Standard Caddy here, going into our fourth winter with it. Big old farmhouse about 2500 sq ft. Poorly insulated but have replaced most windows and doors. Didn't even have ductwork to upstairs, so we added it up and through the attic ... less than ideal. And cold air return from upstairs is only through a floor grate and down stairway. Caddy runs parallel to a high-efficiency propane ... cold air through a 6" filter then splits to two furnaces, plenums rejoin, louvers just after the Caddy to keep A/C air from running backwards through Caddy too much. Can't tell you how much wood I burn because I don't count it. Just cut it and pile it up. Sometimes oak sometimes box elder and everything in between. Also winters have been very variable, and I can't get my wife to keep her paws off the gas thermostat to give the wood a chance to burn. But I'd say 6--8 full size pickup box loads square to the top of the cap in the worst winter here on the Minnesota/Iowa border, and that before we changed out original rattletrap windows and put a garage on the north side.
I sketched some cutaways because the flows through this thing are complex.
The box is lined with firebrick on four surfaces. No brick on the door side of course, and none on the top. The top is a stainless baffle with fiberglass insulation on top of it, then the steel top of the box above. There is a tiny metal plate about 2"x6" over a grate in the floor where you are supposed to shove ashes down into the ash box. It's a hassle, so I cut the tiny grab loop off it and I just leave it alone. I shovel ashes out of the box itself. I also discovered the coals don't burn all that well on the bottom of the box, so I laid a cast grate from another old woodburner in the box. It's just long enough to rest inside the door with a gap under it at the front but not at the back. The front of the box has a tiny hole to let air in for the coals when the draft is closed.
Airflow through the drafts is complex and some of this is guesswork:
- Main intake, controlled by thermostat (no fan) is above the firebox door. Behind it are three intakes. There is a hole in the center of the door that lets air into the center channel even when it is closed.
- There is also the tiny jet I mentioned below the door.
- And there are two square intakes about an inch square below the bottom corners of the firebox door that are always open.
Inside the box, two big steel channels go up the back of the box and then across the top of the sidewalls to the front.
Those channels have four holes in them, with four stainless tubes that bridge across the top of the box. Those tubes are perforated, so the combustion air is preheated in the channels and shot out onto the baffle. This definitely makes the furnace a gasifier. When burning hot, and even moreso when the draft first closes, the hot air shooting out those tubes ignites the gasses and they look just like the burners in a gas furnace.
-The stainless baffle sits on those tubes. In normal operation, that baffle is shoved to the back and gasses move to the front of the baffle then to the back of the box above the baffle to exit. When you clean the heat exchanger (very convenient with a door on the front of the furnace), you first slide the baffle to the front so the soot from the side exchanger tubes drops back into the box. For the center tube you either pull the soot out the front of the furnace or shove it back into the stovepipe. Mine has a t behind it for easy cleanout.
- When the draft is open, the primary combustion air (cold) drops right down the inside face of the glass door (because it's cold) and then flows back across the coals and wood. The wood and coals in the box burn from front to back because the lower oxygen gets used up ... and then the gasses burn from back to front.So the total airflow sequence is ...
1) Front to back across coals and wood
2) Back to front across baffle and jets to burn gasses
3) Front to back across top of firebox to complete combustion before exit
4) Back to front through two heat exchanger tubes that are completely exposed in plenum
5) Front to back through center heat exchanger tube in plenum
6) Exit furnace through cold air return, slightly preheating the returning house air
My minor gripes about the furnace:
-Ashbox is useless and should be eliminated.
-Airflow into coals is less than effective. Should use a grate with slight airflow below, while keeping major airflow across coals as currently designed. This would also help with ash cleanout.
-Stainless baffle is heavy but it still warps a lot in the heat. Some of my gasses now escape around the edges ... not a serious loss.
-Stainless tubes are held in place by stainless pins that distort and break. Instead they should have a right-angle notch so you slide them in and then twist to secure ... with the twist in the opposite direction that you'd push them when sliding in firewood.
- Slight leaks when blower is running ... I can feel the air blowing out around the door frame.
- Leaks into the firebox when blower is running ... if it kicks in while loading the box, you might get a little ash and smoke blown out the door. But perhaps this is by design, so the blower boosts the flame a bit, dunno.
If I had more money I'd have done a water-to-air system, and in a new house I'd do water in floors with a forced air system for backup and A/C. Having a gasifier turn itself on and off is a waste of its design. Better to burn a full box of wood at top gasification temperature all the way, and capture that energy in a water tank heat sink to use as needed.
Hope your new setup is working well for you.