5 Tips on Designing Vertical or Slinky Geothermal Loop Fields

Designing a geothermal ground loop field requires balancing flow rates, pipe sizing, and installation costs to hit your target performance. Here are five tips for getting vertical bore and slinky loop designs right.

Key Points

• Vertical and slinky (horizontal) loop fields are the two most common geothermal ground heat exchanger (GHEX) configurations
• Target flow rate for vertical bores depends on pipe size: 2.8–3.2 gpm for 3/4″ loops, 4–6 gpm for 1″ loops, and 5–9 gpm for 1.25″ loops
• For slinky (horizontal) loops, 3/4″ pipe is strongly recommended — larger pipe sizes are difficult to handle in the field
• Both vertical and slinky designs can deliver equivalent heat pump performance if designed to the same entering water temperature (EWT) and flow rate
• Choosing between vertical and horizontal is primarily a matter of available land area and installation cost

Here are a few tips on on vertical and slinky bore design.Vertical Bore Design

1)      The target (optimum) flow rate versus pipe size is:

• 2.8 – 3.2 gpm per loop for ¾” loops
• 4 – 6 gpm per loop for 1” loops
• 5 – 9 gpm per loop for 1.25” loops

Staying within those flow ranges per loop will keep you well below the maximum recommended flow rate for head loss (4 ft per 100’ of pipe length, Figure 5.4) and above the minimum flow rate required for turbulent flow.  For the vertically-bored design, I recommend using two loops for 6 gpm per loop with 1.25” pipe.

2)      There were a few questions regarding the maximum recommended flow rates shown in Figure 5.4 and the optimum flow rate ranges I provided during the PowerPoint lesson.  For example, I recommend sticking to the suggested flow rate range of 5-9 gpm per 1.25” loop.  Keep in mind that we shoot for an optimum head loss of 1’-3’ of head loss per 100 ft of pipe length when selecting pipe size for a given flow rate.  The upper bound shown on the graph (at approximately 12.5 gpm per 1.25” loop) shows the absolute maximum recommended head loss through pipe of 4’ of head loss per 100 ft of pipe length.

Keep in mind that with 1” and 1.25” pipe, our run lengths are usually longer (deeper bores to represent more GSHP capacity).  If your head loss is in the neighborhood of 4 ft per 100 ft of pipe length, the pressure drop through a 400 ft bore (800 ft run length) can be significant (4 ft of head per 100 ft of pipe x 800 ft run length = 32 ft of head in the loop itself!).

Slinky Design

1)      As mentioned in the lesson for this week, I recommend sticking with ¾” loops (one loop per ton for design) for horizontally trenched systems.  Pipe sizes that are larger (1” and 1.25”) are very difficult to handle and can really give you headaches in the field when trying to construct the slinky loops.

2)      For ¾” slinky loops, I generally recommend using somewhere around 600-800 ft pipe lengths in the trench (the “Pipe Length per Trench” field in LoopLink).  You can lower GHEX design lengths without dramatically affecting performance by using a 27-28 degree minimum EWT for this project.  On much of the design work that we do, we’ll use anywhere from a 25-28 degree minimum EWT for slinky loops, especially when soil temperatures are less than 50°F.

3)      The objective in GHEX design is to design to provide the same performance with the lowest up front cost.  The only thing that the GSHP unit cares about from the GHEX is water flow rate (gpm) and EWT.  If a vertically-bored GHEX and a horizontally-trenched GHEX are designed to provide the same EWT and water flow rate, heat pump performance will be the same.  At that point, choosing one over the other is a matter of available land area and installation cost.  Your operating costs will only change with a change in system efficiency (e.g. – when you use a different minimum EWT).

Frequently asked questions about geothermal loop field design

What is a geothermal loop field?

A geothermal loop field, also called a ground heat exchanger (GHEX), is a system of pipes buried in the ground that circulates fluid to exchange heat with the earth. It is a core component of a ground source heat pump (GSHP) system, allowing the heat pump to transfer heat to or from the ground depending on the season.

What is the difference between a vertical bore and a slinky geothermal loop?

A vertical bore geothermal loop involves drilling deep boreholes straight down into the earth and inserting pipe vertically. A slinky loop is a horizontal system where coiled pipe is laid in shallow trenches. Vertical systems require less land area but have higher drilling costs. Slinky systems are less expensive to install but require more surface area.

What pipe size is recommended for vertical geothermal bore loops?

For vertical bore loops, the recommended target flow rates by pipe size are 2.8–3.2 gpm per loop for 3/4″ pipe, 4–6 gpm for 1″ pipe, and 5–9 gpm for 1.25″ pipe. Staying within these ranges keeps head loss within acceptable limits while maintaining turbulent flow for efficient heat transfer.

What pipe size is recommended for slinky geothermal loops?

For slinky (horizontal) geothermal loops, 3/4″ pipe is strongly recommended. Larger pipe sizes such as 1″ and 1.25″ are much harder to work with in the field when constructing slinky coils in trenches.