Installing a geothermal well field means drilling a grid of vertical bores under a parking lot, athletic field or lawn, setting a sealed pipe loop in each one, tying them together with buried header piping, and putting the surface back the way it was. Most of the work happens outside the building, and when it's done there's little to see on the surface. Here is how it goes, step by step, and what a facilities director or building owner should expect while it's happening.
If you're new to geothermal, start with what geothermal HVAC is and how it works. This post covers the part underground.
Before anyone drills: the site survey
The well field has to fit the site, the building's load and everything already in the ground. Before a layout is drawn, we look at:
- Where the field can go. Parking lots, athletic fields, lawns and green space are the usual spots. Ground that's due for rebuilding anyway, like a worn-out parking lot, is the best candidate, because the surface work gets done once.
- What's already underground. Water, sewer, storm, gas, electric and communications lines all get located and marked before the layout is final.
- Access for the rigs. Drill rigs, pipe trailers and support trucks need a way in, room to set up and clearance from overhead lines and trees.
- Where the header runs. The piping from the field has to reach the building's mechanical room, so the route to the building is planned with the layout.
The test bore
On a commercial job, the first hole is usually a test bore. It's drilled and looped like a production bore, then tested to measure how well the rock and soil under your site carry heat. That number, together with the building's heating and cooling load, sets how many bores the field needs and how deep they go. It's the difference between sizing the field from a rule of thumb and sizing it from your ground. It also tells the driller what the rock is like before the full rig schedule is set.
Drilling the bores
The U.S. Department of Energy describes vertical loops as holes about four inches across, drilled roughly 20 feet apart and 100 to 400 feet deep, and notes that vertical systems are the usual choice for large commercial buildings and schools because they need far less land than horizontal loops. On a specific job, the test bore and the load calculation decide the actual depth, spacing and count.
Each bore goes like this:
- Drill. The rig drills to depth through the soil and into rock.
- Set the loop. A U-shaped loop of high-density polyethylene pipe, with a factory-fused bend at the bottom, is lowered to the bottom of the hole.
- Grout. The bore is filled with grout from the bottom up. Grout seals the hole so surface water can't travel down it, and it gives the pipe solid contact with the surrounding ground so heat moves well.
Drilling produces cuttings and drilling fluid. These are contained on site and hauled off or handled as the job's plan calls for, so they don't run into storm drains or onto neighboring property. The work area is fenced off while rigs are running.
Pressure testing and flushing
A geothermal loop is meant to stay sealed for decades, so it gets tested more than once. Loops are pressure-tested before they go into the ground and again after they're installed, and the whole system is tested once the header is connected. Before startup, the loop is flushed and purged of air and filled with water or water mixed with a food-grade antifreeze. Nothing in the system is pumped out of the ground or into it.
Header trenches and the vault
Once the bores are in, the loops are joined together with horizontal header piping in shallow trenches. The headers usually come together in a manifold, often in a buried vault, where circuits can be balanced and isolated. From there, supply and return mains run to the building's mechanical room. Pipe joints are heat-fused, so the buried piping is one continuous material with no glued or threaded joints in the ground.
Trench work follows OSHA's excavation rules. Per OSHA's trenching fact sheet, trenches 5 feet deep or more need a protective system such as shoring or sloping unless they're entirely in stable rock, spoil piles have to stay at least 2 feet back from the edge, and a competent person inspects the trench each day before anyone goes in. Most header trenches are shallower than 5 feet, but connections at the building and the vault can go deeper.
Restoring the surface
When the piping is buried and tested, the site gets put back. A lawn is graded and seeded or sodded. A parking lot gets its base and paving. An athletic field is regraded and its surface rebuilt. Because the loop is sealed and needs essentially no maintenance, the surface on top can be used like any other parking lot or field.
The one thing we leave behind on paper is a record drawing of where the bores, headers and vault are. Keep it with the building's plans, so future projects on that ground know what's underneath.
Timeline and disruption
A typical commercial well field takes a few weeks to drill. The real schedule depends on the number of bores, the rock and how many rigs are on site. Here's how the phases usually break down:
| Phase | What happens | What you'll notice |
|---|---|---|
| Survey and test bore | Utility locates, layout, one test bore and the heat test | One rig for a short stretch |
| Drilling | Bores drilled, looped and grouted | Rigs, trucks and a fenced work area; equipment noise during work hours |
| Headers and vault | Trenching, fusing pipe, pressure testing | Open trenches inside the fenced area |
| Connection to the building | Mains run into the mechanical room | Work at the building wall and inside |
| Restoration | Backfill, grading, paving or turf | Normal sitework |
For schools, the best window is usually summer, when the lot or field is empty. Drilling can also run during the school year if the work area is fenced off from students and traffic. For churches and businesses, the well field can be phased so part of the parking stays open. Indoors, setting the heat pumps is like any HVAC replacement and can be scheduled separately from the drilling.
Drilling and the federal credit
The federal credit for geothermal heat pump systems depends on when construction begins. IRS Publication 6045 lists geothermal heat pump property as eligible if construction begins before January 1, 2035, and IRS Notice 2018-59 lists installing ground heat exchangers as an example of on-site physical work that can count toward beginning construction. That makes the well field schedule worth discussing with your tax advisor early. For how the credit is figured and paid, see how the credit works for schools and our geothermal questions page.
Next step
A feasibility review looks at your building's load, the ground you have to work with, where a well field could go and what it would cost compared with a conventional replacement. To see the cost side first, try the geothermal calculator. Then request a free estimate or call 636-327-4238.