DUMMERSTON — It was 5 degrees outside when we woke up one morning. Snow devils danced across the field illuminated by early morning winter sunlight. Inside, the blower on our 1980s vintage wood/oil combination furnace was humming away. It was a cozy 68 degrees, our preferred house temperature.
Nothing is extraordinary about these observations, except for one small detail - not a single drop of fossil fuel or even so much as a matchstick of wood is being consumed. Instead we rely on the solar energy stored in the earth's crust to heat our home.
When we moved to our 18th-century farmhouse, we had a strong desire to reduce our home heating carbon footprint to as low as we could get it - zero if possible. We explored a lot of heating alternatives. Some seemed impractical, especially for a retrofit to a historic home. Others involved technologies that we felt had not yet arrived. And others were good on the renewable side of the equation, but still put at least some carbon and other pollutants into the atmosphere.
Of all the feasible technologies, using a ground source heat pump to extract the solar heat stored in the earth's crust seemed the most practical to us.
This technology, which has a long history of success in northern Europe, relies on simple, well-established heating and refrigeration principles and equipment. We could use our current hot-air heating ducts, and the system would provide us with air conditioning when run in reverse in the summer. We could also keep our current wood/oil combo furnace intact as emergency backup.
Now that the system is installed and working, we often find ourselves needing to explain this little home heating and air conditioning zero carbon emission miracle to our friends. Here's what we tell them.
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The earth's crust, once you get below the frost line here in Vermont, is a pretty constant 51 to 54 degrees. This heat comes not from the earth's magma (too far away); rather, it is stored from the sun (farther away, but strong enough to heat the earth's surface).
The trick is getting this heat out of the earth and into the house. There are different systems available for doing so, and the particular one we chose, because of its high efficiency, came from Advanced Geothermal Technologies in Reading, Penn. and was installed by The Good Heat Company.
This system circulates a refrigerant in sealed loops in the ground to pick up the heat stored there (or coolness in summer, relative to seasonal temperatures). There are 8 loops each 70 feet long in holes drilled at an angle so that the disturbed area on top of the ground is only about 10 feet by 10 feet.
The refrigerant comes from the ground loop at around 54 degrees. It passes through a compressor (just like the one in your refrigerator or air conditioner, only bigger) where compression heats it to 150 degrees or so. It then continues to a coil in the furnace where the blower blows air across it and into the ductwork. The air leaving the furnace is about 110 degrees - hot enough to warm the house.
As the air blows across the coil with refrigerant in it, the refrigerant cools down to about 24 degrees and loops its way from the furnace back into the ground to pick up more heat. The whole process is reversed in the summer to provide air conditioning. The compressor unit is housed in the basement and is very compact and as quiet as a refrigerator.
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Some of our friends still don't quite understand how we can go from 54 degrees ground heat to 150 degrees at the other side of the compressor. Obviously, these are the friends who didn't pay attention in physics class, so we try to stay non-technical.
We begin with a question: “Have you ever noticed the heat that comes out from behind an old refrigerator when it's running or how hot it is at the back of an air conditioner when it's running - way hotter than it is in the house or even outside?” Most say, “Yes.”
“The principle,” we explain, “is exactly the same.”
The refrigerant picks up heat from the ground. It then gets compressed and gasified to raise it to even higher temperatures. The air blowing across it takes the heat out and returns it to the ground colder than when it started.
“Aha, I think I get it,” our friends say with hesitation in their voices, “but how can you claim zero carbon emissions when you're using electricity to drive your compressor?”
We have a two-word answer: “Cow Power.”
By subscribing to 100 percent Cow Power we are using zero-to-low-carbon and renewable energy sources for our electricity. Ultimately, we'd like to generate our own electricity as wind, solar photovoltaic, or other technologies become more practical and affordable as this will truly be zero carbon, but right now buying electricity generated by renewable resources like manure methane, wind, and hydro seems the best choice.
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“So,” ask our friends, “what happens when the power goes out?”
Of course, the same question arises with oil or propane, but our friends like to challenge us and seem to forget that fossil fuel furnaces rely on electricity as well. We could use a generator, but still have the option of burning a small wood fire in our backup wood/oil combination furnace.
Lastly, they ask, “How expensive is it, and what's the payback?”
The expense depends on whether you can re-use your existing ductwork and furnace. This aside, it is definitely more expensive than installing a new furnace and, regrettably, the government now provides no incentives for installing ground source heat pump systems. We got a meager $300 tax credit.
The payback as the price of oil continues to rise (and the price of cordwood along with it) looks better and better. Based on the 2007-08 heating season, our total extra electrical costs were $1,054. This is our total heating bill. Not a bad price for heating a 2,400-square-foot house with average weatherization.
Typically, we might expect to use 1,000 gallons of oil annually, which would cost $3,300 at $3.30 per gallon, so we came out at about one-third the cost of heating with oil. In 2008-09, with oil prices at $5 per gallon, we will still be paying around $1,000 versus $5,000. At this rate the investment will pay back in under 6 years.
There is another payback. This is the one of knowing that we are no longer contributing to global warming in the heating our home. It is immediate and strong.
It's the least we can do for Mother Earth in return for the privilege of waking up to 5 degree days with snow devils dancing across the fields as we look out across the valley to Black Mountain covered in snow.