Designing Your Closed Earth Loop

A geothermal heat pump's earth loop installation is not just a matter of burying some pipes and hooking these pipes to your geothermal heat pump. Your earth loop must be matched to the home, and the geothermal heat pump, that it is coupled to. So, the first step in sizing an earth loop is having a correct heat loss/gain calculation done. We need to know how many BTUHs your earth loop needs to supply to your geothermal heat pump, so it can heat your home correctly. This also tells us what the flow rate through your earth loop will have to be.

Next, we need to know about the soil that the earth loop will be buried in. How fast can it transfer heat (the rate of thermal diffusivity) into, or out of, each lineal foot of the earth loop? To accurately determine your soil's thermal diffusivity, we need to know what your soil conditions are, including the type/s of soil (clay, sand, gravel, silt, top soil, and many others), and the moisture content of the soil. You will need to dig a couple of test holes so a soil analysis can be done. You can either do this soil analysis yourself, with our instruction, or you can hire a soils engineer.

The thermal diffusivity of the soil, along with the BTUH transfer needed by the geothermal heat pump, will together determine how many lineal feet of pipe (of a certain diameter) will need to be installed in the ground. For example, for horizontal loops: moist clay soil will usually need 600 lineal feet of pipe per ton, very dry soil needs at least 1200 lineal feet of pipe per ton, sometimes more, and saturated soil (where water seeps into the trench as you dig) needs 425 lineal feet of pipe per ton.

Once we know how many feet of pipe the earth loop will need, we must calculate its total pressure loss. This includes the friction inside the pipe, the manifolds, the pipe and fittings that will be inside the home, the hose kit, and the geothermal heat pump's water-to-refrigerant heat exchanger (some have much larger pressure losses than others). We also have to add the friction loss caused by antifreeze viscosity, and the friction loss at the lowest temperature in the heating season. Once we know the total pressure loss, we must adjust the loop (for example, by using smaller diameter parallel loops, or different pipe and fittings inside the home) if the pressure loss would cause high pumping costs.

Designing the earth loop is a balance between how much power the circulating pump will use because of the diameter of the pipe, and how much the pipe will cost because of its diameter. If the diameter of the pipe is small, you will save money on the cost of the pipe; but since the pipe diameter is small, the pumping power needed will be high, and this will greatly increase your energy bills. On the other hand, if the diameter of the pipe is large enough so that the smallest pump can be used, the energy cost that the very small pump saves will never pay for the high cost of the larger diameter pipe.

It is equally important for the geothermal heat pump, earth loop, and air ducting (or radiant heat piping) to be designed correctly.

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