Free Thermal Energy From The Earth, Air, Or Water

For a homeowner, one of the biggest ongoing expenses is heating (and, with growing frequency in our area, cooling) the dwelling. We humans have tried all sorts of things—wood, coal, fuel oil, natural gas, electricity—and still do. Fortunately, the best, cleanest, and most affordable yet—green energy, from the earth, outside air, or nearby water—is now available for all home types.

Instead of relying on conventional furnaces or standard HVAC (heating, ventilation, and air-conditioning), a relatively new device called a “heat pump” offers sizable energy-cost savings and an attractively short payback on the owner’s capital investment.

There are three types of heat pumps: geothermal, air-to-air, and water source. Heat pumps work by collecting heat from the air, ground, or water outside your home that’s substantially different in temperature from what’s inside the home then concentrating or directing it for use.

Of the three types, geothermal and air to air systems are more typical around here. Of the two,  geothermal is more efficient for several reasons. Because water transfers heat from the earth below the frost line, which is generally about 55 degrees Fahrenheit, and water has a much larger capacity to hold heat than air. The earth also holds a constant temperature, which air obviously does not. About 70 percent of the energy needed for heating will come from this free, nonpolluting resource.

A geothermal heat pump employs a large underground pipe system called a heat exchanger. This can be horizontal or vertical. Either requires substantial digging to install and this will be the major expense. The cost of the equipment is likely to be amortized in a few years and the small operating expense is the electricity for the heat pump unit that includes a small pump, a fan for forced-air heating, and a supplementary electric heater. The ideal time for switching to this system, therefore, is when constructing a new building or performing major renovations. Do it for the savings on heat alone in this area; consider the air conditioning a bonus. And consider also making the system the source of your hot-water needs, possibly through a “desuperheater” that recovers waste heat from the pump. Other advantages of a geothermal system are that it is environmentally friendly, an energy miser, reliable, and no fuel is required. If you use solar energy for the electricity needed, your heating and cooling becomes even cheaper and is completely green.

When compared to fuel-oil or natural-gas heating, geothermal HVAC systems can be three to six times more efficient on cold winter nights, according to the U.S. Energy Department.1 And geothermal HVAC systems are typically 25 to 50 percent more efficient than traditional air-source heat pumps. For every unit of energy a geothermal heat pump system uses, at least 3.5 units of energy are supplied as heating, somewhat less as cooling.2 In other words, a geothermal heat pump multiplies the energy it uses, making it 350 percent efficient, while the most efficient fossil-fuel furnaces only return 97 percent of the energy they burn as heat.

An air-to-air system extracts heat from the air to heat your home in cold months, or heat from your home’s interior itself to cool the house during hot months. There are multiple configurations and types of air-to-air systems: ductless, ducted, short-run ducted, split, mini-split, packaged, multi-zone, and single-zone. The US Department of Energy has lots of good information to help you narrow the choice: energy.gov.

Some of the many advantages of air-to-air are low installation costs, ease of use, and significant heat transfer capacity. When their seasonal efficiency rating (SCOP) is 3.0 or more—3kW of heat generated for every 1 kW of electricity—they are highly efficient.

A significant disadvantage kicks in when the outside air drops much below freezing. The pump’s electric power consumption increases to ensure the house stays warm. But in a typical winter around here, the days of deep cold often number no more than a week or two. Another possible drawback comes from the constant influx of air from outdoors: the pump also brings in the dust, pollution, and odors the air carries.

A water-source system works like a geothermal one, except the pipe is looped over the bottom of a nearby body of water that’s at least ten feet deep, to avoid freezing solid, and large enough to be unaffected by your thermal energy transfer. This situation is rare, but properly designed and installed provides results similar to geothermal installations.

If one lives in New York State and is considering upgrading to any heat-pump system, NYSERDA offers a financial incentive through the NYS Clean Heat Program. It works through your local utility.

For further reading:


A geothermal heat pump, this one from Trane. It installs outside the home, attached to the buried heat exchanger. Others live in the basement.

This air-to-air unit is a “mini-split,” meaning heating and air-conditioning coils are in the same enclosure. it installs inside with an outside aperture.