For adding geothermal heat to an existing house, it's still true what I've been telling you. For top efficiency, you need to distribute the heat by forced air. This is a downer for folks with existing hot water distribution, also called a hydronic system. (Not hydroponic, although that would be nifty, too, as you could grow lettuce year 'round.)

That said, for new construction -- and for a few existing homes -- not only can you distribute the heat by water rather than air, but you might prefer it. The efficiency (COP) is not quite as high, but it's close. And, in a December 15 press release, the EPA announced it now has Energy Star ratings for the water-to-water heat pumps that make hot-water distribution of geothermal heat work.

Here's the catch for home owners with existing hydronic distribution: your existing boiler delivers heat in the range of about 150-200 degrees Farenheit; geothermal will get you about 120 degrees water temperature. With a 70-degree interior air temperature, the temperature differences are, say, 175-70=105 degrees vs. 120-70=50 degrees. That means, in theory, you would need roughly twice the length of hot-water baseboard radiators that you have now. Not totally out of the question, but possibly a bit impractical.

However, if you are in a position to do it, the best way to heat the house with water of that temperature is radiant floor heat. Radiant floor heat is one of the most comfortable types of heat; just ask the ancient Romans whose health-giving baths were heated from below with fires. (Except you can't; they're dead. The baths weren't that health-giving, although the Romans probably did die with warm feet, something only martyrs like Joan of Arc typically experience.)

No, really, putting ancient Romans aside (which is now much easier to achieve than it was, them being dead and all), radiant floor heating is the cat's pajamas: nice and warm -- and not nearly as fuzzy. In fact, radiant floor heating is also the preferred way to distribute solar heat, if you should decide to go that route.

Radiant floor heat is achieved by running a lot of tubing just under the floor surface. This works great for concrete floors, and works pretty darned good for wood floors too.  (Wood floors only impede heat flow to the tune of an R factor of about 1.5.) You add radiant floor heat to a wood floor by snaking the tubing back and forth between and through the joists. (Not a fun job for do-it-yourselfers, though. That tubing is stiff and can kink easily. I know, I've installed it.)

So -- new construction? No problem. Existing construction? Do-able on the ground floor or any place you can get access to the floor joists. Second floor of a stick-built house? Not so much, unless your idea of a good time is ripping up the first-floor ceiling or second-floor flooring.

As I say, the efficiency or COP of a hydronic system is not quite as high as for forced-air. EPA Energy Star standards for water-to-water (hydronic) heat pumps in 2009-2010 is 3.0 versus water-to-air (forced air) COP of 3.3. The forced-air system would be 10% more efficient. For cooling, the advantage goes the other way: the EER for hydronic is 15.1 and the EER for forced-air is 14.1.

Cooling the floor, however, has its own issues unless you live in a dry climate. Here in the northeast, there is plenty of humidity in the summer air, so you must control humidity in the house, or moisture will condense on or in the floor. In forced-air systems, the moisture condenses at the centrally located cooling coils and can be pumped out. In radiant systems, your floor IS the cooing coil!