Gravity Heat: the Complexity of Energy Efficiency

Recently, we’ve been looking into a few upgrades to our ol-timey Gravity Furnace. Whenever we ask for recommendations, we always hear the same advice: “just replace the thing!” 

And that is the generally the advice you’d hear from just about anybody in business that makes money by selling new equpment.  
In fact, a very similar project that I’ve been admiring for years-another ecological uprade of a victorian home–heralded the replacement of their gravity furnace as their number 1 
“green” improvement! 
But given the particulars of our home and furnace, when I “do the math” I find that replacing our old “octopus” unnecessarily adds to our negative ecological impact as well as increasing our “net” heating cost over the life of our home. 
For those who aren’t familiar with a “gravity furnace,” they’re the simplest kind of central heating you can have, relying not on fans to circulate heat, but on the simple fact that heat rises and cold air sinks.  
And while that system comes with certain draw backs and inefficiencies, it also has some advantages, which from a certain perspective, outweigh the negatives:  

1. Fuel Adaptability: in its lifetime, our furnace has run on wood, coal, oil and gas! If current trends towards economic instability and increasing resource scarcity continue this alone makes our “octopus” very attractive as a second furnace (we also have a modern forced air furnace in for parts of our home.)  
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. Financial 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. 

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