Ground source heat pumps last longer, require much less maintenance and are far more energy-efficient than conventional air-source systems, such as a furnace or boiler. GSHPs are also more environmentally friendly, having a lower refrigerant charge per ton of cooling.
The only possible drawback is the installation cost. But the small farm with a tractor handy takes care of that problem.
Did you know that more than 50,000 new GSHP systems are installed every year in the United States, in every possible climate and circumstance?
GSHP are ideal for colder climates, too, maintaining a constant operating efficiency down to well below 0 degrees F. The coefficient of performance is a uniform 3.0, while air-source
system efficiency declines uniformly to a COP of 1.0 at 0 degrees, the same as an electric strip heater (pure electric resistance heat).
A household GSHP can also be the heat source for domestic hot water. The hot water costs next to nothing in the summer because the GSHP extracts heat from the house directly into the water heater. Ground-source systems are so efficient because they transfer heat to and from the ground, which is always cooler than the outside air in the summer and warmer in the winter.
How a GSHP Works
The GSHP system equipment inside the house is similar to a conventional air-source heat pump: ductwork plus a fan blowing air across finned cooling coils. However, instead of transferring thermal energy to/from the air at an outside condensing unit, the GSHP exchanges heat with water in a closed-loop buried in the ground.
A small, quiet and very efficient pump circulates the water in the loop.
The ground loop is most often small-diameter piping ready-molded in loops, similar to a slinky laid flat. This loop can be buried vertically in a deep trench created by a backhoe (16 inches wide by 8 feet deep) or laid horizontally in a horizontal ditch made by a bulldozer (4 feet deep by 6 feet wide).
If the farmhouse has a pond nearby, the ground loop can be put in the water for an even more efficient system. First, water evaporates from the pond’s surface in the summer, cooling the body of water. Then, in the winter, the dark bottom of the pond maximizes solar energy absorption, heating the water.
Both processes make the system more efficient, saving on your utility bills.
Got a Tractor?
These trenches and the labor involved account for most of the higher cost of a geothermal system versus conventional heat pump systems. A farmer with a tractor and backhoe can dig the ditches and lay the pipe, leaving the GSHP contractor to fill the tubing, seal it and pressure test the installation.
The ground loop field can also be created by drilling vertical wells, roughly one 125 feet well per ton of cooling. The contractor puts the loop piping in each well then seals the pipe in tight with a bentonite-soil mixture to maximize heat transfer from the piping to/from the ground.
You can have any combination of these arrangements for the ground loop to fit the topology of your land: horizontal and vertical slinky, vertical wells and pond/stream/river water. You can tap the ground loop to cool and heat air in the house or the barn, provide domestic hot water, or preheat an industrial-grade water source to clean in the milking barn or other facilities on site.
Loop the Loops
A unique feature of this water loop is that you can have any number of auxiliary systems on the loop to heat and cool all year. For example, a water heater takes heat out of the loop, raising the loop temperature. The AC adds heat as the house cools off.
GSHP loops work because of a good thermal connection to the ground. Moist earth is always the ideal and the most efficient. You can even install the ground loop under or integral to the septic field.
If your area is prone to drought conditions, it’s best to put the loop as deep as possible—perhaps the vertical well arrangement, if possible.
The ground loop has another profound influence on the operation of the HVAC system: It adds thermal mass.
When you add thermal mass to a system, it’s like adding weight to a moving vehicle and momentum. Take your foot off the gas, and you keep moving! The heavier the vehicle, the farther you go without the addition of any further energy.
Let’s say you have a bare-bones house, 2-by-4 wood framing and fiberglass insulation—nothing special. When you add thermal mass to the interior—furniture, beds, cabinets, carpets, bookshelves, appliances, brick veneer—you improve the operation of the heating and cooling.
Why? Because once the interior is at the temperature set point, the air doesn’t heat up in the summer until the entire volume of the house—all the mass inside the building envelope—changes temperature. The same is true in the heating season.
The temperature inside a building with a high thermal mass—the extra mass should be inside the insulation—always lags the outside temperature. In the summer, the temperature inside rises much slower than the outside temperature. Moreover, when the sun goes down and gets cooler outside than inside, the direction of heat flow reverses.
Most of the outdoor heat never even gets inside the building envelope!
The ground loop adds all the thermal mass of the earth inside and around the loop piping. Soil is very heavy—especially moist soil!
A vertical well system has so much added thermal mass that the thermal variation is not only daily but seasonal. For example, extracting all that heat from the house in the summer causes the ground loop field temperature to increase many degrees above normal.
So when you switch to heating later in the year, you have all that extra heat save up to extract at a much higher efficiency—vice versa in the summer.
This article originally appeared in the September/October 2021 issue of Hobby Farms magazine.