Geothermal Heat Pump Sizing
Conducting a heating / cooling load analysis of your home is the only way to properly insure that the correct size (output capacity) of heat pump is installed in your home. A quick rule of thumb however is to refer to the chart below for general sizing. The 45, 55 and 65 designations of BOREAL equipment indicate the nominal BTU output of the machines:
- Boreal 45 = 3 ton / 43,000 BTU. Heats / Cools up to 1800ft²
- Boreal 55 = 4 ton / 54,000 BTU. Heats / Cools up to 2700ft²
- Boreal 55 = 5 ton / 65,000 BTU. Heats / Cools up to 3500ft²
Water Wells
Pumping water from drilled water well is a common method of coupling the Geothermal Heat Pump (GHP) to the earth extraction of thermal energy. If a well already exists, this is the least expensive method of providing a source of energy for the heat pump. The well should have the following characteristics:
- DRILLED well of 5” diameter or larger so that a submersible pump can be accommodated
- Well casing properly sealed or grouted into rock
- Water flow preferably entering at a depth of 75 ft. or more to insure reliability during dry weather
- Temperature of water at least 40°F
- Well must be able to supply requirement of BOTH the heat pump and residence usage at the same time with maximum drawdown from static level
Water pump system
- A submersible pump is generally required to produce the volume if water required and for longevity
- If the pump is also supplying the household requirements it must be able to pump the required water flow listed above at a minimum of 30 psig. at the dynamic pumping depth of the well
- Always select a pump by referencing the pump manufacturer's chart
- Use a minimum of a 30 gal. equivalent air bladder tank to prevent short cycling of the well pump
The earth connection
Water based ground loops
Horizontal
A plastic pipe horizontal ground loop is a collectors system which is buried in the ground at a depth of approximately 6 ft. A successful configuration will have the following characteristics:
- Pipe is type 3408 (SDR 11 160 psig.) high density polyethylene
- 500 ft. of pipe is required for each ton of capacity in a horizontal configuration
- A typical configuration consist of a two pipe per trench system wherein a 250 ft. x 4ft. wide trench is dug and a 500 ft. roll of 3/4 " plastic pipe is rolled out as a "U" tube with 4 ft. of horizontal pipe separation
- Multiple parallel loop systems must employ a reverse return (first pipe out is last one back) header technique to insure even flow through all loops
- Horizontal loop systems may also be constructed as a single continuous length of 1 1/2 " plastic pipe of the length shown in the table below
- Installers have generally gravitated to the multiple loop parallel system since the 3/4 " pipe is easier to handle, less expensive and produces a lower pressure drop in the system allowing for the use of a smaller circulator pump
- A geothermal loop system does not allow the use of mechanical joints of any kind outside the building. All underground joints are made by fusing the plastic pipe with socket or butt fusion techniques
Vertical
A vertical ground loop system is similar to the horizontal loop except that the tubing loops are installed vertically in 4-6" boreholes. Special "U" bends are available to allow the insertion of (2) plastic pipes down each borehole. See sample chart or comsult Boreal ® Installation Manuals on depth and spacing information. Vertical loops are an effective solution in locations where available land is at a premium.
Additional loop equipment required
In addition to the plastic pipe itself, the geothermal loop system also requires one or two (depending on the system size) small circulator pumps to move the loop fluid from the ground to the heat pump. The loop fluid consists of water and a freeze depressant such as methanol to allow the fluid temperature to drop below 32°F during winter operation. Your local loop installer or government environmental branch can advise you of an acceptable antifreeze suitable for GHP use.
