My house was built in 1959 and had the original double pain Anderson windows in it when we bought it in 2015. I've been replacing them one room at a time for the past 5 years. I've added a substantial amount of insulation to the attic, and I've re-insulated the exterior walls as I've remodeled each of the bedrooms and the bathroom. The next rooms on the list are the kitchen and living room in that order. The living room has 16 feet of windows that are 7 feet tall, and the kitchen has 8 feet worth of the same windows. I won't see a noticeable change to my heating/cooling cost until all 24 ft of those windows have been replaced. I'm currently saving for the kitchen remodel. It will be a major remodel project, and it's too much for me to do on my own (north of $50K). Realistically, I won't see a noticeable change to my heating and cooling needs until I've completed $75K to $100K worth of remodeling projects. I'll get there, but it's going to take another 7 or 8 years. In the mean time, I will need to do something for heat.
My current unit is somewhat similar to the Fire Chief/Clayton design. I almost bought one of those when I installed mine. You could buy them at the end of the season for $750 back then. Money was pretty tight at the time so I went with a used furnace off Craigslist for a few hundred dollars. My thinking was that I would pocket the savings on my fuel bill and use that to replace the furnace when it wore out. I had no idea it would last for so long, or that things would be SO much more expensive now. The furnace is in the basement, and it's NOT a walk out basement. I was able to wrestle the unit down into the basement with the help of a much stronger friend. We are both noticeably older now, and trying to get the old unit out of the basement, or getting a new one down there would involve a trip to the ER. Installing a new furnace in the basement will involve modifications to the foundation and basement wall, and digging an external entry way. The stairway will be removable (not easily, but removeable none the less), allowing heavy and/or bulky items to be lowered into the basement via mechanical or hydraulic means. Another sizable remodeling project, but one that will facilitate some critical needs 20+ years from now.
So my options are paying a professional welder to rebuild my existing unit where it sits, installing an OWB, or going back to using only the NG unit that was installed in 1994. If I went with an OWB, I would likely treat it like buying a vehicle and look for a lightly used unit (since I'm not in a hurry). The biggest deciding factor at this point is wood consumption. I have access to plenty of good firewood, but it's an hour drive each way. I make it work because I have a loader, a 4wd ton truck, 13K winch, and a 12,000 lb equipment trailer that allows me to load, transport, and unload 2 cord per trip keeping everything in log form until I'm able to process it in the back yard at my convenience. Adding 2 more cord (50% increase) means an extra Saturday or two worth of work. Not a big deal. If it doubled to 8 cord, that's not a possibility. If an OWB is only half as efficient as an add-on furnace, it would make more sense to put the money into digging a new basement entry. I just can't believe anyone would ever bother installing an OWB if it's only half as efficient as the add-on wood furnaces that were built in the 1970's.