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May 11, 2008

Geothermal Installed!


Heating with our new electric geothermal "furnace" will cost less than half what it costs to heat with oil give current oil and electricity costs, even though electricity at $.20/Kwh here is much more expensive than in many parts of the country. This is an environmental investment which can be cost justified even without any subsidy; the whole installation was less than $20,000. Pictured below is the "furnace" – actually it's a heat pump. It's about three feet high and it's relatively noisy when it runs.


Here's how it works: the compressor part of the heat pump does the same thing that the compressor in an air conditioner or a refrigerator does; it squeezes a gas which cause that gas to heat up. In a refrigerator the heated gas runs through coils in the back or under the unit so it can dump that heat into the air; in that case, when the gas expands, it's cooler than it was before the whole process started and it makes the inside of your refrigerator cool but it also makes the air around the coil hot.

Since this heat pump is used for heating, the heat from compression is used in a heat exchanger to heat water. That water ends up heating the house (more on that below). Just as in the refrigerator, the gas is now cooler than it was before the whole process began because we harvested some of the energy in it to heat the water that heats the house. Water in a closed loop that runs from the heat pump over a hundred feet down into our well is used to warm the gas back up again. This exchange cools the water but, as it circulates through the well, which is a constant temperature of about 58 degrees Fahrenheit, the water warms up so that it can, in turn, warm the gas again. The well has to be deep enough or have enough flow through it or some combination of depth and flow so that it doesn't get chilled.

In warmer climates the same heat pump would be used to chill water or air and pump the excess heat back into the well. We don't have air conditioning so don't need that.

The heat pump is quite literally pumping heat out of or into the ground – that's why it's called geothermal. It does require some ground to go into; doesn't have to be a well but, if you have one, that makes it cheaper to install.

Our house is heated by circulating hot water in the floors. This heat pump can heat water to 120 degrees; but, on the coldest days, we might have to have water at 140 degrees leaving the utility room in order to assure that the furthest rooms get warmed. We could have put in an even bigger heat pump but this would have been overkill most of the time. Instead we have a small gas furnace which, when necessary (lots of computing here), steps the water up from 120 to 140. We'll experiment this winter and see how often – if at all –it's really necessary to post-heat the water. Another advantage to having the furnace is that it can run on our standby generator and keep the house warm even during a power failure. The compressor would require a monster generator because of the surge when it switches on.

The tank below is the buffer; the water flowing into it has just returned from warming the house so it, itself, has cooled. The water in this tank circulates through the heat pump which does its best to keep that water at 120 degrees. This water next goes through the furnace; so long as the water is as warm or warmer than the computer tells the furnace its output needs to be, the furnace doesn't fire.

In 2005 the United States used over 65 billion gallons of oil to heat residences. Switching to traditional radiant electrical heat is still more expensive than oil in most parts of the country – although that may well change. But geothermal electric heat use less than a third of the electricity required for radiant electrical heat. Where there is enough ground to put the coils, it's an alternative that's practical today. Look for it in much more new construction.

More on the economics of geothermal heat here although, at the time (last summer), I thought oil would only be $3.00/gallon this winter. This time next year, I hope to have some actual numbers to report.

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