The Ground as Heat Source and Sink
The ground can be treated as a large heat source or as a heat sink. In other words, one can extract heat from the ground or reject heat to the ground. The temperature only a few feet below the surface varies half as much as the ambient temperature. Below 10 feet the temperature remains fairly constant in most places.
There are three general methods of using the ground as a heat source or sink: the well, the horizontal field and the vertical field.
The Well: The oldest method, and, in some places, the easiest, is to dig a well, then pump the water up and through the heat pump before piping it to drain. Many local codes will not permit this approach and will require you to have a second well some distance away to discharge the water back into the ground. This all assumes your location has a readily accessible, adequate and reliable flow of ‘sweet’ water. ‘Sweet’ meaning it has no undesirable characteristics, such as dissolved salts that will corrode away both pumps and heat exchangers very quickly. Local knowledge and test holes can be invaluable.
The horizontal field and vertical field refer to pipe loops in the ground that transfer heat to or from the ground.
The Vertical Field:
1. The field has been prepared and planned, and then vertical bore holes are drilled.
The vertical depth for the boreholes ranges from 50 to 500 feet, depending on ground conditions and the cost to drill the holes to these depths. Boreholes must be spaced well apart to avoid having them thermally affecting each other. The effect is minimized with a row of holes, but this is not always an attractive alternative. A rule of thumb is 20 feet apart, but local conditions, such as underground water flow, can reduce this distance. A test hole can be bored and used to test the heat transfer characteristics of the local soil conditions to help determine the number of wells and spacing required.
2. Durable U-shaped plastic pipe loops are lowered into the boreholes.
3. Each borehole is back-filled with excavated material or with a special mixture to enhance heat transfer with the ground.
4. The ends of the pipes are connected to headers, which are routed back to a building to pumps within the building. The pumps are connected to piping that is circuited to one, or more, water coils, each on one side of a heat pump.
Vertical ground source systems have the following advantages:
e They utilize smaller areas of land than the horizontal system.
e Their performance is quite stable(when spaced and sized properly), since the ground temperature does not vary with the seasons.
e They use the lowest pumping energy and the least amount of pipe.
e They often provide the most efficient performance.
Vertical ground source systems have two disadvantages that vary according to location:
e They are generally more expensive to install than horizontal systems and can be prohibitively expensive in hard rock areas.
e The availability of qualified contractors is very limited in some areas.
The Horizontal Field: This method involves burying pipe loops in trenches or open pits at a depth of at least 4 feet. There is a variety of pipe loop arrangements that are designed to take advantage of local conditions.
Horizontal systems have the following advantages:
e They are relatively easy to install with readily available, non-specialist, equipment in areas without rock.
e For rural residential systems, the land requirement is usually not a restriction.
e They usually have a lower installed cost than vertical systems and they are potentially easier to repair.
Disadvantages
e They require a much larger land area.
e They have a more significant variable system performance than the vertical arrangement, due to greater variations in ground temperature that arise from seasonal temperatures, rainfall and shallower burial depth.
e Their efficiency is generally lower than the vertical arrangement, due to fluid temperature and slightly higher pumping requirements.
Correctly sizing a heat pump for winter heating and summer cooling can be a difficult task. In many climates with cold winters, the winter heating load can be much higher than the summer cooling load. Installing a heat pump that is big enough to do both tasks is often a mistake. If the unit is oversized for summer cooling, it will cycle excessively and dehumidification will be very poor to non-existent. The maximum over-sizing above summer load should not exceed 25% for reasonable summer performance. The winter heating load that is not supplied by the heat pump is best provided by supplemental heat.
One relatively new opportunity to deal with this issue is the two-speed compressor unit. Two-speed units may allow for correct sizing for the summer load by cooling at low speed, while high speed may allow the winter heating load to be more closely met. For heating in these climates, it is very efficient to use a radiant floor system. This is because the temperature difference between the ground and the heat pump heating-supply temperature is lower, thereby, providing a significantly higher efficiency.
An extension of the idea of using “natural” sources for heating or cooling is the idea of using natural ventilation from operable windows. This will be covered in the next section.