Geothermal Heat Pumps vs. Air to Air

Why Geothermal Pumps Are So Much More Efficient

The heart of a typical geothermal system is a ground-source heat pump that cycles water through an underground piping loop. The water piped through this loop uses soil temperature to warm or cool the heat pump's refrigerant. Significantly, the heat pump is located indoors, like a furnace, which provides advantages we'll get to shortly.

While this equipment may sound exotic, its operation is fairly easy to understand when compared to that of conventional air-to-air heat pumps. A conventional heat pump is really just a central air conditioner that can reverse the flow of its refrigerant. The compressor is located outside the home, and, in the heating mode, it's able to extract some of the heat present in cold, outdoor air and deliver it indoors to a condensing coil. Unlike conventional furnaces, heat pumps don't have to create heat, they just harvest existing heat–and therein lie the savings.

Below about 10 degrees Fahrenheit, however, too little heat is present in the air and a backup heat source is needed to make up the difference, or, in many cases, take over entirely. Even within a heat pump's effective operating range, efficiency is directly tied to ambient temperature.

The colder it gets, the less heat is available and the less efficient the system becomes.
In contrast, a ground-source heat pump, with its underground piping loop, is able to tap a warmer, more stable heat source. The soil below frost level–4 ft. to 6 ft. deep–stores the sun's energy at a more or less constant level, with temperatures keyed to latitude. Subsoil temperatures range from the low 40s in the North to the low 70s in the South.

For purposes of comparison, we'll use the 55 degrees F soil temperature common to much of the central US. This area of the country suffers some extreme temperatures, but also has a fair number of mild days, so it's a reasonable choice.

With a ground temperature of 55 degrees F, the system needs to boost the heat a mere 15 to 20 degrees to reach a comfortable indoor temperature. Compare this to the 40 to 60 degrees maximum differential that an air-to-air heat pump may handle, and even greater differentials expected of standard furnaces, and the logic comes into focus pretty quickly. The only influence outside air temperature has on the equation is in the home's ability to retain heat. Houses lose heat faster on colder days, so all systems work harder in cold weather. But while a ground-source heat pump may need to run more often on these days, it doesn't run less efficiently.

The geothermal principle works about as well for air conditioning. Instead of an outdoor compressor laboring against the heat of the day having to use hot air as its heat-shedding medium, a ground-source heat pump operates indoors, using ground temperature as its starting point. The result is a 20 percent to 40 percent savings over conventional heat pumps and air conditioners.

Of course, lower soil temperatures will reduce heating efficiencies and warmer soil will cut into air-conditioning savings. On average, however, ground-source heat pumps deliver three to four times the energy they consume.

Another big difference between a standard heat pump and a ground-source pump is that a ground-source model is installed indoors. At first blush, this might seem unworkable, if only because of the noise. But these systems are quiet, almost as quiet as your refrigerator, which, after all, is really just another kind of heat pump. The noise we associate with an air-to-air unit comes from the large fan needed to pull air over the compressor coils. But ground-source compressors use water, not air, so they don't need fans. They have, instead, a quiet circulation pump and a compressor that runs at a lower pressure, both sealed in an insulated cabinet about the size of a washing machine.

This sealed environment also pays dividends in other ways. Compressor fins on outdoor units are more prone to casual damage–and the compressor fan draws in huge amounts of dirt and debris that reduce air flow and damage bearings. Further reductions in efficiency can be caused by corrosion or by an out-of-level condition that results from a settling of a compressor that's mounted on the ground. All these factors cause a compressor to work harder and run hotter, up to 450 degrees F on a really hot day, which is hot enough to do damage. Cold starts on cold days also take their toll.

But an indoor unit in its sealed chamber has none of these problems. As a result, the oldest models have been in place for 20 years and they seem to remain efficient with age. This may explain a frequently noted disparity between lab-certified efficiency ratings and field performance. In side-by-side tests comparing ground-source heat pumps to conventional heat pumps and air conditioners, the latter units gradually lose ground. In all likelihood, the difference is the environment they're installed in.