Not to rain on your parade or tell you what to do but I would avoid this particular temptation. Reason being that even if it works (physics says its possible) what do you do with the condensate that will likely be classified as hazardous waste? Toxic sludge basically from a cord wood fired appliance. No one is going to want you dumping that anywhere near them. Not sure what disposal of hazardous waste costs in your area, here it's high. Then there is the accelerated corrosion of the appliance flue/chimney to consider & cost out. Was the appliance ever designed to be operated in a condensing mode? The cooler the exhaust gets the more resistance it will offer to the fan doing the pushing. Is the fan designed to resist this higher level of corrosion? Plenty of factors to consider when moving from conventional or draft induced/assisted to condensing.
It is much in the future or never and a fix for a problem that I do not have. The basement is already running warm, radiant in the floor, and I cannot imagine how much useable waste heat I would get from two tanks running a passive surface temp of 110 deg F or so. They would have to be right under my bed for me to get the heat. I am getting condensation in the outside riser of stainless UL all fuel that collects and drips out the bottom cleanout cap of the tee. Have seen a little ice mound under it, less than the size of a gallon of milk. It washes the inside and the wet ash collects in the cleanout cap, very small quantity. Most of the ash has been staying in the secondary chamber of the Froling and I've taken out 7 gallons ash (three 2 gallon trips) from the boiler since October. I've been checking it and it's clean right now.
It was mainly a concern for if wood got scarce and all I had was an arm load of wood to keep from freezing or burning the furniture. A condenser between the boiler and the flue would also keep the mess in one place and the flue a lot cleaner, but it's staying clean now.
There would be no change to the Froling, which is running with the LK Acaso 810 loading unit and stays fully above the condensing temp. I just brushed the secondary chamber after 3 months running and the factory steel came right up with one swipe. Looked at the top of the firetubes at the fan chamber and there was nothing worrysome, just light grey ash coating.
I know there will be unforseeable issues. I see three components to handle, separation of H2O, very fine dusty ash, and some tar. The concern would be the accumulation of flammables, and the weight of the tank with a ten year collection of fine ash and some, minimal, tar varnish. If a 50 gallon steel tank lights off the tar inside it, it is not going to burn through, and it seems to be slow burning. I see it accumulating but not lighting off.
The H2O condensate is not a disposal hazard. Maybe slightly acidic with some ash minerals. Disposal hazards are usually solvents and heavy metals. Coal ash may be a problem, not clean firewood.
I think a cordwood flue gas condenser could be taken to the next level with some experience and factory engineering. Could get into patentable ideas. I'm sure the geometry of the condenser chamber makes a difference, easy separation of H2O from ash would be necessary. The chamber would have to be rated for a residue burnout, but it should not have a fire, that's what the secondary gasification chamber is for. Remember, the stainless HT 103 flue is several thousand dollars that could otherwise be invested in a listed condensing accessory and PVC vent, when the products and the code approvals mature, like they are for gas condensing boilers. IMO, that would be an objective. The condenser could be a preheater for DHW or low temp return water. Not needing a chimney would be an advantage.
It's an area that calls for investment and a huge market. I'm a bit afraid that if everyone on oil switched over to wood, there would be no standing trees anywhere. There is a market for more expensive, more efficient equipment, trading capital in equipment for fuel and labor.