Ask Mr. Green: A Prius or a Furnace? From the Sierra Club

This week we'll get the insights of Bob Schildgen (aka Mr. Green), who writes Sierra's popular "Hey Mr. Green" environmental advice column.

Hey Mr. Green,

I'm thinking of replacing my 25-year-old gas furnace. I'm also thinking about getting a Prius, since my old Corolla gets way lower mileage. But my funds are limited. So tell me, which would give the most bang for my buck in terms of shrinking my carbon footprint: the furnace or the Prius?

--Mel in Detroit, Michigan

While the neighbors might be impressed by an eco-correct Prius, an inconspicuous new furnace may be a more cost-effective path to a dainty footprint. This possibility is obviously much stronger in places with very cold winters, like your upper Midwest.

Your old furnace, likely only 65% efficient (or less), might burn 400 more therms of natural gas than a new model that's 95% efficient. So, with natural gas at $1.29 per therm, you'll save $516 per year with a new furnace. Since burning a therm of natural gas emits the equivalent of 11.8 pounds of carbon dioxide, the new furnace would eliminate some 4,700 pounds of emissions per year. Even if you buy a high-end $10,000 furnace, that breaks down to 15 cents a pound to stifle CO2 over its 15-year life span.

Now for the Prius: Driving it 10,000 miles a year for 15 years will generate 75,000 pounds of CO2 equivalent. Your Corolla will emit roughly 150,000 pounds over that distance, so the Prius could save a net of 75,000 pounds (plus $445 a year on fuel). With the Prius's base price of $23,520, that's 31 cents per pound of CO2.

It clearly pays to do a cost-benefit analysis tailored to your situation before making any big energy purchase, be it furnace, car, or solar panels. It's a pain, I know, having spent hours with the numbers above, but it's worth it. If you're not up to the task, tap an energy auditor or buy your geekiest friend a beer-brewing kit in exchange for crunching your numbers. 

Call 916-624-0808 for an energy assessment.