Horizontal Loops

If sufficient land area suitable for drilling or trenching is available, then horizontal loops may be the most economic. A horizontal system will use a number of horizontal trenches. Piping system designs range from a single pipe, to multiple pipes arranged vertically in a narrow trench to multiple pipes in a wider trench.

The ground heat exchanger may be either series or parallel piping. Each has both advantages and disadvantages.

Series System
Advantages:

  • Single flow path and pipe size
  • Higher thermal performance per foot of pipe since a larger diameter pipe is required

Disadvantages:

  • 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.

Parallel System
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.

Advantages:

  • Lower cost pipe
  • Less antifreeze required

Disadvantages:

  • Special attention to assure air removal
  • Attention to balanced flow, within plus or minus 5 percent, in each parallel path is required

Examples:
Some examples, depending on system size, of typical horizontal ground heat exchangers are:

  • Single 1¼ to 2 inch pipe, series water flow, 350 to 500 feet per ton nominal length, buried 4 to 6 feet deep
  • Two-pipe using 1¼ to 2 inch pipe, series water flow, 210 to 300 feet of trench per ton with 420 to 600 feet per ton nominal length, buried 4 and 6 feet deep
  • Four layer using ¾ to 1 inch pipe with 1¼ to 2 inch headers, parallel water flow, 125 to 200 feet of trench per ton with 500 to 800 feet per ton nominal length, buried 6 feet deep with 12 inch spacing

Further information, pipe data, pressure drop information, and calculational procedures are given in guides published by IGSHPA. Some heat pump manufacturers also have selection software available.