My small plumbing and heating company recently replaced a client’s inefficient 25-year-old combination wood-and-oil boiler with an efficient new Buderus oil boiler. Oil was the obvious choice of fuel. The homeowners no longer wanted to deal with firewood, and natural gas isn’t available in the area of Vermont where they live. And propane — though available — is ruinously expensive. But we had one important choice to make: Did it make sense to go for broke and put in an ultra-high-efficiency condensing boiler, or should we opt for a slightly less efficient noncondensing version?

Going for maximum efficiency offered one substantial carrot: There’s a 30 percent tax credit for systems with an AFUE rating of 90 percent or more. (The AFUE — or annual fuel utilization efficiency — of a given fuel-burning appliance represents the average amount of useful heat delivered relative to the amount of fuel used over the course of a...

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