Just before Thanksgiving 2009, my family began heating and cooling our 4,400-square-foot suburban home with geothermal energy. As I wrote in a story that appeared in Health & Science last March, we got rid of our 24-year-old oil-burning furnace and traditional air conditioning, and replaced them with a system based on drawing moderate temperatures from beneath the Earth's surface.

We expected better performance, big-time savings on our energy bill and a rapid return on investment, thanks to a host of tax and financial incentives.

Now we've lived with geothermal for a year - a year of historic weather extremes. We've paid our utility bills and applied for tax credits. So . . . did reality match or collide with our expectations? Was this project worth it?

Largely, yes - but with some interesting caveats.

First, let's recap.

Almost two years ago, we began looking at options for replacing our aged HVAC equipment. For a new conventional system, we got bids ranging from $9,500 to $11,000.

We also checked into cleaner, renewable alternatives, including solar, wind and geothermal. We settled on geothermal. Its basic principle isn't complicated: The temperature of the Earth a few feet below our Montgomery County yard (and for hundreds of feet below that) is fairly constant: in the mid-to-upper 50s year-round. The idea is to transfer that temperature into the house, then make the relatively small leap from there to a comfortable indoor level.

The bill for our geothermal system came to $23,950, which included hiring a drilling company and an HVAC firm.

But we knew that our actual cost would be much less: Under the American Recovery and Reinvestment Act of 2009, geothermal projects completed by 2016 are eligible for a 30 percent federal income tax credit. In addition, we would qualify for a $5,000 Montgomery County property tax credit and a $2,000 state grant. Those incentives brought the net cost of geothermal to less than $10,000, rivaling the lowest bid we got for a traditional HVAC system.

And because we'd eliminate oil bills, we expected to save thousands of dollars every winter.

It seemed on paper that we could get a return on investment in less than four years.

Construction started in October 2009. The drilling company buried hundreds of feet of polyethylene piping in our yard and routed it indoors. Inside the pipes, a mixture of water and antifreeze circulates, absorbing the moderate underground temperature and bringing it into a newly installed geothermal heat pump in the basement.