So,my lady might be OK with an indoor rocket mass heater.
My house once had gravity flow furnace,aka an octopus furnace.
It was replaced with a natural gas forced air furnace.
The new furnace doesn't use a chimney,rather it is vented out the side of the house via PVC pipe.
So there appears to be an disused 8" diameter chimney running in a stack from my basement.
I have not inspected the insides of this chimney. When I did check the cleanout years ago,I found a dead bird.
Back then and until recently,I thought the brick chimney stack contained only a exhaust vent for the gas fire place.
Still useful, but restricted in diameter.
Exploring this option led me to rediscover the bigger 8" flu.
I think this could work as the chimney for an 8" batch box RMH.
I could build it right beneath the stack.
But the heat probably wouldn't disperse well over two stories of conventional house.
To combat this, I am considering building a bell up and around the return air ducts of the forced air furnace,and locating the batch box inside or adjacent to this bell.
With the air preheated, the gas furnace could operate normally,and only kick on when needed.
Running an 8" insulated duct from the bottom of the bell to the legacy chimney seems doable.
If there is space on the first floor for a bell near the existing stack, I think you could do the heater core in the basement with the bell space extending up through the floor. This would let the heat rise to the center of the first floor. Were there ducts to the second floor, or did heat rise through grilles in the ceiling?
I would use a batch box in the basement so you need minimal fire-tending and maximum burn time. You would need to cut and resupport the floor framing for safety clearances to the bell surfaces. You would also want to seal all openings in the upper half of the house against leaks so you don't suffer from whole-house stack effect (google it). This would have the beneficial side effect of reducing heat loss from air in- & exfiltration.
10 Podcast Review of the book Just Enough by Azby Brown