I looked into the various ways of getting the water jacket and into the house.
You can run it into either a heat exchanger (I used a radiator out of a 1986 Nissan Sentra) and run it like a conventional forced air furnace.
You can run it radiant in-floor, but it's a lot more complicated and expensive to do that way, but it makes for a much more comfortable house as the floor is always warm and the heat is dissipated upwards from there. Makes for a more natural heating pattern, but not as quick a response time to the thermostat.
You can run it into baseboard heaters. I immediately turned this idea down as we originally heated with baseboard electrics and the house may have been warm, but the floor was still (literally) freezing cold.
There are other ways, but I won't get into them as they are WAY out there in expense and difficulty of installation on an existing house.
As for the boiler itself, I wouldn't recommend using 55 gallon drums. The fire in mine can get EXTREMELY hot. To the point of when I'm burning chopped up pallets, I will actually melt the nails into little puddles. Mind you, mine is forced draft induction, so the squirrel cage fan on the door can act like bellows on a forge, getting the flames almost white hot in there. I've gotten it hot enough to warp the inner panel on the door, which is 3/16" steel. But in any case, if you're even the slightest bit low on water in the water jacket, the exposed steel at the top of the firebox will get VERY hot, and the thin walls of a 55 gallon drum won't last long at all if you get any sort of creosote build up and it burns off.
Transverse heat tubes help considerably when trying to extract as much heat as possible out of the fire into the water. And a minimum of a 3 foot diameter firebox is a must if you're using chopped wood as fuel. Trust me, it's not worthwhile to go any smaller than a 36" diameter and less than 3 feet front to back on these. You really don't want ot be going out there three or four times a day to reload them. And they are HUNGRY. I go through a pickup truck load of oak a week at times. They are definately not the most efficient form of wood heating, but it is SO nice not to have to drag nasty firewood inside the house and worry about what kind of critters are in it. It's also nice not to have to worry about the house catching on fire if something falls or gets knocked over onto a standard indoor fireplace. Also not having to worry about creosote buildup in the chimney is a nice feature, too. Most outdoor furnaces have a chimney length of 36", so creosote isn't a concern at all. The chances of your heat source causing a fire in your house are pretty much zero. The hottest the water coming into the house can get of course is a little over 210 degrees Fahrenheit, because any hotter and it flashes to steam, which the water pump won't move.
These systems aren't great when it comes to heating a very small house, as they tend to let the fire go out before you extract enough BTU's from the water to warrant the fire being stoked again. You want to ideally keep the water between 170F and 180F. My system is one of the smaller ones and it has 150 gallons of water in the boiler alone. It takes a while for it to heat up if the fire goes out and we're asleep. But they are ideal if you have a 2,500 sq ft house, a garage/shop, you use a water to water heat exchanger to heat your potable hot water, and pipe it under the porch for easy snow/ice removal. Then it becomes MUCH more economical to use. For me, I have an ANCIENT house in northern Minnesota where for 6 months of the year we are below zero F. I probably lose around 50-60K BTU's an hour out of this house when there's no wind. So I can suck enough out of the boiler to keep it lit. But if this place was well insulated (which would cost me several thousand dollars I don't have), I'd probably spend more time lighting the thing than I would resupplying it with wood.
If you have any more questions, please e-mail me as I tend to not check in here all that often. darrin(dot)moore(at)gmail(dot)com
