Ecofrugal dilemma

So, it turns out we need a new boiler. This is hardly a surprise; the one we have now is, as best we can tell, the original heating system in our 40-year-old house, so it's actually a bit remarkable that it's still running at all. But when I had the guy in last week to do a tune-up, he advised me that our boiler lacks not only some basic energy-saving features (like electronic ignition and vent dampers) but also a couple of basic safety features (a low-water cutoff and a backflow preventer) that would cost about $1500 to add. Not really worth it for a 40-year-old boiler, so it was time to start shopping for a new one.

So far we've only received one quote, and it seems a bit on the high side, so I doubt we'll be going with that company. However, they did tell me one thing that has me a bit perplexed. The "comfort specialist" who came to our house said that while we could get a new boiler that was 90 percent efficient, it would cost about $3000 more (including installation) than an 84 percent efficient model. True, we could get half of that back through the federal tax credit for energy efficient home improvements, but that would still amount to an extra $1500—and given how modest our home's energy use is now, it's essentially impossible that we'd ever make up that extra cost in energy savings over the 15-to-20 year life of the new boiler.

Now, this estimate may just be completely off. It's possible another company will come back with a quote that shows only a $1000 difference between the more and less efficient models. But assuming that there really is a $3000 difference, what's the right choice here? Is it worth paying an extra $1500, knowing we'll never recover more than a fraction of it, just to know we're really doing all we can to reduce our carbon footprint? Or would we be better off going with the reasonably efficient, but not super-efficient, equipment and offsetting the extra emissions? Right now, we pay less than $100 a year to Carbonfund.org to offset all our carbon emissions for the house and the car combined, so it seems like the difference between an 84 percent efficient boiler and a 90 percent efficient one couldn't account for more than an extra dollar per year. That's a heck of a lot less than $1500, so unless we expect to own this boiler for more than 100 years, offsetting is clearly the better choice financially. But is it the better choice morally, or is it just a greenwashing cop-out?

Or is it really too little a detail to worry about, and am I just a bit too obsessed about maintaining my green cred?