If the soil conditions are not conducive to trenching, a vertical loop may be the choice. Vertical systems are used where land is too rocky for an economic horizontal system, and for commercial or educational facilities.
Spacing vertical boreholes can be in any variety of ways - lines, squares, rectangles, grids - depending on available land areas and system borehole requirements.
The ground heat exchanger may be either series or parallel piping. Each has both advantages and disadvantages.
- Single flow path and pipe size
- Higher thermal performance per foot of pipe, since a larger diameter pipe is required.
- Larger water or antifreeze volume of larger pipe
- Higher price per foot of piping material
- Increased installed labor cost
- Limited length due to fluid pressure drop and pumping costs
Fabricated from smaller diameter pipe, which is generally less costly, special care must be taken in flushing to get all the air out of the piping loop.
- Lower cost pipe
- Less antifreeze required
- Special attention to assure air removal
- Attention to balanced flow, within plus or minus five percent, in each parallel path is required
- Supply and return headers of larger pipe to minimize the pressure loss down the length of the pipe run
- Loops extending from the supply header down the borehole and back, connected to the return header.
- Reverse return or compact headers to allow each loop to have approximately the same flow
- U-bends, a 180-degree fitting used at the bottom of the borehole
- Single U-bend with 0.75- or 1-inch pipe loops, 1.25- to 2-inch headers, parallel water flow, 150 to 280 feet per ton bore lengths, and 300 to 500 feet per ton pipe lengths.
- Single U-bend with 1-, 1.25-, 1.5 or 2-inch pipe, series water flow, 125 to 225 feet per ton borehole lengths, and 250 to 450 feet per ton pipe lengths.
Further information, pipe data, pressure drop information and calculational procedures are given in Section 4 of the IGSHPA Installation Guide. Some heat pump manufacturers also have selection software available.