Thanks to alternety. I will consider these things.
The existing furnace I condemned 5 years ago or so for fear of monoxide was one of those stewpid floor registers that actually sucked in the at the floor on either side of a central wall and exhausted out the wall with a big vane that could direct most heat to one or the other side, all gravity stuff. It used 17,500BTU of gas. I suspect it was perhaps 50% efficient. Truly dreadful, causing massive unpleasant stratification. Like standing head temperatures of 110F, sitting head temps of 70F, and feet at 45F. At any-rate, I will be blocking this off and removing what I can. I will re-sheet rock the wall [both sides] and try to match the plaster. I will patch the floor.
I have tentatively selected a new Goodman furnace:
GCV90704CXA
69,000BTU Full input
64,200BTU Full output
48,000BTU Half input
45,000BTU Half output
21" x 28" x 40"
135 pounds
93% AFLU
It is a two stage. I have done the full energy calc and came up with 51kBTU, which seems about right. I kinda laugh because finding a 50kBTU is like finding "the corner" while standing in a sphere.
I believe I will only need the first stage most of the time except for those rare 20F evenings. This will allow a long even cooking without short cycling.
Twill be mounted in the attic horizontally. I will run 6 registers. The return and furnace will be centrally mounted above where the condemned furnace is under the house.
I will saw off the existing flue pipe that comes up the wall that the condemned heater is in under and where it exits the ceiling in the attic. I will drop a black iron pipe down the old flue pipe from the roof. It will be braced and mounted under the house and in the attic. I will run the condensate drain from the new condensing furnace down the same old flue chase. Then I will foam the top and bottom of the chase.
I'll run a sweeping 90 return from the hallway ceiling return register into the furnace thru an electrostatic filter. The hot air plenum will of course have the six different register ducts leaving it for all rooms to be heated.
Question: Does anyone know anything about 2 stage heat/cool thermostats or have any recommendations?