You guys can have your add on furnaces and OWBs - every time I read about them I'm thankful I don't have one. Complexity has a cost I'm not interested in paying. If the power goes out (which it does here a lot), it doesn't matter. My simple EPA stoves will quietly heat my home as cosy as I need, purely by radiation and convection.
Really, there isn't anything complex about an add-on that shares a single blower with the gas furnace... and you don't have to have electricity to use it. It's a simple matter of routing the air flow from the gas furnace through the add-on before connecting it to the heating ducts... which allows both heaters to run independently or at the same time depending on demand. If installed properly the wood-fired add-on can be used when the power goes out because convection will still carry the heat into the ducts... it just won't be distributed as evenly to spaces farther from the appliance.
A few years ago we had an ice storm that snapped off hundreds of miles of power poles and left us without power for 10 days in February... Believe me, 10 days without power in February is a long time in this climate. I wasn't burning wood for heat at the time (another story) but I had plenty of wood stacked for fire pit use. I fired up the wood furnace and heated the house for those 10 days without any electricity or blower, convection through the heating ducts alone kept the main spaces of the house warm enough but the far bedrooms got pretty cold... no different than it would have been using a free-standing stove. The wood furnace heated/cooked food (we also used the BBQ out on the front porch) and melted snow for much of our water needs, such as flushing the toilet and washing.
That power outage taught me a serious lesson... It sucks to be without electricity! Yeah, I can survive if I have to, but if I don't have to, why should I? Pain in the butt... I had to move all the food from the deep freeze outside to keep it frozen, and finding a place where the refrigerator type stuff will stay cold without freezing was a pain. Washing machine doesn't work, and even if it did the well pump won't run. Taking your bath from a bucket gets real old, real fast! No radio, no TV, no lights (camping lantern 'cause flashlight batteries couldn't be found), no phone (all cordless in the house and there ain't any way to charge the cell short of starting the truck), no water, no computer... Night time was the worst, just you, the wife, the kids and... dark silence... I can only play so many board games by lantern light. And you couldn't buy a generator anywhere within a 1000-mile radius!
You can bet your wife's sweet azz I have a generator now! I start and test it every 60-90 days, and keep plenty of fresh, stabilized, non-ethanol fuel on hand for it! 5500 watts will power my whole house without a problem (with a bit of common sense... ya' can't run the oven, cloths dryer and microwave all at the same time) and will run all night on less than a tank of fuel.