OT....Who here heats with wood?

Thanks.

A second point is that a fair amount of "dirt" comes in with the wood, both soil and bark as well as sawdust and even insects. The outside units avoid this problem and also reduce fire risk.

Your biggest issue if you are going to heat with wood is saving labor and figuring out how to get the wood in the house. I suggest building a wood chute if you plan to heat a lot.

I heat a large farmhouse and a three story barn and workshop 100% with wood.

In regard to what you're proposing, I had that setup once. A hot-air wood furnace coupled to an existing hot-air oil furnace down the basement.
To make it work correctly, you will probably need to fabricate a specical "back-flow preventer" in the plenum on what is called a "parallel" installation. That depends on your particular furnace and plenum setup. Some MUST have the back-flow preventer, and some can get buy with something simpler. With many, if you don't have it, when the wood furnace kicks on, it will suck air from the oil-furnace cold ducts - backwards - and wind up sucking air from the existing hot-air ducts - which does't not work.

I redid my system and built an addition onto the house at 1st floor level - with a stand-alone wood-furnace. It has it's own hot-air ducts and also heats all our hot-water with thermo-siphon to an 80 gallon storage tank. The chimney meets Canadian burn-out standards that are much better then USA standards. Also the room will hold three full cords of wood at at time, and keeps it nice and dry. As a side-benefit, we hang all our winter clothes in there - and even when sopping wet -they are bone dry in an hour. This system is virtually fire-proof. If the chimney ever caught fire - it would still be safe. Also, I can clean it in 15 minutes, from inside - no need to go on the roof. Keep in mind that you can't turn down a wood furnace like you can with a gas or oil furnace. And, even when it IS run low because the house is hot - it will smolder a bit and soot up the chimney. The chimney is going to get dirty no matter how good and dry your wood is - especially in early fall or early spring if when temps are not super cold yet. If I build another house, I will install exactly the same setup. A big plus is loading wood without having to go outside. And, if you bring in wet wood, it will be dry in a week or two -long before you have to burn it. When temps are at zero or below, the room holds more than a month's worth of wood. Biggest hurdle of my installation was installing hot-air ducts that go underground. I used flexible insulated heat-duct with corrugated plastic culvert pipe on the outside of it where buried. If you ever decide to buy stainless chimney pipe, try to get Canadian versions and NOT USA versions.