5 Tips on Designing Vertical or Slinky Geothermal Loop Fields

We’ve found it useful to focus on both articles that will help companies with their sales and marketing AND design and installation. A few weeks ago, I shared a piece – thanks to Ryan Carda – on geothermal flow path analysis for ground loop design that came from a discussion forum from our advanced geothermal design course.  My plan is to share more technical discussions that are happening within the course. If you are installing or designing geothermal projects, these articles will be useful to you if you never take the training. This is my goal.

If you need to learn more about the basics of high performance building and HVAC technologies and principles, we’ve created an amazing free class for you. Our Free Course: High Performance Building and HVAC is the most in-depth, best free course on high performance buildings and HVAC systems available on the internet. You’ll learn from all of the smartest industry experts. The class has 20+ of video lesson, plenty of reading assignments and a number of free tools. It will drastically decrease your learning curve on these subjects. Topics include, residential building enclosures and ventilation, zero net energy homes, passive house design principles, biomass heating, ground source heat pumps and solar thermal. Click here to sign up for High Performance Building and HVAC. 

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).