What is a Grouted, Geothermal Borehole?

The following blog post is republished courtesy of our friend, Bill Martin, President of the California Geothermal Heat Pump Association.  Visit CaliforniaGeo’s website for more great posts like this and see the original here.

What They Are-

Drill rig and service truck for boring and grouting of vertical pipe loops


When you want to exchange thermal energy with the earth, you must make an underground connection to tap it as your thermal battery.  There are multiple kinds of geothermal heat exchangers (GHEXs), but today we’ll look at the type known as grouted geothermal boreholes.  All photos in this post come to you courtesy of Santa Rosa, California’s premium installer, Air Connection

Boreholes for geothermal heat pump purposes range in diameter from under five inches to over seven.  Once drilled vertically downward, they carry a pair (or two) of HDPE (high density polyethylene) pipes with a return bend at the bottom of the loop before being filled to the surface with grout. The grout is a precise mixture of water, sand, and bentonite, which swells slightly against the borehole wall as it sets up.

Where They Go-

They can be placed under driveways, open fields, under parking lots, or even under buildings themselves.  For most residential applications, a single loop of U-bend pipe will be installed to a depth of 200 feet. 

Grouted loop pigtails ready for header trenching and connection


This counts as one ton of heat exchange capacity per hour (12,000 Btus).  Commercial and industrial applications go down to 600 feet in wider diameter bores, carrying either larger pipe or two or more smaller pipe loops for multiple tons of capacity.

Because pulling heat out of the ground or sending it back into the ground can have a concentric thermal influence, outward from the borehole, spacing between boreholes can be critical for renewable thermal capacity to be assured for heating or cooling. 

New construction bore field under future parking lot


For example, too narrow a spacing could be difficult for heat extraction in Fargo, North Dakota, just as it could be for cooling in San Antonio, Texas.  This is why the best bore fields on large jobs are engineered via thermal conductivity tests on-site and computer analysis of the results.  Therefore, mapping of available space for boreholes to supply needed thermal capacity is a must.

Getting To/From The Heat Pump-

Once the spaced boreholes with pipe and grout are installed, their stubs are connected to horizontal supply and return headers (at a minimum depth of four feet) that circulate liquid down to their bottoms and back up to the surface. 

Retrofit bore field and headers under existing parking lot

The headers’ liquid content is pumped from (and leads back to) the geo heat pump(s) in the system.  All pipe connections are heat-fused before burial.  There are no mechanical, tension-based fittings to loosen or fail, underground. 

With proper installation, geo borehole heat exchangers are a perpetual thermal resource, isolated from disturbance, damage, taxes, or underground temperature change.

—Bill Martin