Geothermal technology heats up Harvard home

After 20 years of toting firewood to heat our slightly too-passive solar home, we are done. Over the years we’ve become aware that many folks control the temperature of their houses by adjusting a little instrument on the wall instead of dragging three 50-pound loads of wood inside every winter day.

We thought we would check out this “central heating” thing and maybe move into the 20th century. (We do have a centrally located woodstove with a thermometer on it, but twiddling that instrument made us no warmer. We also have three—count ’em three—baseboard electric units on thermostats.) So we jumped ahead to the 21st century and decided to go geothermal. We’d get our heat from the very rocks of Harvard.

As encouraging as the Oct. 23 Harvard Press article was on the likelihood of local earthquakes, we couldn’t find any pocket of vulcanism to tap into. It turns out that we share the same, boring, temperate 50- to 55-degree soil (that is to say, rocks) of Harvard that all its residents do. So the plan became to suck heat out of that soil and raise that heat up to the human comfort level. Through the rather unbelievable magic of thermodynamics, we could do this without actually adding heat. It also means we could pump excess heat down to the rocks in the summertime. (It’s good to give back to the Earth.)

If the ultimate incentive to get central heat was to save backs, the big incentive to go geothermal now was to save on taxes. As part of the federal recovery package we could make certain home improvements to save energy, and get 30 percent off the total cost in the form of a tax credit. Some credits, like those for energy-efficient furnaces, are limited to $1,500, but not geothermal installations.

This is a credit, not a deduction, so you really do get 30 percent of the installation cost back into your pocket, not just 30 percent off your taxable income. Plus, the state organization, MassSave, will happily coordinate as much as a $15,000 loan for seven years, interest-free, subsidized by the power companies. And, there was no state sales tax. If we were ever going to do it, now was the time.

Our household is pretty geeky, so we, of course, ran the thermal and financial numbers on alternatives: continuing with wood; adding more electric baseboard heat; using gas, propane, or oil; or installing a new type of air-to-air heat pump. All alternatives used new equipment efficient enough to garner a 30 percent, $1,500 maximum tax credit. Having no central heat to start with, we also needed to include a distribution system—ducts for hot air or pipes and baseboard radiators for hot water.

The “numbers” took the form of the various installation costs, offset by credits and the free loan, and the net present value of the cost of fuel for 20 years. (We applied conservative expectations for fuel inflation and economic inflation.) The geothermal system won, with an estimated payback of seven years, and about a 14 percent return on investment after that in fuel savings. That agreed with the payback analysis given to us by the bidding installer, Nexamp, so we felt confident.

Geothermal heat (the nonvolcanic kind) isn’t entirely free. It’s only about 75 percent free. The “fuel” for geothermal is electricity—but, surprisingly, not for heating. You might think we’d be just taking 55-degree heat from the earth and heating it up the rest of the way to, say, 68 degrees, but that’s not the case.

Here’s how it works. (Geeky material alert!) We circulate water in a closed loop through two 320-foot wells, so that it enters our basement at about 55 degrees. A rather conventional process of heat exchange and refrigeration then transfers that heat to the air circulating through our ductwork. It’s the refrigeration process that pumps heat out of the water and discharges it into the air, raising the air temperature.

It is disconcerting to base your heating system on the faith that heat can move from a colder material to a warmer one. A few thoughtful experiments with ice cubes and warm scotch will confirm that heat generally flows the other way. Your cold toes get much warmer, for instance. But then consider how an air conditioner removes heat from a 70-degree condo in, say, Maui, and discharges it outside to the much warmer, 85-degree air. Rational people would then, of course, conclude that they should take the bottle of scotch and move to Maui.

Failing that remedy, however, we warmed the cockles of our hearts by realizing that we would need only about one-quarter the electricity it would take to warm the air with electric baseboard (resistance) heat. We had one friend say, “Wait, that can’t be true. Aren’t you still heating the air with electricity, except that you’re using a motor, not an electric baseboard heater?” No, we’re pumping heat. If we were just adding heat, nothing would be made cooler in the process. In fact, we return significantly colder water to the ground loop than we started with. (In air-conditioning season, we return warmer water.)

Since the refrigeration unit in this system moves heat both ways (heating and air conditioning), it is called a heat pump, not a refrigerator. In a geothermal system it is sometimes called a ground-source heat pump. This implies that there might be other sources than the ground, and in fact the most common source for heat pumps is air. If you come from the South, air-to-air heat pumps may be familiar. (Right now, you may also be wondering why you left.) Those pumps are really very close to being air conditioners installed backward. The problem is that they don’t work very well when outdoor temperatures drop below 40 degrees. In fact, they switch on conventional electric heating elements at low temperatures.

We discovered that there is at least one air-to-air heat pump for northern climates, and it actually does extract heat from winter air. It’s called the Acadia and is made by Hallowell International Heating and Cooling Systems in Maine, where even the moose wear woolly socks. It’s not quite as efficient as a ground-source heat pump because of the lower temperature of the source of heat (winter air), but is very good nonetheless, and its installation is no more complex than installing central air conditioning. Unfortunately, for technical reasons it doesn’t qualify for the unlimited 30 percent tax credit—only the credit up to $1,500. Even with our requirement of drilling wells, the financial incentives still made geothermal more attractive for us.

So what about these wells? And the plumbing? One friend, having seen a system in the home of an engineering professor, thought that we’d have a wall full of pipes and valves to adjust. Another asked if geothermal was where they had to drill 30-foot-deep wells in your basement.

As much as I’d love to see both of those happen (being a geek)—no. Watching a drill rig try to work in our basement would be perverse fun of a sort. We certainly could have had pipes with valves if we wanted an engineering busy board, but in fact we have two small insulated pipes coming through the basement wall, hugging the ceiling, going to the heating system. The heating system is strikingly unremarkable, easily mistaken for a conventional furnace. The actual heat pump that does the magic is only about as big as a garbage disposal.

We did have about a five-week period of construction, indoors as well as outdoors, due to the need for ductwork. If we’d had enough land, we would have been digging out a hole about 30 by 80 feet to lay the ground loop. This being Harvard and we being on ledge, we had to drill wells, which is substantially more expensive.

The result? It’s too early to prove the efficiency, but we have a nice, even, low heat that follows the dictates of a programmable thermostat. Unlike fuel-burning furnaces, heat-pump systems deliver a low heat, and users report that the winter air is far less dry as a result. Since the house still has solar gain, we’re also looking forward to having our solar-heated air recirculated and filtered through the system by the fan.

The woodstove still resides in the center of the house, having earned a quiet retirement, but is still capable of swinging into action in the event of a power failure. We do still have a lot of wood as a consequence of Harvard’s last power-killing ice storm. But my days of traipsing out for 50 pounds of wood once my fingers get too cold to type are, we hope, over.

Dave Kay lives on Mass. Ave. and can be contacted at dave@greenlifeanswers.com. To ask him questions, read his blog, and see videos about this geothermal installation, visit www.greenlifeanswers.com.