2. Dependable: few moving parts, no fans, baffles, computers, plus strong long-lived materials, little to go wrong.
3. Ecological impact of embodied energy: As an annualized impact, this equipment's ecological footprint is tiny, even assuming impractically large efficiency improvements, an honest evaluation including the embodied energy of new ductowrk, etc. makes it unlikely that furnace replacement would "pay off" in the lifetime of a furnace.
4. viability: again, assuming huge efficiency increases only possible with home alterations (wall insulation, increased sealing) that are likely to have a negative impact on the lifespan of our victorian home (Victorian homes weren't made for that kind of air-tightness)--and not including those costs in my calcualtions--a new furnace is unlikely to pay for itself over its lifetime! When I do the math, the payback time ends up being over 40 years, not including maintenance and parts replacement! When it comes down to it, the money we save by waiting to replace this monster can be invested in things that will have a bigger ecological impact and shorter pay-back time.
5. Depressurized heating efficiency in an old house: lab tests of efficiency assume the relative air-tightness of a modern home. In a leaky victorian home, there are efficiencies that come with not having heat "forced" out of the home by a blower. It's hard to measure this factor, but it shouldn't be discounted when calculating supposed benefits of newer furnaces.
6. Reasonably Upgradable: efficiency improvements can be made such as updating the ignition, adding thermal mass, and installing radiator coils in the flue pipe, which was popular in the 70s and can capture 40-50% of the "lost" heat, bringing efficiency into the range of modern heaters. Unlike a new furnace, these improvements would pay for themselves in a few short years.
All things considered, it's impossible for me to justify unnecessary replacement of our gravity furnace on ecological, efficiency or financial grounds. It just goes to show that the conventional advice isn't always right, and fancy "efficiency upgrades" sold by the industry don't always work out to be more efficient when you do the numbers.
Some day, this beast will come to the end of its life and it will be time to replace it. Even then, this analysis shows there may be ecological and financial benefits to eschewing fancy "green" high-efficiency equipment promoted by "green living experts," with so many short-lived parts, fussy electronics and high embodied energy for more "appropriate" tech that's simpler, longer-lived, more adaptable, and less expensive.